Potentially Invasive Plants

No one intends to plant an invasive plant. Yet, many times a season I find myself battling a plant that just does not stay within its boundaries. Worse yet, I have passed these plants along to my friends and family to find out how well they thrive under the conditions in different gardens. Sometimes, the invasive plant or plants have taken over and out-competed everything else in the garden. Many times, invasive plants are some of the most attractive plants for foliage or flower. And, when conditions are right, many natives can be quite invasive, while others may take years to become well established enough to begin spreading out of control; dame’s rocket is an example of such an invasive plant. Further, many plants such as baby’s breath are not invasive in some climates but are extremely aggressive in other climates.

Plants that reproduce rapidly and form stands that exclude nearly all other plants are defined as invasive plants. The Missouri Department of Conservation has identified several exotic species that are capable of spreading aggressively and is working to eliminate the use of these species. Included in this list are Japanese and bush honeysuckle, purple loosestrife, musk thistle, kudzu, multiflora rose, sweet clover, teasel, common buckthorn, crown vetch, autumn olive, garlic mustard, and Sericea lespedeza.

Many plants are beneficial because they can control erosion and are commonly used as ground covers in landscapes. However, gardeners need to be cautious using these plants as they can grow aggressively and have the potential to spread into natural areas where they may crowd out existing vegetation. Commonly used groundcovers that many people may consider potentially invasive include English ivy, wintercreeper, ajuga, periwinkle, Liriope spicata, and certain types of honeysuckle. Wintercreeper (Euonymous fortunei) grows well in shade and competes strongly with understory plants in wooded areas; thus, these plants should be planted with extreme caution.

Many desirable plants such as the obedient plant (Physostegia) and bamboo can be quite invasive, thus driving efforts at propagating cultivars that tend not to spread quite so severely. A clumping, non-spreading white form of the obedient plant, ‘Miss Manners’, is now available, and a clumping form of bamboo is now available for northern climates.

To keep a garden free of invasives, gardeners need to attempt to exclude plants known to be aggressive. Some key words that should promote caution when purchasing or ordering plants are "grows anywhere," "quick coverage," "tolerates dry shade," and "is a prolific seeder."

Many plants that are aggressive can do battle in the worst conditions and may be capable of out-competing an undesirable weed when the weed cannot be controlled any other way. Out-of-control plants can be moved to sites that are less than ideal, thus controlling the overly aggressive plant in a location where nothing else has been able to grow. If a plant does get out of control in the garden, there is no substitute for the old fashioned way of ridding the garden of the weed - hand pulling!! Weeding can be great therapy.

Mary Kroening, Missouri Master Gardener Coordinator and UMC Horticulturalist, (573) 882-9633