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
© University of Chicago Press, for American Society of Naturalists
Gray’s Manual of Botany
Wikipedia (some pages are excellent, others are sparse)
John Semple, University of Waterloo, Ontario, Canada
Flora of North America