How could I have forgotten one of the most conspicuous winter features of dead goldenrod stems: galls of the goldenrod gall fly? The answer: easily, because even though they are dirt common in New York and Pennsylvania (and probably elsewhere), they are not common in central Massachusetts. I had forgotten about them until I stumbled across a place where they were abundant enough for me to notice. So it’s time to talk about old galls.
The goldenrod gall fly, Eurosta solidaginis, produces one generation per year, at least in the northeastern parts of the United States where I’ve seen it. Right now (April), the pupae are living inside of the galls that the larvae produced last summer. The stems were alive and growing when the eggs were laid, and the larvae induced the stem tissue of the host goldenrod to swell, forming the walls of the gall with a hollow chamber inside where the larvae live and feed. When the stems died, the larvae did not – the hardened gall protected them pretty well from physical harm. The galls and flies got cold in the winter, which didn’t bother the galls at all, since they were dead. It didn’t bother the larvae, either, because they have an array of internal antifreezes (complex alcohols) and well-placed water-transport proteins that protect the cells from freezing (some fluids of the larvae might actually freeze, but in locations that don’t matter). At some point, the larvae pupated inside the gall, and waited.
The pupa is inside the reddish brown puparium. It’s surrounded by fungi, and has a groove on it. Is it viable? Only time will tell.
This gall was on a stem that was bent but not fallen in April. The hole shows that something has exited the gall. What was it? The hole is neat, so it’s not the result of birds pecking away at the gall to get to the insect inside, or of predatory beetle larvae boring in to find a meal. Was it an adult fly? Or an adult parasitoid that killed the fly? If we look at the pupa inside the gall, we might be able to tell. If the exit from the puparium is neat, the fly did it. If not, an enemy did it.
Whatever came out of this gall, the gall has been through two winters. That’s a tough gall. If it had fallen to the ground before the fly emerged, decay could have been a problem. I wonder whether the presence of a gall is associated with a tougher stem so that the gall stays above ground long enough for the fly to emerge. Do gall-inhabited stems remain upright more often than gall-free stems? If so, then I also wonder whether the larva stimulates not only the swelling of the gall but also the hardening of the stem?
The exit hole is neat and circular because the larva, before it pupated, bored a tunnel nearly all the way out of the gall. The pupa can’t even move, much less bore its way out, and the adult has weak mouthparts (some would say none at all). The newly emerged fly has to swell its head to push its way through the last thin wall of the tunnel provided by the larva. If all is well, it will find a mate, and then females will find growing stems of Canada goldenrod, tall goldenrod or giant goldenrod on which to lay their eggs.
There are many good sources information about Eurosta solidaginis, so you should have no trouble learning more about them. I will revisit these flies when new galls form this summer.
Warren Abrahamson and Arthur Weis. 1997. Evolutionary Ecology across Three Tropic Levels: Goldenrods, Gallmakers, and Natural Enemies. Princeton University Press.
(If you plug this into Google Scholar, you get a link to all the works that cited this seminal book; some of them involve Solidago)
Of course all plants need water to grow, and goldenrods are no exception. But goldenrods vary widely in their demand for water, and some are only found near particular types of water.
At one extreme, there are goldenrods that thrive in or near wetlands (see previous post on Soil). Some prefer alkaline environments, others prefer acidic, but all require wet soil.
Toward the other extreme, prairie plants grow in full sun with a modest amount of rainfall. But their exposure (i.e., absence of a tree canopy) ensures that atmospheric moisture condenses during radiative cooling at night. Much (most?) of the dew evaporates as the morning warms up, but some of it drips or runs into the soil and helps the plants endure the times between rains.
In these upland environments, there is variation in tolerance and/or preference for levels of soil moisture. Patricia Werner and Robert Platt examined five species of goldenrods in a prairie preserve in Iowa (a sixth species, grass-leaved goldenrod, is now in the genus Euthamia). The species had broad overlap across a range of soil moisture, but each had a distinct peak where they were most abundant (see the graph). Would each species shift its peak if one or more of the other species were absent?
Giant goldenrod, Solidago gigantea, (#5 on the graph) is on the wetter end of the moisture gradient, which is consistent with my experience in upstate New York. Giant goldenrod was in nearly every old field I explored, but nearly always in parts of that field that tended to be wetter than the rest. When the soil elsewhere was firm, the soil around giant goldenrod was softer, or if it was also firm, it was darker from the higher moisture content. Throughout its extensive range, giant goldenrod occurs where the soil holds sufficient water for its growth (but not too much). Solidago canadensis (#4) and S. speciosa (#3) overlapped with giant goldenrod, but grew well in drier conditions.
