Riverwoods Preservation Council

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 Invasive Plants - January / February 2006
   Buckthorn
   Garlic Mustard
   Poison Ivy


Common Buckthorn Buckthorn

Buckthorn is a tree-like shrub first brought from Europe in the mid-1800s as a popular hedging plant. Shortly after its introduction here, it was found to be quite invasive in natural areas. It grows to heights of 15-25 feet and is one of the earliest to leaf in Spring and the last to lose its leaves in Fall. It bears small purplish-black berry-like fruit. Two species are found in Riverwoods, Common Buckthorn (Rhamnus cathartica) and Glossy Buckthorn (Rhamnus frangula), but the Common Buckthorn is dominant here.

Buckthorn is a problem because it out-competes native plants for nutrients, light, and moisture, and also changes the nitrogen content of the soil. The resulting landscape is dominated by the species, which is not conducive to woodland health. Although some residents appreciate its density as a landscape screen, there are other quick-growing shrubs which are without Buckthorn’s troublesome characteristics. It degrades wildlife habitats, both here and in many states, contributes to erosion by creating an impermeable layer of vegetation which limits water infiltration, and serves as a host to various pests. Our oak/hickory woodlands require a thick understory of grasses and various shrubs, but Buckthorn severely inhibits the growth of such beneficial vegetation.

Buckthorn fruit is one of the few invasive species that birds will eat. That makes it difficult to control, since birds eat the fruit and spread the seeds widely. Buckthorn is probably the most widespread invasive shrub in Lake County, and is common throughout Riverwoods. Because it is fast-growing, hardy and inexpensive, it is still sold by some landscapers, as “Tallhedge”.

As an incentive to residents to remove Buckthorn, the village matches (up to a $1000 limit per year) the funds spent by a homeowner to have Buckthorn removed. To qualify, residents must apply to the Village and receive approval prior to performing any work, and then submit proof that the work has been completed and paid for. Effective control requires removal of the plant root or painting the cut stump with a woody plant herbicide such as Ortho Brush Be Gone or commercial strength Round Up (glyphosate). Note, however, that these are dangerous chemicals that must be handled and disposed of in strict accordance with directions. Even the amount of humidity in the air must be taken into account when using them.

Visit these web sites for more help identifying buckthorn:
Minnesota Department of Natural Resources
University of Minnesota Extension Service


(Buckthorn photograph courtesy of John Mr. Randall, The Nature Conservancy)

VolunteersGarlic Mustard 

You can’t spend very long in Riverwoods, especially in Spring, without hearing about this pernicious plant. Garlic Mustard (Alliaria Petiolata) is a plant with a two-year cycle, flowering and going to seed in the second year. Accidentally introduced from Europe, it now is widely distributed in the U.S. and has invaded much of Lake County and Riverwoods. Ranging in height from one to four feet, its leaves and stems, when crushed, emit the odor of garlic. Occurring in both shaded and open areas, Garlic Mustard blooms in May and its small white flowers produce thousands of seeds which are dispersed in June and July, producing a sea of new plants the following year. As a result, the spread is very rapid, displacing many native wild flowers and tree saplings. For this reason, it is considered a major threat to indigenous vegetation and wildlife.

Tall Garlic MustardAs with Buckthorn, the Village has attempted to address the issue by offering a matching funds program to residents who have the plants removed. There are several methods of effective control. Plants may be carefully pulled, so as to disturb the soil as little as possible, preferably with removal of the rather easily dislodged plant root. Alternatively, the plants can be cut at ground level just after flowering and before seeding. The timing of cutting is critical to success, because the biennial seeds remain dormant for 20 months, so any seeds that fall to the ground will generate more plants. In the early Spring, before desired wild flowers have come up, it is possible to carefully spray Roundup on the newly emerging garlic mustard plants, which are usually the earliest to appear. Within a week or two, these sprayed plants will die, never producing their flowers and seeds. All plants, whether pulled or cut, must be placed in plastic bags (never composted) and removed from the area, as seeds on the dead plants may remain viable for at least five years. Effective control requires action over several consecutive years.
(Garlic Mustard photograph courtesy of Brian McCormack, MN DNR)

For more information and photos, visit the WI DNR web site:

