If you garden, you probably spend lots lots time removing certain weeds which you consider invasive. While these plants are no doubt invading your garden, they are more properly called aggressive, although they might actually be invasive as well. It's a confusing topic, but it's helpful to understand the difference in the terms if you plan on communicating with other gardeners with any level of clarity.
Aggressive plants are usually pioneer species that love your garden because the space between your tomatoes and green beans is open land ready to be colonized. Mother nature developed such plants to quickly cover bare ground in the event of a disturbance to prevent loss of precious resources. When a roaring flood scours the banks of a stream, carrying plants and topsoil away, pioneer plants, often annuals, rush in and cover bare ground, protecting it from further loss. These forerunners prepare the area for perennials, shrubs, and eventually trees, as the stream banks return to a condition resembling their pre-flood state.
A traditional garden with lots of empty space between plants, and nutrient rich, fluffy soil, calls to annual weeds like a siren. These weeds may be native or non-native. It's not necessarily appropriate to refer to such plants as invasive however, as that term has a specific meaning. According to the USFS, an invasive plant, or noxious weed, is a non-native plant that poses a threat to the economy, the environment, or to human health. States maintain lists of plants considered invasive within their borders, and what is invasive in one state may not be in another. Furthermore, a plant may indeed be invasive in your state, but has not yet been granted invasive status because it's a new arrival.
There's lots of room for debate because "threat to the environment" is a phrase that can be interpreted various ways. Some people embrace all plants, view exotic visitors as inevitable, and eschew the term invasive. They point to the fact that whipping up animosity towards non-native plants leads to greater herbicide use as landowners go to battle against the invaders. The concept of the invasive plant may be a boon to the chemical industry, but using this fact to argue that we need to accept all plants with open arms does not follow logically.
In the scenario above in which pioneer plants colonize an exposed stream bank, these aggressive plants support natural succession, and are an integral part of land healing itself after disturbance. In urban lots devoid of topsoil and life, or heavily chemicalized and abandoned agricultural land, invasives may play a similar role. They are often able to survive and thrive in such wastelands, and create conditions that lead to soil regeneration and the presence of more desirable plants in the future. The problem with many invasives is that they are not playing this role; they are invading areas where natural succession is proceeding beautifully, or they are entering mature woodlands that are already in a high state of complexity. In such cases they impede succession rather that support it, decrease species richness, and promote monocultures where polycultures were developing.
Take the case of Euonymus fortunei, commonly known as wintercreeper, and easily found for purchase at garden centers. Originating from a neighbor's bank planting, wintercreeper entered my woodland and within a few years covered large areas of ground. It climbed mature trees, and produced millions of seeds from great height, allowing it to spread deeper into the forest. The Euonymus now competes with with trees for sunlight, and has encased their trunks with vast masses of vines and leaves. On the ground, a thick mat of it covers wide areas where various species of native wildflowers and herbaceous plants would otherwise be thriving.
On the fringes, where the forest is trying to expand into abandoned farmland, the situation is even more dire as trees face the double threat of Japanese honeysuckle and multiflora rose. These two invasives use the lowest branches of young trees as ladders, often bending the saplings down to the ground before completely engulfing them. This has a enormous negative effect on the forest's ability to regenerate itself.
Many people believe that planting trees is part of the solution to climate change. In areas of adequate rainfall like the eastern US, forests would naturally expand given the chance. Invasives impede this progress. Acknowledging this fact and working to simply release trees from the grasp of invasive plants is much more efficacious, and cheaper, than attempting to plant a forest. I spend lots of time clipping the strangling vines near the ground, and cutting off low tree branches. Once trees reach a height of about 10 feet they have a much better chance of escaping the vines and making it to maturity.
Another contentious question is whether exotic invasives support wildlife and thriving ecosystems equally as well as natives. Entomologist Doug Tallamy has explored this issue extensively and the answer is resounding no. The arguments put forth in his book Bringing Nature Home are highly compelling and I urge anyone who is on the fence about this issue to read this book. He presents not merely his opinions, but the results of research conducted by him and his students in the field, much of it concerning the connections between plants, caterpillars, and birds. He points out that while exotics often appear to provide shelter and berries for birds, native plants lead to higher populations because they support caterpillars who feed on one type of plant. These specialists provide the nutrient-dense food birds need to migrate and raise their young. Without them, birds may visit your yard for sugary berries, but will not breed there.
The issue of native versus exotic is not totally black and white. I am not a purist who destroys all foreign plants, and grows only natives. I am simply mindful of how each plant fits into the ecosystem as a whole, and evaluate plants on this basis. If you are considering using a plant that has a reputation for being invasive, take the time to find out why. Choosing to grow it in your garden may mean you alone are making a decision that will affect your entire community, since this plant will be forced upon your neighbors as it spreads. Our individualistic society where everyone considers it their right to do whatever they want on their own land might support such a decision. A responsible neighbor, especially one who is concerned about "people care" and "earth care," will think hard before making such a decision for others, human and otherwise.
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