Friday, September 19, 2008

Is Your Thistle a Wildflower or Weed?

We have had numerous comments from concerned visitors that we have invasive thistles at Powell Gardens. The concerns are over three invasive species of thistles that are on the noxious weed list: Bull Thistle (Cirsium vulgare), Canada Thistle (Cirsium arvense) and Musk Thistle (Carduus nutans): all not native to the United States (or Canada!). I did carefully check out the visitor comments just in case, but I can't find any invasive thistles on the grounds. I did photograph the two native species, which we actually have planted in the butterfly garden as well as growing wild.

This is the native Field Thistle (Cirsium discolor) and should actually be called "Prairie Thistle." This thistle is native to our region and throughout the Midwest. It is a premier plant for insects and wildlife and IS NOT INVASIVE nor on the noxious weed list. It supports such a menagerie of native bugs and birds that most of its seeds are consumed. Goldfinches love the seeds and use the fluffy seed down to line their late summer nests. Many bees and butterflies use the flowers for nectar and the Painted Lady butterfly's caterpillar eat the leaves!
This is a Prairie Thistle we planted in the butterfly garden with a late Great Spangled Fritillary imbibing nectar from the blossom.
Here a female Cloudless Sulphur is back lit while nectaring on the other native species of thistle on the grounds: the Tall Thistle (Cirsium altissimum), which is also NOT INVASIVE nor on the noxious weed list.
Most of the native thistles have dense hairs on the undersides of their leaves, giving them a woolly white appearance. All the noxious species do not have this trait and are also much spinier. You wouldn't see my hand without a glove holding them!
The flowers of the two native thistles are nearly identical but they have very different leaves: the Prairie Thistle has distinct lobes like a red oak leaf.
The Tall Thistle has leaves without lobes or minor lobes. Both native species are biennial (living only two growing seasons). Thistles in flower now will die as they produce their seeds. Those seeds will produce a rosette of leaves in the summer of 2009 and will bloom in late summer of 2010!
Ironweeds (Vernonia spp.) have been mistaken for thistles by our visitors. This is the Curly-top or Arkansas Ironweed (Vernonia arkansana) in the Butterfly Garden. It is also native on the grounds.
I also spotted a Missouri Ironweed (Vernonia missurica) still in bloom. Yes, it looks similar to a Canada thistle but is not spiny and does not spread by underground rhizomes but stays in a distinct clump.
The gardeners at Powell Gardens know the difference between native and exotic thistles and we do our best to remove noxious weed from the grounds. We do promote our native wildflowers whereever we can!

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