Poison Ivy Poison Ivy

This plant, which thrives in Riverwoods, may be seen as an individual plant or shrub growing close to the ground or as a vine climbing high into trees and onto walls and fences. Often reddish stems can be up to an inch in diameter, though when that large, they appear as woody, hairy brown vines. Poison ivy looks similar to several other plants. The plant has three leaves, with two of the leaves immediately opposite one another and the center leaf on a slightly longer stalk. In the Riverwoods area, the most distinguishing characteristic is that the two leaves opposite one another are each roughly in the shape of a mitten, with a large lobe and an adjacent smaller lobe. The size of the leaf, its glossiness, and the color of the leaf underside and stem, are not reliable identifying features.

As anyone who has had a major encounter with this plant can attest, it is a most unwelcome inhabitant of our area, producing an annoying and at times painful (and occasionally dangerous) blistering rash. The oily toxin urushiol causes this reaction and the severity (which sometimes requires medical attention) varies with individuals, and from year to year in the same individual. In most cases, sensitivity is developed only after one or more prior exposures to the toxin.

Because reaction to the toxin occurs when the toxin penetrates the skin, quick action - generally within about 5 minutes of exposure - will often avert a problem. Otherwise, redness and swelling typically appear within 12 to 48 hours after contact, followed by blisters.

If exposed to poison ivy, follow these steps:
  1. To avoid contaminating your home, remove contaminated clothing and, if possible, wash or rinse affected areas before entering the house.
  2. Cleanse your skin immediately with generous amounts of rubbing alcohol (isopropanol). If rubbing alcohol is not available, use soapy (or even plain) water. Be careful to clean only the contacted area, to avoid spreading the toxin to other parts of your body.
  3. Take a shower. Don’t reuse a soap bar used for the initial cleaning, as it may have been contaminated.
  4. Wearing disposable gloves and using rubbing alcohol, wipe off shoes, clothing, tools and anything else that contacted the toxin. Immediately and carefully discard the gloves after decontamination is complete.
    Contrary to popular myth, no toxin is contained in the rash and blisters, so they are not contagious and will not spread. The rash and blisters appear only on the body parts that came in contact with the oily toxin, and typically disappear in two to three weeks. Mild cases may be relieved with wet compresses or soaking in cool water, and oral antihistamines can also reduce itching. Other helpful products include baking soda, calamine lotion, zinc oxide, and kaolin. For more serious cases, corticosteroid medication (hydrocortisones) may be advisable. Over-the-counter sources are sold under brand names such as Cortaid and Lanacort.
  5. In severe cases of exposure, seek the prompt advice of a physician, as treatment must begin within a few hours after exposure to be most effective. A prescription corticosteroid may be appropriate. Because topical corticosteroid is not considered effective once blistering has begun, an oral version of the medication may be indicated.

All parts of the plant, including leaves, stems and roots, are poisonous at all times of the year. Oil from the plant remaining on clothing and footwear can remain toxic for a year or longer. While dogs, cats and other pets are not sensitive to poison ivy, they can transmit the oily toxin on their hair. Other potential carriers are garden tools and anything else that comes into contact with the plant. Even smoke can contain the toxin and inhalation of toxin-bearing smoke can cause a medical emergency. For this reason, Poison Ivy plants should never be burned.

The best time to attempt to control poison ivy is from May through July, when the plants are flowering and most dangerous. Foliage can be sprayed with a general herbicide, such as glyphosate (sold under such brand names as Roundup), but again, remember this is a non-selective herbicide, and generally kills any vegetation it contacts. For that reason, take care to avoid desirable plants. Cut large vines a few inches above the ground, and immediately apply glyphosate to the freshly cut area. Since the chemical travels to all parts of the plant, doing so should kill the roots. Note that the vine and leaves will continue to contain the toxin, so they either should be left in place or removed and disposed of carefully. Since Poison Ivy is quite persistent, multiple applications of glyphosate may be required. Manual eradication is also possible, so long as all parts of the plant – leaves, vines and roots – are removed.
(Poison Ivy photograph courtesy of the U.S. Army Corps of Engineers)

© Riverwoods Preservation Council- - Page last updated: December 2009