Gray goldenrod, S. nemoralis, (#1 on the graph) grew on the driest soils in Werner and Platt’s study, which is pretty impressive. I often find gray goldenrod in New England in the driest soils near roads. They are always short, but they still manage to flower and produce seeds.
Missouri goldenrod, S. missouriensis, (#2 on the graph) requires a bit more water than gray goldenrod, but also has a reputation for being tolerant of drought. In the Dust Bowl of the 1930s, when many grassland plants were dying for lack of water, Missouri goldenrod was said to survive well, and even spread into areas left bare after the demise of other plants. Or maybe it was just more conspicuous when other plants were gone. Whether it spread or merely survived, it depended on its deep roots (over two meters down) to harvest whatever moisture was available deep below the surface. It’s an impressive contrast to those species that are restricted to wetland habitats.
Proximity to water
Some goldenrods are most abundant near the seacoast, perhaps benefitting from climate moderation. Seaside goldenrod, S sempervirens, ranges from the Canadian Maritimes into the states along the Gulf of Mexico. Its leaves tend to be a bit leathery, and it almost certainly has some degree of salt tolerance from the sea spray blown inland by storms. It also does well along the shores of lakes, though it seems not to have gotten to these shores on its own. People have planted it far from the ocean, perhaps for its long-lasting flowers.
Wand goldenrod (S. stricta) is another coastal species, found from New Jersey through the Gulf of Mexico, growing in wet sandy soil. Pine barren goldenrod (S. fistulosa) grows in similar habitats as far south as Louisiana. Carolina goldenrod (S. pulchra) is also in similar habitats, but only in three counties of North Carolina (who knows why?). On the face of it, sandy soil shouldn’t be wet, but there are plenty of places where the water table is near the surface (coastal sands can be excellent aquifers) or where there is something impervious below the sand.
Elliott’s goldenrod (S. latissimifolia) is yet another coastal species, growing from Nova Scotia to Alabama in wetlands, some of which are brackish, a clear sign of salt tolerance. None of the habitat descriptions mentions sand, though, so its specific habitat seems to differ from some of the other coastal species.
Small’s goldenrod (S. pinetorum) and twisted-leaf goldenrod (S. tortifolia) grow on sandy soils, but farther inland where the sand is a good deal drier than the soils that wand and pine barren goldenrods need.
Hairy-seeded goldenrod (S. villosicarpa) always grows near estuaries, only in North Carolina, especially in places where the trees have been felled by hurricanes, leaving a lot of sunlight until the trees grow back. This combination of conditions occurs in just a handful of locations.
Solidago spathulata has somehow earned the common name of coast goldenrod, perhaps because it occurs only on dunes and headlands in a few places along the coast of California and Oregon. Like its eastern congeners, Small’s and twisted-leaf goldenrod, it manages to survive in dry sandy soils.
Solidago simplex, sticky goldenrod, grows in a wide range of harsh habitats, but one variety (gillmanii) is found only on the dunes around Lake Michigan and Lake Huron. We’ve seen this combination before: sand and large bodies of water. But the water here is entirely fresh, just to add to the variety of habitats exploited by goldenrods.
But wait, there’s more. At least two other species live in close proximity to water, but only water that is flowing in streams. They are riparian. One is rock goldenrod, S. rupestris, scattered in river valleys from Tennessee to Pennsylvania. The common name suggests that it prefers places where the river has washed away most of the small particles like soil (!). Such places are not great for other plants, so these goldenrods might benefit from the absence of competitors.
The other species, plumed goldenrod (S. plumosa), probably depends on riverbanks scoured by floods where few other species can survive. I say “probably” because there are hardly any plants of this species still alive, and those are found only along the Yadkin River in (you guessed it) North Carolina, where dams might be reducing or eliminating the scouring floods on which it depends.
So what can we conclude about water and goldenrods? Some need a lot, some can survive on a little, and many are in between. Some need to be near large bodies of water, perhaps for the reduction in seasonal temperature fluctuation, perhaps for the humidity, perhaps even for a bit of salt. Some depend on disturbance, perhaps to open up the canopy and let in sunlight, perhaps to keep the number of competitive roots down to a tolerable level.
Goldenrods are definitely diverse. Since they shared common ancestry in the distant past, they have diverged to live in different places, under different conditions, and with different requirements. Sure, there are other groups of plants with an even wider diversity of habitats and adaptations, but goldenrods are pretty impressive. Before I began posting about them, I had no idea how varied they were. I already liked goldenrods. Now I like them even more.
