Invasive Plants of the Preserve, presented Sunday morning, 12 June 2022 by Anne Pearce, Invasive Species Outreach Specialist at UW-Madison. We had a small but very engaged group for this Sunday morning walk. Anne began by providing the official definition of “invasive species,” that is, a non-native species that has the potential to cause harm (ecological or economic). People often use the phrase more broadly, but although a native species may be aggressive, or even “weedy,” it is not technically invasive.
Anne began by showing some invasive plants that are right near the entrance to the path towards Picnic Point. We saw reed canary grass (Phalaris arundinacea), and learned to recognize it by its large ligule where the leaf sheath meets the blade. This species covers large areas where it crowds out other kinds of plants and becomes the only species in the stand, especially in wet ground. Another plant that is very near to the trail entrance is European Highbush Cranberry (Viburnum opulus var. opulus (a.k.a., Guelder-rose), an invasive species which is difficult to distinguish from the closely related native American highbush cranberry (Viburnum opulus var. americanum, sometimes still sold under the older name Viburnum trilobum). The best way to distinguish between them visually is to use a hand lens (loupe) and examine the small glands that are at the top of the leaf stalks (petioles), just below the leaf blade. If the glands are concave, like tiny bowls, the shrub is the European invasive, whereas the glands are convex it is the native one (see more at https://extension.umaine.edu/cranberries/highbush-cranberry/). Unfortunately, those along the path toward Picnic Point are the invasive kind.
In beds around the trees along the stone wall we saw some other invasive species. There were two different bedstraw species (Galium spp.), one with Velcro-like hooks along the stem, and one without. The one with Velcro-like hooks (Galium aparine) is native, can stick to clothing, and is also known as cleavers. Smooth bedstraw (Galium mollugo), which lacks the sticky hairs, is a non-native, invasive species. Both species can be problematic weeds in gardens and farms. Another invasive species found in the same area near the entrance is known as goutweed, bishop’s weed, or bishop’s goutweed, Aegopodium podagraria, which is invasive by means of vegetative growth of rhizomes. It has been used as a ground cover because it spreads quickly, and is still sold for that purpose. However, the same characteristic makes it problematic. It is difficult to eradicate from gardens, and can even escape into forests if a garden with this plant is adjacent to a forest.
Bishop’s goutweed illustrates one of the ways that plants can be invasive; in this case by spreading vegetatively. A different way that a plant may become invasive is to spread by seeds, especially if the fruits are eaten by birds, and then dispersed after passing through the birds’ digestive tracts. We saw and discussed several shrubs that are in this category. The first was European buckthorn, Rhamnus cathartica, which has fruit that is eaten by birds, and that then causes diarrhea (notice the word “cathartic” within the name?), causing the seeds to be spread in the birds’ feces. Similarly, several species of invasive bush honeysuckles (Lonicera × bella, L. morrowii and L. tatarica) are infamous for being dispersed by birds. Thanks to a lot of work by both staff and volunteers, much progress has been made in reducing the numbers of buckthorn and honeysuckle at the preserve, especially along the paths that are most visited, such as the path out to Picnic Point. One other shrub that is not yet as problematic as buckthorn and honeysuckle, but which has the potential to be just as invasive, is the winged burning bush (Euonymus alatus). It is commonly grown as an ornamental, and is beginning to appear in woods, similarly to the other invasive woody species. There is a similar species, wahoo, Euonymus atropurpureus, which is native to our area, and would make a good substitute for the non-native species. In response to a question about removing and replacing invasive shrubs, Anne suggested a decision tree available at https://woodyinvasives.org/ to help homeowners decide whether they should remove invasive species from their yards and gardens. She also distributed a handy brochure that lists non-invasive species that can be used as substitutes for invasive ornamental species. The brochure is also available here: https://woodyinvasives.org/wp-content/uploads/2021/06/LA-Brochure_WEB_FINAL.pdf
A few other invasive species that we learned about were either found along the trail or discussed in spite of our not seeing any. One participant spotted a single plant of greater celandine, Chelidonium majus, so that Anne could describe how to distinguish it from the similar looking native wood poppy (Stylophorum diphyllum). The plants are both in the poppy family and are not easy to tell apart by their leaves, although the native species has larger flowers. They are easiest to distinguish when they have fruit (seed pods), which are narrow and point up in the invasive celandine, but wider, hairy, and drooping in the native wood poppy (see images at https://bplant.org/compare/586-8527). We did not see porcelain berry (Ampelopsis brevipedunculata), but discussed how problematic it is becoming in certain Madison neighborhoods. It is also difficult to distinguish from its close relative wild grapevine by leaves alone, but can be easily distinguished when its upward-facing, multicolored, pastel fruits (that look like porcelain) are ripening, which do not look like downward-hanging wild grapes. We also heard about the annual garlic mustard pull in early spring, during which many volunteers help to remove this invasive plant from the woods of the preserve.
The participants in this walk are undoubtedly better prepared to recognize and fight invasive plants after this interesting morning. Thanks, Anne! Report by Eve Emshwiller
Links to illustrations of highbush cranberry glands here:
These both come from this page: