Good Oak News

Wednesday, August 28, 2013

Native Thistles: Pillars of the (Natural) Community


A great spangled(?) fritillary butterfly enjoying some nectar from a native pasture thistle.

Yesterday I took the crew out for a field day to some great natural areas so they could learn some plants and see what real, healthy, managed prairies and savannas look like. Along the way we were fortunate enough to see a lot of native thistles in bloom. We saw many butterflies and bees feeding on these thistles, and I explained to them that, in addition to being excellent pollinator plants, the seeds were valuable food sources for small birds (goldfinches love thistles) and that the fluff from thistle seeds is used by hummingbirds and other small birds for nesting materials. Later, researching online I found that goldfinches are so enamored with thistles that seeing them actually has an effect on their hormone levels.

Then last night I read this article about the reintroduction of the rare swamp metalmark butterfly into a restored wetland in the Chicago area. It turns out the swamp metal mark caterpillars can only eat our native swamp thistle and pasture thistle. These valuable members of our local natural community have a lot to offer, but unfortunately they have suffered not only from habitat loss that effects all of our native species, but also from persecution due to their similarity to the weedy exotic thistles.

Exotic Thistles: America's Least Wanted

Most people think of thistles only as weeds. And it's true that some of most common, troublesome and difficult to eradicate weeds include Canada thistle, plumeless thistle, bull thistle, and musk thistle. These are all exotic species, introduced accidentally as agricultural weeds which are now problems in cultivated and natural areas alike. There are a few more exotic, weedy thistles too, such as European marsh thistle, but they are less common in this part of the state. In any event, these weedy thistles have long been recognized as pests and are on top of many noxious weed lists. There are only 3 plant species listed under Wisconsin's noxious weed law and one of them is Canada thistle.

In Illinois, both Canada thistle and musk thistle are listed as noxious weeds (not sure why they skipped the more common bull thistle), and they only bothered to put 7 other plant species on this list. Canada thistle is in fact listed as a noxious weed in 43 states!

When we at Good Oak are managing natural areas we always make controlling thistles a priority. Every farmer has a deep seated hatred of thistles, and we don't want people thinking our prairies are just 'weed patches'. Because of this common disdain for thistles, and some local weed laws (more on this later) native thistles are rarely included in commercial prairie seed mixes, despite the many benefits they can offer to wildlife.

Native Thistles: Rare Natural Gems

Wisconsin has five native thistles. Let me introduce you to them:

Pasture thistle (Cirsium discolor) is found in mesic prairie remnants, and sometimes more disturbed sites like woodland edges and yes, even pastures. You will occasionally find it in some better quality restored prairies.

Pasture thistle flower.
Pasture thistle.
The pale underside of the leaf on pasture thistle.

It looks a little like bull thistle at first glance, but the underside of the leaf is distinctively white, which is where this species gets its scientific name. A lot of overlapping silvery hairs create this bright white appearance. Bull thistle can be pale on the underside, but if you're looking at pasture thistle you will definitely think "whoa, that's white!"


Tall thistle (Cirsium altissimum) is a plant of open woodland and savannas. Note that the leaves aren't very thistle like with only small thorns. In fact, without the flowers you might not think it was a thistle at all. They have more surface area to gather more light in these semi-shade environments. The stems are covered in fine hairs, and tall thistle shares the white leaf undersides with its cousin pasture thistle.
Tall thistle.
Pasture thistle and tall thistle are the only two species that you have any chance of finding outside of a high-quality natural area. If you simply remember to look for the white underside to the leaves, you can always differentiate these two native thistles from the various weedy exotic species. These two species also bloom later than the exotic thistles. Bull thistle and Canada thistle for example, bloom from late-June through July, whereas tall and pasture thistle bloom from late July through August.


Swamp thistle (Cirsium muticum) grows only in good quality wetlands. The flowers are a darker, more vivid magenta color compared to most other thistles.
Swamp thistle.
Swamp thistle flower.


Hill's thistle (Cirsium hillii) is a state threatened species which you which you will only find in dry sandy prairies in the upper midwest. It's our shortest native thistle, typically around 18". It's also densely hairy to help it retain moisture in its dry sand-prairie habitat. The flower color is more of a true pink rather than magenta, even reddish.... unfortunately my poor-quality cell-phone photo below is not color accurate.
Hill's thistle.