Patricia A. Werner and William J. Platt. Ecological Relationships of Co-Occurring Goldenrods (Solidago: Compositae). The American Naturalist, Vol. 110, No. 976 (Nov. – Dec., 1976), pp. 959-971
Sunlight is essential for all plants, and in my previous post, I talked about species of Solidago that do best in full sunlight, a habitat type that is rare in regions dominated by forest. In the middle of the continent, of course, sunlight is easy to find. But there is more to a species’ habitat than sunlight.
The sunny habitats span an enormous range of temperature and moisture across the North American continent. Canada goldenrod (S. canadensis) grows in open places in nearly every state and province north of Mexico. In Alberta, summer day length peaks near 18 hours, while in Texas, it is a little over 14 hours. Temperatures differ accordingly. In Ohio, mean annual rainfall is roughly a meter. In New Mexico, it is one third of that.
Canada goldenrod’s temperature tolerance, growth rate, flowering time and drought resistance vary enormously, perhaps because of phenotypic plasticity, or because of local adaptation, or both. In any one location, of course, it will occur in locations that have what it needs (for example, relatively dry soil farther east, relatively wet soil farther west). But even with some microhabitat preferences, this species is one of many goldenrods, and many plants in general, that have extraordinarily wide environmental tolerance.
But most goldenrods live in fewer places and in a narrower range of conditions.
Some species of goldenrods do best in shade. Solidago caesia (blue-stemmed goldenrod) and S. flexicaulis (zigzag goldenrod) are two of those in eastern North America. They are not huge plants, but they can be quite abundant, nearly always beneath a canopy of trees. They bloom in summer and autumn, so they are not spring ephemerals that bask in sunlight before the trees leaf out. No, these are shade-loving plants, in stark contrast to many of their congeners.
Some other shade-dwelling species of note live in eastern mountain ranges. Solidago ouachitensis, as the name implies, is found only in the Ouachita Mountains of Oklahoma and Arkansas, preferring forested areas on north-facing slopes. Not to be outdone, gorge goldenrod, S. faucibus, lives up to its name by living down in the gorges of the middle and southern Appalachian Mountains. Probably the most extreme example is S. albopilosa, whitehair goldenrod, found only in the Red River Gorge of eastern Kentucky, only under overhanging rock ledges, and only in sandy soil that is shaded, but not deeply shaded. Perhaps it is no surprise that this species is the most endangered of all the goldenrods in North America.
Some of the sun-loving goldenrods can grow also in the woods. Susan Beatty and her students often found S. juncea (early goldenrod) and S. rugosa (wrinkleleaf goldenrod) under tree canopies in upstate New York. These two species are common in neighboring old fields, and there seeds drifted in among the trees and managed to take root. In Gray’s Manual of Botany, Fernald notes that S. ludoviciana (Louisiana goldenrod) is another species that can grow in “open woods,” though when they do, the plants are “mostly sterile.” But even if unable to flower, goldenrods can usually spread vegetatively (a topic for a later post).
The term “open woods” is used often to describe goldenrod habitats, but it covers an enormous range of plant communities. At one extreme, the openings are like those mentioned in my previous posts, gaps in the canopy resulting from the death of one to many trees. Those gaps are doomed to close, but for a while, there is enough sunlight for non-forest plants to grow. Solidago rugosa seems particularly good at finding such gaps.
In other places, “open woods” means that the trees are spaced more widely than in a typical forest, forming a woodland without a completely closed canopy. Pines and oaks are common trees in such woodlands, and goldenrods do well in the many spaces between the trees. As the distance between trees becomes even greater, woodlands becomes savannas, and there is far more open space than shade. Then sunlight is rarely limiting.
While some goldenrods seem to care hardly at all about the quality of the soil, others are quite picky.
Fernald (Gray’s Manual of Botany) notes that several species are found in “rich” soil, though I’m not sure precisely what that means. A soil can be rich and dry, so richness is not moisture. My best guess is that he was referring to soil depth, especially depth of the organic layer. If I am right, then talus slopes would be the least rich, with far more rock than soil. Only the crevices between the rocks provide access to soil, so the plants are inevitably sparse. Wright’s goldenrod (S. wrightii) often grows on talus slopes, as do many of the high-elevation goldenrods (see below).
