A whistle for thistle
By Alyssa Nyberg
Although field thistle (Cirsium discolor) is a common “weed” along our Northwest Indiana roadsides, it is a favorite flower of many butterflies, including the Monarch. From August through October, field thistle provides a valuable nectar source for such butterflies as the fritillaries, swallowtails, painted ladies, and sulfers. The flowers aren’t the only valuable part to butterflies. The leaves of the field thistle are the sole food of the painted lady butterfly (Venessa cardui) caterpillars.
Field thistle can grow from two to seven feet tall. During the first year of growth, field thistle remains a basal rosette of prickly, lobed leaves. Each leaf is roughly nine inches long, three inches wide, green above and white wooly below. Sharp spines line the edges and tips of each leaf. The second year, a flowering stalk emerges. Leaves grow alternately up the stem on either side of the stalk. At the top of each stalk are one or several flowers which can be pink, lavender or even white. The flowers grow up to two inches wide, providing a large colorful landing pad for the butterflies.
You will most often see field thistle growing in full sun. It is found in a wide variety of habitats, including prairies, woodland openings, meadows, pastures, and railroad and roadside right-of-ways. Although it occurs in disturbed and degraded areas, field thistle is also found in high quality prairies.
What to look for
Field thistle should not be confused with Canada thistle or bull thistle, which are both non-native, aggressive species introduced from Europe. Although we delight in seeing field thistle at Kankakee Sands, we remove Canada thistle and bull thistle from our restorations and roadsides. To tell the difference, look for the wooly white underside of the field thistle leaf. Neither Canada thistle nor bull thistle has this feature.
In just 20 years, Monarch butterfly populations have plummeted 90%, due to habitat destruction and loss of nectar sources. With the Monarch butterfly population declining at such an alarming rate, allowing our native field thistle to grow and flower seems an easy and prudent thing to do. We like easy! Come on out to Kankakee Sands to enjoy the beauty of the orange and black Monarchs feeding on the pink flowers of field thistle. It might just make ya whistle!
If you think you have the invasive Canada thistle or bull thistle on your property, please feel free to contact our office for help with identification and/or information on how to eradicate them.
Alyssa Nyberg manages the Native Plant Nursery at Kankakee Sands, coordinates outreach activities for the site and mows around field thistle when she sees them.
The Nature Conservancy’s Kankakee Sands of Indiana and Illinois is 10,000 acres of prairie and savanna habitat in Northwest Indiana and Northeast Illinois, open every day of the year for public enjoyment. For more information click here or call the office at 219-285-2184.