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Swallow-wort threatens monarchs

Swallow-wort has been reported in small populations in Southwest Michigan since 2012, and is still being found in new areas. This invasive creeping vine grows very quickly and very densely, meaning that it can swamp and smother other vegetation and trees. It also is poisonous to monarch butterflies, which are attracted to the vine because of its distinctive seed pods. The butterflies will lay eggs on the vine, but the larvae will be poisoned when they hatch, killing entire generations of the butterflies. Because of this, it is highly encouraged to manage populations as soon as they are found.

But, because of these seed pods, winter is a great time to find new populations of swallow-worts! The vines hold on to the thin, milkweed-like pods throughout winter, meaning they are easy to spy now that the leaves have fallen off. In the spring, the vines will grow dark green, waxy, opposite leaves, which can be narrow- to oval-shaped. June through August, swallow-warts create small, 5 petaled flowers that are dark purple for black swallow-wort and light cream to pink in pale swallow-wort. The milkweed-like pods will form in late summer, and appear green and thick until they dry to brown and eventually split, dispersing their winged seeds.

Like many invasive species, swallow-wort can be difficult to remove once established. With very small or new populations, plants can be hand-pulled or dug. Remember to never compost or throw away invasive species, as this can increase the spread! Instead dry or burn any removed plant material entirely. Mowing and cutting the plant is not advised, since this can increase sprouting and re-growth. If pods are noticed late in the summer, these should be removed by hand and burned. This will discourage spread and increase. For long term management or to remove established populations, chemical control is generally necessary.

The SWxSW Corner Collaborative Invasive Species Management Area (CISMA) is a grant funded program to manage invasive species in Berrien, Cass, and Van Buren Counties. With funding from the Michigan Invasive Species Grant Program, the CISMA aims to help landowners and stakeholders in the Southwest Michigan area manage invasive species. If you believe you’ve seen swallow-worts, please report this to the Midwest Invasive Species Information Network (MISIN) at, if you have questions about invasive species or invasive species treatment please contact the Cass County Conservation District at 269-445-8641 ext. 5 or If you’re interested in volunteering for the Cass County Conservation District please contact Korie Blyveis at or at the office number listed above.