Dune thistle (Cirsium pitcheri) is our rarest thistle, so rare in fact that it is Federally Threatened. It only grows on a handful of sites in Wisconsin, on stabilized sand dunes on the Lake Michigan shoreline. Its range is limited to the Great Lake shorelines of Wisconsin, Illinois, Indiana and Michigan, within a few-hundred feet from the shore. Its flowers are a pale pink, and its foliage looks nothing like other thistles with hardly a thorn to be seen. In fact the frosted-grey looking leaves look more like sage than any thistle.
Dune thistle flower.
Dune thistle's basal rossette (foliage).

Thistles: Nature's Grocery Store

As it turns out, thistles are extremely valuable plants to wildlife. Most of us think of flowers and we think of pollinators, and thistles are extremely attractive flowers to many kinds of bees, butterflies, hummingbirds, hawk moths, and many, many other small creatures. Spiders, assassin bugs and other predators set up shop on thistle flowers; pollinators can't resist the flowers so they are great spots to ambush prey.

Two species of bumble bee and dozens of small green beetles collect nectar and pollen from a pasture thistle.

As much as it pains me to admit it, this Delaware skipper is enjoying a nectar drink from a Canada thistle. Even the exotic invasive thistles can provide some value for wildlife.
Butterflies love thistles. Fritillary butterflies like this one seem to be particularly enamored by flowers in this genus.

In addition to pollinators and the predators who love them, many insects feed on the foliage of thistles, such as the above mentioned swamp metal mark butterfly. Some of these insects are specialists that can only feed on thistles and no other food source. Others are more generalist and can feed on other, similar plants. John Hilty from Illinois Wildflower has put together this list of insects that feed on thistles on his amazing website.

With at least 23 species of insects that feed on the foliage and (I don't think I'm going out on a limb here to say) hundreds of insects (and one bird) that feed on the nectar and pollen, you can see they play an important role in our ecosystem. Even more so if you consider all the insects, spiders, amphibians, reptiles, mammals and especially birds that in turn feed on the insects that feed on the thistles. Thistles aren't only a pillar of the natural community, they are a big part of the foundation of the natural food pyramid.

Separating Good from Bad: 

Our noxious weeds laws are put in place for a reason. These weeds cause economic and ecological damage. Canada thistle in particular is extremely difficult to eliminate requiring an expert, targeted approach for success. But to be absolutely clear, none of our native thistles are aggressive, weedy species. They will not spread rapidly like Canada thistle or infest over-grazed pastures like bull thistle or plumeless thistle. They really can't tolerate the heavy disturbance associated with modern agriculture and dairy production and pose no threat of economic loss to farmers. Unfortunately, many weed laws treat all thistles the same, outlawing the exotic weeds and the already imperiled native thistles alike.

For example, the City of Madison's weed ordinances which can be found in Chapter 23, section 29, paragraph 2 states that “The term 'noxious weeds' as used in this section includes the following: Canada or other thistles...” while leaving out many more harmful weeds such as, oh, I don't know, just about everything listed on the state of Wisconsin's invasive species rule NR-40.

The state of Iowa lists any and all all plants in the Cirsium and Carduus genera (what we might call all of the true thistles) as noxious weeds, regardless of origin. At least Iowa is thorough in its noxious weed list to include harmful invasive plants like purple loosestrife, common buckthorn and poison hemlock among others. By comparison Madison's weed ordinance seems somewhat haphazard.

I don't have much say in Iowa or in other municipalities around the midwest that throw a blanket ban on any thistle of any kind, but I would like to help make an effort to improve Madison's noxious weed law. If we can be clear about what plants are an economic, ecological and/or health threat to our citizens then we come up with a sound list of plants that should be banned in our community. The state's NR-40 list is a good place to start. Throw in a few hazardous plants that are already on the list such as poison ivy and the ragweeds, throw in a special request for reed canary grass which has some economic value, but is an ecological horror show, and I think we would have a good law. Then if only we could get the city to enforce it...

3 comments:

Nature in the Burbs said...

Interesting write up and great caption -- Thistles: Nature's Grocery Store. Thanks for the link to my blog too!

Susan J Hobart said...

How do I figure out which thistle we have in our oak savanna?

FrankOnABike said...

Hi Susan, if you read this, I apologize for not responding. I only recently got post notifications set-up.

Anyhow, you're going to need a good plant ID book. Most oak savannas have a history of grazing so they could still have the native thistles, or they could the weedy plumeless or bull thistles.

Look for the pale color on the underside of the leaf. Both pasture thistle and tall thistle will appear almost white on the underside. Those are the good guys.

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