Soil pH is another soil characteristic that is important for several species. Houghton’s goldenrod (S. houghtonii), Ohio goldenrod (S. ohioensis), and Nevada goldenrod (S. spectabilis) all occur in wetlands that are alkaline. Western rough goldenrod (S. radula) and Gattinger’s goldenrod (S. gattingeri) grow in limestone soils, but not in wetlands. In contrast, bog goldenrod (S. uliginosa) prefers stereotypically acidic bogs.
Gorge goldenrod, mentioned earlier, often grows beneath hemlock trees (Tsuga canadensis). Not only do these conifers produce deep shade, they also alter the soil chemistry (as do essentially all plants, but hemlock is more significant than most). Susan Beatty (also mentioned earlier) found that the forest understory community below hemlock tended to be measurably different from that under other species of trees. Something about hemlock-modified habitats seems to suit gorge goldenrod.
In Oklahoma and Texas, high plains goldenrod (S. altiplanites) can grow on gypsum soils, high in calcium sulfate but otherwise nutrient-poor. Farther west, Guirado goldonrod (S. guiadonis) grows in wetlands on serpentine soils in southern California. These soils are low in nutrients, but also high in heavy metals, a seriously stressful combination of conditions. These two species are outliers among goldenrods, but a clear testament to the potential for evolutionary adaptability in Solidago.
North American goldenrods can be found from Alaska and Nunavut in the north to at least the Isthmus of Tehuantepec in the south. But no one species lives everywhere, and some have distinct temperature preferences.
Solidago leiocarpa and S. spithamaea live at high elevations, the former in the northeast, the latter in the Great Smoky Mountains. Another species (S. multiradiata) has at least three common names, all of them suggesting cold: alpine goldenrod, northern goldenrod, and Rocky Mountain goldenrod. The distribution map is impressive – all across Canada (plus Maine), then south into the Rockies, Cascades, Sierras, and some sky islands in the southwest, but always at high elevations. None of these species live where there are prolonged high temperatures.
There are no true desert goldenrods, but some of them can tolerate intense sunlight at subtropical latitudes. None has evolved succulent tissues anything like those in drought-adapted Senecio, but they are certainly tough. Within temperate latitudes, goldenrods can be found in all but the hottest places.
Water is supremely important for all plants, and goldenrods get their water from the soil in which they grow. But it’s not just soil moisture that matters for some species. I will devote another post to goldenrods and water.
The most conspicuous goldenrods in many parts of North America are those growing in fields that once were used for farming, so-called old fields (Catherine Keever was one of the first ecologists to carefully document old-field succession). Some of them (perhaps most) grew well in the grasslands and prairies that preceded European-style agriculture in the middle of the continent, places with lots of sunlight because woody vegetation was grazed and/or burned away, or couldn’t grow in the dry conditions. With plenty of light, rainfall, and good soil, some goldenrods can grow two meters tall, while other species are shorter.
When Europeans began to colonize the east coast of North America, most of the land they encountered was forested. As they spread agriculture and cities westward, up to 90% of the forest was cut in any one location. We can see the results of deforestation in the sediments of lakes and other bodies of freshwater. Most trees and many weeds are pollinated by wind, not animals, and their pollen blows all over the place, including into the water. The outer covering of the pollen grains is highly resistant to decay, and the grains remain identifiable (to some degree) for thousands of years. Researchers (Margaret Davis being one of the most notable) can extract, identify and determine the age of the pollen laid down in these sediments. What do they find? When settlers removed the trees, tree pollen decreased precipitously. At the same time, ragweed pollen shot up like a rocket, so much so it the Ambrosia pulse (Ambrosia is the genus of ragweeds) is now used to mark the arrival of European-style agriculture in any part of eastern North America. Ragweed thrived in the sunlight and exposed soil of farms. Before that, it was extremely rare.
Goldenrod pollen is heavy and sticky, great for glomming onto passing insects, but terrible for blowing in the wind. There is no pollen record for goldenrod. But the sun-loving goldenrods might have behaved much like ragweed, their sun-loving herbaceous neighbors. Or not, because their seeds disperse differently. Goldenrod seeds (fruits, technically) disperse in the wind, while ragweed fruits have no hairs (pappus, technically) but its fruits contain some oils that can attract birds.
But where were these sun-loving species before the land was cleared, before there was abundant available sunlight? We don’t find them today in the deep shade of the forest (with few exceptions), nor was there more than a trace of ragweed pollen for most of the time the forest was present after the ice age. These weeds must have had difficulty finding places to live when the forest was intact.
But their lives were not impossible. There are always some places where trees die and light reaches the forest floor for a few years until other trees fill in the gap. Sometimes, the gaps are no larger than a single tree. Other times, they are huge when a storm is intense (a hurricane, or a tornado, or just a nasty regular storm). Or maybe a pest (disease, insect, etc.) had its way with some trees, maybe a lot of trees.
