Imagine a horror movie depicting a plant species that grows everywhere, consuming cars, buildings, and everything in its path. Although this is strictly science fiction, there are plants that are quite aggressive, taking over a garden, forest, or turfgrass areas in a short period of time. An example of this is the kudzu vine (Pueraria lobata). It was widely grown in the southeast United States as a fodder plant, to control erosion, and sometimes as an ornamental. Due to its rapid growth rate it has become a rampant weed. Fortunately, kudzu is not hardy in Iowa. Another example closer to home is ground ivy (Glechoma hederacea), sometimes called "Creeping Charlie". It was introduced as a fast-spreading ground cover for shady areas. Unfortunately, it doesn't stay where it is planted and has become a major weed problem in many lawns and gardens.
Many of the plants which are considered invasive or aggressive spread by underground stems called rhizomes. This enables them to invade new territory quite rapidly. This method of propagation makes it difficult to keep the plants in bounds. Cultivation doesn't always reduce the plant population and, with many plants, can make it worse.
Another way in which plants can become a weed problem is by self-seeding. Fortunately, many seedlings can easily be controlled by hoeing or pulling them out.
Gardeners do not need to avoid planting species that spread rapidly, however, they should be aware of their growth habits prior to planting and take measures to prevent them from becoming a weed. Below are some garden species that have aggressive growth habits and some tips to keep them in the desired location.
Bishop's Goutweed, sometimes called Snow on the Mountain, (Aegopodium podagraria `variegatum') grows 8 to 10 inches tall with leaves that are light green with white margins. It is used as a ground cover in sunny or shady locations. Plant it where it can be restricted, such as between a sidewalk and the house or use a deep edging to prevent it from moving into turf areas.
Bugleweed (Ajuga reptans) grows only 6" to 9" to form a dense, mat-like growth. It's spreading characteristic makes it a natural as a beautiful ground cover.It prefers a shady location, however it will tolerate some sun. Plant ajuga in areas where cover is needed, such as on a bank or in shady areas. It should not be used as an edging near grass areas because it has a tendency to move easily and quickly into these areas.
Roman Wormwood (Artemesia pontica) is a dense plant growing 18" tall with silvery foliage. It can be invasive, particularly when grown in sandy, loose soils. It should be restricted to sunny, expansive areas where a fast-growing ground cover is needed.
Lily-of the-Valley (Covallaria majalis) is an easy plant to grow. The 6-18" plants produce a raceme of white, bell-shaped, fragrant flowers in mid-spring. It needs room to spread, so it should not be placed in the perennial border. It is an excellent choice as a ground cover beneath shade trees and other shady areas where the aggressive spread is desired. It should be grown in full or partial shade.
Gooseneck Loosestrife (Lysimachia clethroides) is a unique plant that grows 2 to 3' tall and produces racemes of white flowers that curve creating a "gooseneck" appearance. Gardeners should be prepared to lift sections of the plant each year to retard its spread. If not kept in bounds it will soon appear everywhere in the garden. It prefers a partial sun location but will grow in full sun.
Purple Loosestrife (Lythrum salicaria) grows 3-5' tall. It produces reddish purple spikes 1' long in mid-summer. It will freely self-sow and takes over wetland areas and chokes out other vegetation. Although the cultivated species does not have the aggressive characteristic of its relatives, the planting of purple loosestrife should be discouraged.
Hollyhock Mallow (Malva alcea) grows 3 to 4' tall with large pink flowers borne on long-blooming spikes. Its self-seeding habit creates a quick spreading perennial which can be a problem in a small garden.
Beebalm (Monarda didyma) grows 2-3' tall. It produces scarlet-red tubular flowers in whorls creating a dense head. It may require dividing every three years to prevent its rapid spread. It is desirable in a garden because the flowers attract bees and hummingbirds. It grows well in full sun and will spread faster in a shady area.
Lamb's Ear (Stachys byzantina) grows only 12" tall with silvery-white foliage that has a velvet-like texture. It is used as a border or ground cover. Although it will invade turfgrass and other garden areas, it can easily be controlled by removing stray plants.
(Mentha sp.) Many mints, such as spearmint and catmint can become a weed problem in just a few years. It is a good idea to plant mint in a container and submerge the container in the garden. This will prevent the plant from becoming a weed in your garden.
This article originally appeared in the July 1, 1994 issue, pp. 106-7.
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