Tom's Blog

Friday, July 26, 2013

Compass plants out of their normal habitats

Compass plant (Silphium laciniatum) is a classic prairie species which does best in mesic habitats. Because it is deep rooted,  loam soils (such as the black soils of Illinois) are especially suitable. Although it grows well from seed, the first few years at a site it grows only vegetatively. Except in unusually favorable locations, it requires five or six years to flower. Once well established, it is long lived.

Here are two examples of compass plant developing outside its principle habitat.

Our  south-facing slope at Pleasant Valley Conservancy is very rocky. It is impossible to dig with a shovel and stakes and rods cannot be installed. Soil is practically nonexistent. The principle species in this prairie remnant is little bluestem, a very shallow-rooted grass. However, this year we have five or six compass plants in full bloom scattered across the slope. How do they manage it?

The bedrock here is Tunnel City sandstone, which the hydrogeologists tell us is a high permeability rock that is an important source of water for the Wonewoc aquifer. Presumably, compass plant is able to send roots down through this fractured rock, following vertical fissures, until it reaches a water supply. Although we have had scattered nonflowering plants for some years, this is the first year we have seen many flowering plants. Presumably the high rainfall has helped make the difference.

The other example is illustrated in the photo below. This compass plant seems to be thriving in deep shade under a shagbark hickory. The tree is growing at the edge of a fairly open oak savanna. All the other compass plants in this savanna are in full sunlight. Note that the tree is south of the compass plant so that the only open area is north-facing. This plant will almost never be in full sunlight.

The size of this tree shows that it has been here a fairly long time, so the plant has come up under the shade. Obviously, this compass plant does not need full sun to develop. I suspect the reason this species is not found more often in shady environments is because it is unable to compete with other species.

This year weather conditions have been outstanding for prairie and savanna development. It will probably be a long time before we get another year like it. Enjoy!


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