One of these enemies might have been beavers. These busy rodents build dams where there isn’t already a pond, and the resulting flood kills trees that were living near the stream. Beavers also gnaw down trees to reach the twigs that they store for winter. In short, beavers can make major modifications of the landscape.
After some number of years, beavers are likely to deplete the supply of preferred trees within easy access of the pond. Then they move. Or they might starve in a severe winter, or some predator might manage to catch them. However it happens, when the beavers are gone, the dam will fail, the pond will drain, and the land will revert to meadow.
I’m just speculating here, but could the areas thinned by beavers, and/or the meadows that grow when beavers were gone, have been habitat for those goldenrods that thrive in lots of sunlight? There is no pollen record to test this hypothesis, so for the moment, I haven’t been proven wrong.
All organisms, including goldenrods, have chromosomes in their cells. Their chromosomes are organized in pairs, and the two chromosomes in a pair typically have the same genes in the same physical order, though the precise makeup of each gene (the sequence of bases in the DNA) often differs a bit between the genes (alleles) on those two chromosomes.
When goldenrods reproduce, the sperm or egg carries only one chromosome from each pair because both types of gametes are the products of meiosis, a series of cell divisions that separate the pairs of chromosomes. When egg and sperm meet, the resulting embryo (and plant, if the embryo survives and grows properly) will have pairs of chromosomes, one member of each pair from the mother, one from the father. The number of chromosomes in the gametes is the haploid number (represented by n) and the number of chromosomes in an adult is the diploid number (2n), twice the haploid number.
Most species of goldenrods have nine pairs of chromosomes in each of their cells, a diploid number of eighteen.
But there are also goldenrod species with 36 chromosomes, some with 54, at least one with 90, and at least one that has 108 or 126. Some species have 18, 36 or 54 chromosomes, some other species have 18 or 36, some other species have 36 or 54, and some other species have 18 or 54. (Different published sources sometimes report different chromosome numbers for the same species. They are not wrong! They are telling us that we need to examine the chromosomes of more plants.)
You have probably already noticed that every number is 18 or a multiple of 18. Somehow, some plants have ended up with more than the diploid number (2n) of chromosomes. “Somehow” is a phenomenon called polyploidy, and it has occurred in essentially every lineage of vascular plants, and multiple times in the genus Solidago.
How can the number of chromosomes increase, and why in multiples of 18?
Here is one way: First, let’s say a goldenrod is producing egg cells in its ovary and sperm cells in its pollen grains, but for some reason, meiosis fails, and the gametes (egg and sperm) end up with 18 chromosomes (2n) instead of 9 chromosomes (n). These would be the result of nondisjunction, a double negative that means the pairs didn’t separate when they were supposed to. Then, let’s say that this plant is self-fertile (that is, pollen from its own anthers can pollinate ovules in its own ovaries). If the diploid sperm fertilizes the diploid egg, the result is an offspring with 36 chromosomes (4n), a tetraploid plant. This could be one way that some goldenrods ended up with 36 chromosomes. This is one type of autoployploidy, polyploidy produced within a species.
Here is another way, which seems even less likely, but we have evidence that it has happened in many kinds of plants. First, let’s say that nondisjunction happens in individuals of two different species of goldenrod. We already know that many species of goldenrod can hybridize, but on rare occasions, they do so with diploid gametes, and the resulting 4n hybrid would have 36 chromosomes (tetraploid). This is called allopolyploidy, polyploidy that results from hybridization.
Just in case you were wondering, polyploids are often sterile, but they can still reproduce vegetatively (a topic for a future post).
But it is also possible that they might be fertile and capable of producing seeds.
But could tetraploid and diploid goldenrods breed with each other? Let’s look at the chromosome numbers for a clue. If a plant with 18 chromosomes crossed with a plant with 36 chromosomes, and if both produced gametes by meiosis, the result would be 9 + 18 = 27. That’s a triploid number (3n), and no goldenrods have yet been found with that number of chromosomes in the adult plant. Triploids are rarely fertile, and you have seen a triploid plant if you have ever eaten seedless grapes. They can’t produce fully-formed seeds because triploids produce weird gametes (if they produce any at all) that rarely fuse.
But perhaps you noticed that 2 x 27 = 54, and there are goldenrods with 54 chromosomes. Maybe (I’m just speculating here) plants with 18 and 36 chromosomes each produced gametes by nondisjunction, so that the gametes had 18 chromosome from the 18 chromosome parent, and 36 chromosomes from the 36 chromosome parent. The result could be 18 + 36 = 54. Several species of goldenrods have plants with 54 chromosomes, and at least three species appear to have only 54 chromosomes (6n or hexaploid).
