Spotted Knapweed

2016 November 19

I photographed these purple flowers on alongside the road leading uphill to our house on July 28, 2016, although we can find them pretty much all summer.


There appear to be two distinct populations of this plant along the road. Down at the bottom of the hill, the plants have good-sized blossoms and fairly broad, unbranched leaves:



But, further up the hill, they have smaller blossoms, and the leaves are thinner with frequent “fingers” projecting from the sides:



I’m pretty sure that these are both Spotted Knapweed, Centaurea maculosa, although they certainly do look like different varieties. The only part of the plant that looks “spotted” to me is the husk of the seed-head that remains after the flower drops its petals, so I presume that’s where the name comes from. This name also suggests that there are “non-spotted” knapweeds somewhere, but this plant genus originated in Eurasia and the others haven’t made the jump from the “old country”. Yet.


Anyway, this is another of the more notorious invasive plants in the area, and the edge of this road is a good example. About 11 years ago, the road was extensively re-built, with a lot of excavation of the adjacent hillside. This cleared out the existing vegetation. And, the first plant to come in and colonize the area was spotted knapweed. So now, there are long expanses of the hill beside the road that are pretty much knapweed monoculture. In these next pictures, the pale purple strip between the coarse rocks and the tree line is almost entirely spotted knapweed:




There are several reasons why it does so well: first, the seeds are little windblown things that rapidly colonize disturbed soil. Second, they are rugged perennial plants that don’t need much topsoil and can tolerate dry conditions well. Third, they taste nasty and have a dry, scratchy texture, so that nothing much wants to eat them[1]. And fourth, they produce scads and scads of seeds.

It can force out other plants, but where it really thrives is in disturbed locations where the topsoil has been removed and nothing much else grows. There have been some attempts at biocontrol, but without much success. Mostly the things that feed on it do well just to slow its spread, nothing seems to really knock it back. We see a lot of spittlebugs on the knapweed every spring, and I found a Knapweed Gall Fly once, but their effects on the actual growth of the knapweed appear to be minimal.

[1] Sheep (and probably goats) will eat it if they have to, but I don’t think they like it all that much.

3 Responses
  1. Carole permalink
    November 19, 2016

    Sounds like it doesn’t support m/any chewing insects which are what birds and other critters are looking to eat and is crowding out the plants that do support wildlife. 🙁 They are creating food deserts.

  2. Kathleen permalink
    November 21, 2016

    We do have more than one knapweed (that look similar) in eastern US, but I confess I do not know how to tell them apart.

  3. Mark Sturtevant permalink
    November 23, 2016

    If both populations of plants are the same species, then the differences can be from a couple different causes. First, there could be a genetic difference, and this character is referred to as a polymorphism. Or, the differences are not genetic but rather caused by growth in different environmental conditions. In which case one would describe it as phenotypic plasticity.

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