Documents: Hot Horticulture Issues:

Invasive Plants
by Leonard Perry
by Leonard Perry


In extension I serve as an advisor and consultant to the greenhouse and nursery industry, primarily in Vermont but throughout the region and beyond as well.

I give presentations on my research to the industry, and to home groups. In Research, my focus is "herbaceous perennial production systems".

His website is at  Leonards zone of gardening: home with my trials, generally USDA 4a. Campus in Burlington is 5.

September 23, 2007

Invasive plants generally are defined as those plants that are non-native to a particular ecosystem, and whose introduction is causing harm to the environment. Plants native to a particular habitat or area coexist with other plant and animal life in a balanced manner. They are kept in bounds by site factors such as soil type and climate, or pressures such as insects, diseases, and feeding by animals. Remove these plants to an area without these pressures, or with more ideal conditions, and they can begin to take over or become invasive.

Some say that invasive plants are not “bad” as often termed, as in their native habitats they fill a role. Invasive plants are just stronger competitors when in a more ideal environment for them with less pressures. Basically they are weeds—plants out of place where we don’t want them. In this case the emphasis is not on our gardens, but on our natural areas.

On the first part of the definition, note that they are not native to a particular ecosystem. Often, native plants are defined in a broader geographic sense or with a date, such as plants existing in this country when the original settlers arrived. This definition doesn’t really work for invasive plants, as a plant native in one part of the country may be invasive in another area with different ecosystem. The focus should be on a particular ecosystem, and on the behavior of a plant, as some such as prairie ecologist Neil Diboll, suggest.

One interesting study by researchers Houlahan and Findlay in Ontario wetlands found four invasive plants were no more likely to dominate these areas than four native species. Their results pointed to behavior of plants too as the main concern. Their conclusion was that dominant species in a plant community, regardless of their geographical origin, should be discouraged.

On the other hand, an introduced or non-native plant may not be bad.

There are over 3,500 plant species originally not native to this country by one estimate that have escaped cultivation into the wilds to become “naturalized.” Of these, about 1,000 are considered invasive, and about 700 considered a serious threat to agriculture. Many of our attractive wildflowers are in fact non-native, and have become “naturalized.” Many more introduced plants haven’t escaped cultivation, including most of our food crops. Many of the ornamental plants in landscapes don’t produced seeds or aggressive roots and stay put.

An example of a non-native plant that may or may not be invasive, in this case depending on climate, is the ornamental miscanthus or eulalia grass. While seed invasive in warmer climates, this heat-loving plant doesn’t produce seeds in the colder northern areas so usually is not a problem there. In this and other cases, plant behavior is a function of location.

The second part of the definition is the real cause for concern by an increasing number of people. By taking over natural areas, invasives crowd out less adaptable or less vigorous native plants. In some cases, particularly in very specific habitats, threatened or endangered plants may be at risk. I’ve seen estimates of 42 to 57 percent of federally endangered or threatened species in this country at risk from invasive plants. An example is the swallow-wort vine which threatens several thousand acres of plants in the shallow limestone barrens (rare alvar habitats) near Lake Ontario, including 23 rare plant species.

A study in Rhode Island by Darcy suggests the swallow-wort may harm Monarch butterflies. The butterflies normally lay eggs on milkweeds, to which this plant is related, and in this study laid eggs on this plant as well. The problem is that the resulting caterpillars need compounds in the milkweeds to survive, so perish when they hatch on this plant. However, a study at Cornell by Ditommaso and Losey found this not to be the case, but that the loss of milkweeds from the aggressive swallow-wort vine may be the larger issue. The points here are that rigorous ecological research has just begun on invasive plants, often with contradictory results, and with much more to be studied to find out under what specific conditions invasive plants may cause harm.

Another example of contradictory information is a study at Cornell by Blossey that found no effect of certain forest invasive plant species on native plants, although this is generally not believed to be the case. Norway maple, burning bush, honeysuckle, buckthorn, and barberry seeds are spread by birds to natural areas where they out compete native plants, or shade them out in the case of the dense canopies of the Norway maple in forests. For these reasons, these plants are becoming prohibited from sale in some states.

A study on the invasive herbaceous plant garlic mustard, by Harvard researcher Stinson and others, showed this plant suppressed growth of canopy tree seedlings by disrupting their beneficial association with fungi (myycorrhizae) in the soil. Also, garlic mustard destroys spring woodland wildflowers such as trillium and bloodroot by outcompeting with them for light, moisture, nutrients, soil and space. In one locale, garlic mustard is threatening the already rare West Virginia white butterfly by killing out its food source-- toothworts. In addition, chemicals in the garlic mustard appear toxic to eggs of this butterfly when laid on them.

Along roadside and in fields, attractive wildflowers may be overtaken by plants such as giant hogweed, swallow-wort, and wild chervil. Beyond the beauty of desired wildflowers is their function for pollinators, butterflies, and insects. Many of these wildflowers feed the many beneficial insects that keep the ones we don’t want from getting out of control. Over 97 percent of insects are beneficial, or do no harm, or serve as food for birds.

Arguably the most known terrestrial invasive plant, and one reason for active programs in many states, is the purple loosestrife (not to be confused with root-spreading loosestrife species). This is the attractive spiked purple flower seen in wetlands in late summer in masses. It is spread there by seeds in water and wind. Several cultivars (cultivated varieties) have been developed that were originally thought sterile, so not a problem. Later studies have shown even these can be pollinated by wild plants to produce seeds.

The purple loosestrife, compared to native wetland species such as cattails, is less desired by specialized wetland birds such as bitterns and black terns and so they are declining in areas taken over by this invasive plant. Also it changes the nesting habitats for turtles, and the nutrients in water for those organisms fed on by fish and birds.

Since the purple loosestrife is native to Europe, it is there researchers went to find biological controls that share the same habitat. After years of trials of these insects, to make sure these introduced pests had no other effects on the environment, four have been introduced successfully in various states and are helping to control purple loosestrife through feeding. The whole story on this plant and these controls can be found online ( ).

There are local and national publications where you can learn more about various aspects of invasive plants, as well as websites such as the one from the National Agriculture Library ( ).

  • New Eden
  • Kids Garden
  • Plant a Row Grow a Row