Characteristics of invasive plants

Characteristics of invasive plants

Indicators of invasiveness

Creating effective policies to prevent plant invasions means understanding what makes a plant invasive so that we can decide what categories of plants to exclude from the country. Ecologically, it is important to incorporate research into the policy. Without proper research into the issue, we may find that our criteria are incomplete and we are inadvertently allowing invasive plants into our ecosystems. Politically, scientific research is also important. The more the nursery and landscape nursery is confident that there is objective proof behind the restrictions being put on them, the more likely that they are to comply with the restrictions, as I have discussed earlier. However, lack of proof should not prevent likely invasive plants from being restricted. It takes years to determine whether a plant is conclusively invasive or not, by which time the plant may spread beyond control. Determining common characteristics of invasive plants and basing control decisions on that research, rather than researching a particular plant, is a more effective way of deciding invasiveness.

Characteristics of invasive plants

Horticultural invasive plants are good competitors in their new ecosystems because gardeners often select plants for the very characteristics that can make them invasive. Successful invaders often share a few traits that allows them to outcompete indigenous plants. Invasive plants may have any of the following characteristics (Gould 2000, Mehrhoff 1998).

Rapid growth allows invasive exotics to take advantage of uninhabited patches of the landscape, often from disturbance. It also means that the plant might be a great groundcover or a quick way to fill gaps in a garden.  

Early maturity allows a plant to produce seed, and thus invade areas, at a young age, meaning the plant flowers after only a short time in the garden - great instant gratification. Plants with early maturity dominate the landscape through establishing populations and reseeding before native plants have a chance to produce seeds and spread.

Invasive plants often produce large quantities of seeds and fruit, which means they have a large number of offspring which can establish in uncolonized areas. This is a desirable characteristic for some horticultural plants because their seeds or fruits are attractive. This is the case with Celastrus orbiculatus (oriental bittersweet), which produces a large quantity of red and yellow fruit.

Effective seed dispersal methods help invasive plants spread over wide ranges. Celastrus orbiculatus has colorful fruit attracts birds, which disperse its seeds over a great distance. In fact, birds spread the seeds of most invasive woody plants (Mehrhoff 1998). Wind is another method commonly used.

Some plants, like Eichornia crassipes (water hyacinth) don't need to be pollinated because they spread through vegetative reproduction, wherein new plants sprout from roots or stems of the parent plant. This can be a desirable trait in the garden because these plants will fill in empty spaces without the need for new seed.

Invasive plants often are able to utilize local pollinators so that they can produce seed in foreign environments. The spread of nonindigenous insects (and other pollinators), like the European honeybee, also allows nonindigenous plants to spread.

Invasive plants often leaf out earlier, stay green longer, or bloom at an earlier or later time of year than natives, meaning that they can shade out natives or take advantage of pollinators with no native food sources at that time of year. This difference in timing is called different phenology.

Generation of dense shade, a trait of Hedera helix (English ivy) and Acer platanoides (Norway maple), is a characteristic of a good groundcover as well as a trait of invasive plants that allows them to reduce competition by reducing available light to undergrowth. Dense root mass also allows invasive plants to become dominant by preventing other plants from establishing roots.

Invasive plants often use allelopathy, which is the production of chemicals that inhibit the growth of other plants. Plants that do not need specialized pollinators have an easier time invading than those that require very specific pollinators.

Pest-free plants are clearly advantageous to gardeners because they don’t require pesticides and have a better chance of survival. Invasive plants often are pest-free because they have managed to migrate to a new environment without the natural predators from their home environment. For most plants, new environments provide constraints to which they have not adapted, so they are not able to naturalize and spread outside of cultivation (where humans help them overcome their less-than-ideal conditions). But with invasive plants, they have overcome their limitations and are actually better adapted to their new ecosystem because they have not got natural predators, competitors, and pests to keep them in check.

Indicators of Invasability

Ecologists are continuing to develop characteristics of invasive plants so that they may determine whether new plants may become invasive if they are introduced to new ecosystems. No set of characteristics covers all invasive plants and no one plant has all of the characteristics - there are always exceptions, but there are a few characteristics which seem to point to invasiveness. Rejmanek and Richardson (1996) pinpoint a few different characteristics of plants that tend to indicate invasibility.

Latitudinal range of herbaceous species in their native habitats determines whether they’ll be able to invade new habitats.

For woody species, r-selected species with short juvenile periods, short intervals between large seed crops, and small seed mass tend to indicate invasive characteristics. An "r-selected" species is one that tends to be monocarpic, producing great numbers of offspring all at once and then dying (Gotelli 1995). Because each generation produces a lot of seed at a young age, the r-selected plant can quickly spread across an area.

Sarah Reichard (1997) has developed a model that uses the traits of the new species and compares them to the traits of other known invaders. Her criteria are structured into a decision tree which ultimately groups plants into prohibited, accepted, and further testing lists. These criteria include invasion in other parts of the world, membership in a family or genera already invasive in North America, fertility, ability to spread vegetatively, short juvenile period, and/or seeds not requiring pretreatment. She recommends that those plants falling on the further testing list should be monitored for at least five years before they are released publicly.