There are at least two species of goldenrod with 90 chromosomes (10n) and at least one with 108 or 126 (12n or 14n). Polyploidy has gone crazy in these species, but still the numbers are multiples of 18. These species (S. faucibus, S. lancifolia, and S. glomerata) all live in the southern Appalachians of Kentucky, Virginia, North Carolina and Tennessee. Is there something about that geography, bedrock, soil, climate, evolutionary history, etc., that has produced more polyploid descendants than elsewhere? I suspect so, but don’t know how to figure out why.
In contrast, all the species in eastern Asia, whether on the continent, peninsulas, islands, or mountains, have 18 chromosomes, the typical diploid number in Solidago. Is there something about that region that has limited the occurrence of polyploidy? I wonder…
One more thought about the origin of polyploid hybrids. There is a polyploid species of cordgrass (genus Spartina) in Great Britain that seems to have arisen not through fertilization, but through merging of cells in the underground portions of the plants (roots and/or rhizomes). These polyploids are fertile, breeding well amongst themselves, but not with any of the other cordgrass species in the saltmarshes where they live, all of which are diploid.
Could any of the goldenrod polyploids be the result of underground mergers? They have abundant roots and rhizomes, and they can grow vigorously in crowded stands. I don’t know, but I’ll just throw this hypothesis out there.
But if goldenrods with different numbers of chromosomes cannot interbreed, then those “species” that have individuals with two or three different numbers of chromosomes might actually be different species, if we define species as groups of organisms that can successfully interbreed. If so, we might be able to distinguish these species only by checking their chromosome numbers, not something that works well with a field guide to wildflowers, or even a dichotomous key intended to be used by experts. If you already thought it was difficult to identify the different species of goldenrods, it might be a whole lot more difficult than you thought.
John C. Semple 2016. An intuitive phylogeny and summary of chromosome number variation in the goldenrod genus Solidago (Asteraceae: Astereae). Phytoneuron 2016-32: 1–9.
The bright, abundant flowers of goldenrods are a conspicuous part of their life cycle. Pollinators visit the flowers for nectar and pollen, and as they move from flower to flower and plant to plant, they spread pollen. Pollen grains contain sperm cells that find their way into the ovule within the ovary, where there is an egg nucleus. The merging of sperm and egg nuclei produces the first cell of an embryo, which will remain dormant inside the seed until the time and place are right for germination.
We will revisit pollination and fertilization when they are actually happening in the summer.
In sexually reproducing organisms, species are usually defined as groups of organisms that can breed with each other. For most goldenrods, that’s exactly what they do: reproduce with members of their own species. But quite a few species manage to breed with other species and form hybrids. Apparently, pollen moves easily between them, and then the sperm and egg cells are able to fuse to produce viable offspring. Hybrids occur rarely enough that the species remain mostly separate, but they occur often enough for botanists to notice.
Among those botanists was Merritt Fernald in the first half of the 20th century. He compiled the eighth edition of Gray’s Manual of Botany in 1950, a century after Asa Gray had produced the first edition while he was on the faculty of Harvard University. Fernald recognized sixty-nine species of Solidago (excluding Euthamia, which was then a subgenus of Solidago, but is now a genus of its own). Of those sixty-nine species, Fernald stated that twenty-seven form hybrids with other goldenrods, and he listed the species with which each species did so.
We have learned a lot about goldenrods in the last seventy years, but the species in Fernald’s compendium are nearly all still recognized as “good” species in sources such as Flora North America, though some of the names have changed. I think his list of species is worth examining further.
Among the species in Gray’s Manual that were said to hybridize, I think it’s interesting that some species hybridize with only one other species, while others hybridize with as many as eight species. I expected that hybridization would occur with closely related species, and it often does. Using the subsection classification within Solidagopresented in Flora North America, eighteen species hybridize within their own subsection (presumably species that are closely related), but seventeen can cross with species beyond their nearest relatives.
Canada goldenrod (S. canadensis) and wrinkled-leaf goldenrod (S. rugosa) are the champions, hybridizing with eight and seven other species, respectively. Next is silverrod, Solidago bicolor, with six hybridization partners. Silverrod has white petals around yellow stamens and pistils, suggesting that the difference between white and yellow petals does not prohibit crosspollination. Who knew?
