Ways Native Plants Can Help Control Invasive Plants

In the wild, plants compete with each other for limited resources such as sunlight, water, and soil nutrients.  When you plant native plants in your garden, you provide a seed source that will help these plants to spread out into wild areas.  This can have the biggest impact when you live directly adjacent to wild areas, but even if you don't, you can grow plants with wind- or bird-dispersed seeds that are able to travel a long distance from the parent plants.

As the seeds of native plants spread out from your garden or property, you will increase the supply of seeds of these plants, and as they sprout, the seedlings will compete with invasive plants.  The greater the seed source of native plants, the fewer invasive plants will be able to survive.

Even a single small garden, or a small cluster of plants, can make a huge impact.  For example, when I grew Rudbeckia hirta, a black-eyed susan species native to my area, I planted only five plants, and within one year I noticed that many of the plants coming up in cracks in the sidewalk were now this species, rather than the invasive common plantain (Plantago major), Canada thistle (Cirsium arvense), or Prickly lettuce (Lactuca serriola).  I also found that growing a few Common evening primrose (Oenothera biennis) plants also changed the balance of weedy plants in the surroundings.  The effects of large-scale plantings can be tremendous.

Direct competition is not the only way through which plants can hinder each other's growth.  Many plants produce chemicals which inhibit the growth of other plants.  This phenomenon is called allelopathy.

Plants are not equally effected by the allelopathy of a particular plant.  Plants co-evolve to tolerate each other's allelopathy, growing together in plant communities.  Allelopathy is common both in native and invasive plants.  People often think of allelopathy as one of the reasons why invasive plants become invasive, as the plants in the new region are not adapted to their particular blend of chemicals.

However, allelopathy can also be a factor in helping native plants inhibit the growth of invasive plants.  In North America, two well-known native plants which have strong allelopathic effects on other plants are black walnut trees, and goldenrods (Solidago sp.).  If you garden with native allelopathic plants, you can help to protect the area from invasive plants.

Not only will allelopathy help these plants to hold their own against invasives, but as these plants get established it will help shift the balance of plants back towards the native species with which they co-evolved.

Many insect herbivores are specialists, eating only a certain group of related plants, such as plants in the same species, genus, or family, or plants in a series of related species, genera, or families.

Many invasive plants have close native relatives.  Although initially, many invasive plants are eaten by few or no native insect specialists, there is a phenomenon in which insects "jump host", from one plant to a new plant that they did not previously eat.

An example of this, the Ailanthus webworm moth is a moth native to South Florida and the American tropics, which originally ate trees of the Simaroubaceae family, such as Simarouba glauca and Simarouba amara.  The Ailanthus altissima tree, or "tree of heaven", was introduced into North America from East Asia, and it became invasive, spreading throughout the continent.  Ailanthus has rapid growth and is strongly allelopathic, and these two factors combine to result in it causing extensive harm to some ecosystems.

Although few native insects originally ate the Ailanthus altissima tree, the tree is a member of the Simaroubaceae family, and where its range came to overlap with the native trees of this family in South Florida, the Ailanthus webworm moth jumped host, and started eating the Ailanthus altissima trees as well.  This moth has now expanded its range far north into North America, following the distribution of the invasive tree, and under certain conditions it can hugely limit the growth of these trees, sometimes even killing full-grown trees.

By planting native plants that are closely related to invasive plants that are common in your area, you can help attract native insects that specialize on those families, genera, or other groupings of plants.  As more people do this, it will increase the populations of these insects, and increase the amount of contact between these insects and the invasive plants, which will increase the likelihood with which these insects will jump host and begin eating invasive plants.

Most herbivorous insects do not depend only on a single species for their survival, but rather, utilize different plant species throughout their life.  Many insects feed on different plants in their larval form and their adult form.  For example, butterflies and moths typically eat leaves or other plant parts as larvae (caterpillars), and then as adults they often drink nectar from flowers.

Another way native plants can help control invasive plants is by providing support for insects in one part of their life cycle, which would eat invasive plants during another stage of their life cycle.  As an example, planting Rudbeckia laciniata and Solidago sp.  in an area where Ailanthus altissima is growing, can attract the Ailanthus webworm moth, which is attracted by these flowers and supported by their nectar in its adult form, and once the moths are in the area (and are well-fed), they will be more likely to find Ailanthus altissima trees and lay their eggs on them.

Having a diversity of native plants can also attract and support generalist herbivores, which will often feed on invasive plants as a part of their diet.