Purple Thistle (Cirsium horridulum) is a common sight throughout the summer months. This plant is considered invasive by many, in fact in Iowa and Arkansas it is illegal to grow this plant. Believe it or not there are gardeners out there that prize this plant for it's beautiful purple flower and unusual foliage. These cultivated plants easily escape the confines of gardens and spread with great vigor throughout the landscape. It takes no time at all before the area looks like something out of a horror film that could be aptly titled "invasion of the thistles". I can certainly appreciate the gorgeous bloom that this plant produces, but it does not off-set the horrors of the prickly leaves or the invasive quality of the plant itself. Anyone having come in contact with this plant can attest to the horror of being stuck by these bristles. Their species name of horridulum means "somewhat bristly" boy is that an understatement. I think the name hints at how horrible this plant is! We fight a battle with these plants each season, my husband is constantly spraying them, only to realize he missed some and has to spray yet again. There are always a few that get by us each year and they end up going to seed and we are faced with them again the following summer.
All is not bad though, the plant is interesting in a few respects, one being its ability to move towards anything that disturbs it. For example if you were to brush against a leaf, the leaf would turn towards you in a protective manner. Insects traveling on the plant may find themselves inadvertently curled up in a leaf, as the plant tries to protect itself. This movement is called thigmonastic motion. This plant is also the host plant for a few butterfly's including the Painted Lady which has the nickname of Thistle Butterfly and the Little Metalmark. Dozens of other butterflies are attracted to the nectar of the bloom, such as the Black Swallowtail, Palamedes Swallowtail, Palmetto Skipper, Delaware Skipper,Three-Spotted Skipper and others.
Thistles are quite large, reaching heights up to three feet with another foot added when the bloom shoots upward. The leaves can range in length from 8 inches to 2 feet. So these plants are very impressive. The bristles are so sharp that even grazing cattle know to leave them alone. I'm sure wrapping your lip around one these leaves is an experience you wouldn't soon forget.
This plant will grow in a wide variety of habitats, including waste ground, roadside ditches, open fields, and pastures. They can tolerate moist or dry soils, in fact not much will deter them. If you choose to plant them in your garden to try and attract butterflies or because you find the plant to be interesting, please be aware of their tendency to be aggressive and keep them in check. Do not let them go to seed. If they aren't controlled, you will have nothing but a garden of thistle in short order, and the neighbors may not like it when these plants suddenly show up in their yards from seeds born on the wind straight from your garden.