According to the notes in Gray’s Manual, hybridization is not always symmetrical. Was this a mere oversight, or did botanists know that the pollen of species A could fertilize species B, but the pollen of B could not fertilize species A? This kind of asymmetry is not as strange as it might seem. There are complex interactions between pollen grains and pistils. Is the stigma receptive, can the pollen grain germinate (yes, it germinates), can the pollen tube grow down the style and into the ovule (yes, there is a tube for the sperm cells to follow), can the sperm enter the ovule, then find and fertilize the egg? Any of these might work in one direction, A to B, but not the other, B to A.
Some of the hybrids alleged by Fernald seem, on the face of it, highly improbable. The stems of Solidago caesia, blue-stemmed goldenrod, grow at an acute angle to the ground, spreading laterally rather than vertically. Their flowers form in clusters in the upper axils of the leaves, dotted along the stem. Yet this species is supposed to hybridize with Solidago canadensis, a tall species with vertical stems and a large branching inflorescence at the top of a conspicuously tall plant. I dismissed Fernald’s claim – until I saw a plant on the Cornell University campus that was only somewhat bent from vertical, and that had both axillary and terminal flowers. That plant made me stop and pay attention to the malleability of goldenrods.
Researchers have learned quite a bit about goldenrods in the seven decades since Fernald published his edition of Gray’s Manual. A few of the hybrids turned out not to be hybrids at all, but merely reproduction within a highly variable species. And some species have experienced hybridization in the past that is not obvious by merely examining the structure of the plants. How do we know? Chromosome numbers. That’s a topic for another post.
Goldenrods are beautiful, but I know that many people like go beyond the esthetics to learn how wild species can benefit or harm humans.
One benefit of goldenrods is honey from honeybees. Honeybees (Apis mellifera) are indigenous to Eurasia and Africa, but not North America. All American honeybees were introduced from other continents. There are feral honeybees, as well as vast numbers overseen by beekeepers, and they visit a huge array of plants to gather the nectar that they reduce down to honey.
In many parts of North America, goldenrods bloom abundantly in mid to late summer (more on blooming in later posts). Hundreds of species of bees, including honeybees, gather nectar and pollen from goldenrods. Only honeybees store so much nectar, as honey, that adults can remain active through the winter in cold temperate climates. They spend nearly the whole winter huddled in the hive, keeping the queen and the colony warm, using the stored honey and pollen to generate metabolic heat. Only occasionally do they venture out for a cleansing flight to dump their feces and sometimes debris from the hive. They need a brief warm spell for a successful flight, and if they don’t get it, the hive can be in trouble. I lost a hive when too many bees froze in their attempt to cleanse. They were probably also stressed by endotracheal mites, so that was the end of my beekeeping – it was just too hard to keep them going.
Pollinators in New England face a changing flower-scape during the growing season. Howard Ginsburg (I know him as Howie) documented the change some years ago (Foraging ecology of bees in an old field. Ecology 64: 165-175: 1983
Of course, plants bloomed throughout the season, but not in equal abundance. After an early pulse in June, floral abundance drops a bit in July, and then surges with late bloomers like goldenrod. Some bees forage throughout the season, and others for just some of it, but there are nearly always some bees on flowers, and they are joined by a wide array of other insects (flies, butterflies, moths, beetles, wasps, and more).
Honeybees collect nectar from as many plants as they can, and many of the early flowers produce nectar that results in a light-colored honey. It is sweet, and the flavor is, shall we say, mild. Beekeepers can get lots of early honey, and that’s what most people come to expect when they consume honey, whether local or commercial. Some beekeepers are lucky enough to have their hives near basswood trees, and they get an excellent crop from flavorful midseason nectar. Honey from buckwheat is deeply dark and distinctive, if your bees are near a field of buckwheat. I have no idea where any such fields exist.
I do know where goldenrods exist: across the entire continent. The nectar in these late-blooming wildflowers is abundant, and it produces a darker honey with a wonderfully complex flavor. But the season is about to end, so it is risky to take too much late-season honey from a hive if the bees are to have enough for their winter survival. All hives have late-season honey because goldenrods are ubiquitous, and with care, people can collect some of it to savor, share, or sell. If you want to experience something deep and different, look for late-season honey, and thank goldenrod for the bulk of the flavor.
One final note: if you let any honey sit long enough, it will darken and get more interesting. If it was fully condensed by the bees, it won’t spoil because the sugar content is too high (and the water content is too low) for anything to live in it. If it was not fully condensed, it might ferment and gain an alcoholic fragrance among the floral notes. Either way, it’s good stuff.
In winter, goldenrods are not golden. They are brown or pewter gray, the seeds lingering or gone, the cells dead, with no green remaining.
Yet they stand – at least some – and remind us of the tall weeds of summer. They stand because they are strong, as solid as their name Solidago. This winter in New England, one of our early snowfalls was wet and heavy, dragging many of the goldenrod stems to the ground. But not all. Some could withstand a weight that bent birch trees low.
Alas, these dead stems will not sprout again. They were the growth of the past year, not the future stems of the goldenrod. As the seeds mature and the stems die, the plants turn brown. Then they slowly fade to gray as the seeds disperse and the fragile remnants of the flowers fall away.
But spring will bring another chance for the stems to harbor life, another year to be part of the field community.
Once it becomes sufficiently warm (a warmth that you might consider cold), spiders will emerge from their winter dormancy and use the bare branches of dead inflorescences to build their webs. Two kinds of spiders use old goldenrods frequently, small orb weavers (the spinners of stereotypical spoke-and-spiral spider webs), and dictynids, a family of small, inconspicuous spiders that construct a tangled mishmash of a web and hide in the middle. I will try to get pictures of these spiders in the spring and share them. For now, imagine what they might look like on a dewy morning, the silk supported by the branches of dead flowers.
For much of the summer, dead goldenrod stems are among the tallest structures in a field, and there sit the spiders, waiting for their food. Flying insects, rising above the bulk of the green and growing plants, occasionally blunder into their webs. For a small spider, even a small insect is a good meal. Tall, dead stems give these spiders an excellent location for feeding.
There is another advantage for being on dead stems. They don’t attract one of the major enemies of spiders: spider-hunting wasps. Some wasps grab spiders and paralyze them with a sting. Then they store the helpless spiders in nests or cells where the wasp’s young will feed on the defenseless prey. Quite a few spiders live on flowers of goldenrod, and the wasps also feed on nectar, so blooming goldenrods provide one-stop shopping for the wasps.
But dead goldenrods have no nectar, and few spiders, so they are likely to be overlooked by the wasps. That’s a good thing if you’re a small spider.
Late in the summer, another kind of creature will sometimes climb up on dead goldenrod stems. These are the immature stages of small beetles in the genus Exema. The larvae feed on goldenrod leaves, but they look like the turds of large caterpillars because they use their own turds to build a dark brown case around themselves for protection (visual and physical). They hide inside their own excrement and grow slowly during the summer. As the season wanes, they are ready to pupate, and that’s when they seek a location that won’t be disturbed. Some of them wander on to the dead stems of goldenrods from years past. Perfect. The gray stems don’t have pollen or nectar to attract parasitic or predatory insects. While the enemies of the beetles go elsewhere, the pupae on the dead stems can develop in peace.
Standing dead stems do not remain standing forever. When they finally topple – and they all topple eventually – they will decay back into the soil. It’s part of the life cycle of a perennial herb.
Goldenrods (species in the genus Solidago) appear to be indigenous on at least three continents, North and South America, and Eurasia, and perhaps a fourth, northern Africa. John Semple at the University of Waterloo has a map of this worldwide distribution on his website: https://uwaterloo.ca/astereae-lab/research/goldenrods.
There is one widespread indigenous species in Eurasia, S. virgaurea, though it is so widespread and has so much recognized variation that there could well be multiple species. There are also a half dozen indigenous species in far eastern Asia.
There is no universal agreement among botanists about how many species of goldenrod exist in North America, but they all agree that there are a lot of them. While you might think they all look alike (and many do), there are some surprising differences among all these species, and I hope to explore some of these differences in future posts.
Let’s look at something seemingly simple: where do they grow? The Natural Resources Conservation Service of the United Stated Department of Agriculture (USDA) maintains a database on plants in the United States and Canada. For each species that they recognize, they have a map of where it grows.
So let’s look at goldenrods. I’m using the common and species names from the USDA.
For each species, you might have to scroll down a bit for the map. But then, you can play with the scale bar and zoom in on each state. When you zoom in, many of the maps show the occurrence of the species in each county. Some of the county distributions are quite surprising.
What you will see in these maps is the result of considerable speciation and adaptation among goldenrods. Some are widespread generalists, some narrow specialists. Specialization might include temperature, soil moisture, soil type, shade tolerance, elevation, latitude, proximity to the coast, or some combination of these. We will explore some of these specializations in future posts.
Have fun with the maps. If the maps don’t appear in your first browser, try another.
Atlantic to Pacific: these species stretch all the way across the continent (sometimes just barely).