Dear Stump:
I need your help! While I have always had to battle invasive species (I currently have garlic mustard, buckthorn, Manchurian honeysuckle, and Japanese barberry) they seem to have completely taken over the understory of my woodlot this spring. What can I do to stop the spread of these invaders? Thanks!
Sadly, there are no good answers. Managing a woodlot is a labor of love and battling invasives, once they have become established, is a never-ending battle. I feel your pain, both figuratively and literally, as I have been pulling these weeds out of my woodlot by hand this spring. Invasive species are problematic because they have competitive advantages that allow them to become established and outcompete native plants for basic site resources like sunlight, space, soil nutrients and water.
Habitat, niche, and adaptations
The term habitat is used to describe the resources that plant requires to become established and thrive, niche describes a plant’s role in the habitat, while adaptations describe physical or behavioral traits that help it compete and survive in its habitat. Native plants develop over time in cooperation or competition with other plants and become adapted to their habitat. They develop physical and behavioral traits that allow them to be successful and thrive. Their populations may wax and wane as climatic conditions, disturbances (like fires or windstorms) that can alter the landscape, and plant cohorts change but overall they are established in their niche and have adaptations that allow them to be competitive and even prevent the establishment of other plants species. Invasive species are such a serious problem because the adaptations that allow them to be competitive in their native habitat give them competitive advantages that allow them to outcompete native plants for their niches. Once these plants become established they often take over and in some cases completely alter their new environs and create conditions that favor their growth and survival.
Competitive Advantages?
Although we don’t often think in these terms most of us are aware of them and even manage our woodlots to promote or reduce certain species based on their competitive advantages. Aspen is an early succesional species. It likes bare mineral soil and full sunlight. Clear-cuts are often used to regenerate aspen because it caters directly to aspen’s competitive advantage; the ability to colonize disturbed site where full sunlight reaches the ground. Sugar maple, on the other hand, can thrive in the deep shade of a northern hardwood forest. We use select harvest systems that create small canopy gaps to encourage the growth of sugar maple and similar species since their competitive advantage gives them the ability to grow quickly and become established in these small canopy gaps.
So why are some species more troubling than others?
Because their competitive advantages have developed in extreme conditions and they have developed multiple ways of insuring their ability to survive. Let’s take a look at a few of the worst ones we are currently dealing with in our woodlands.
Garlic mustard is particularly nasty because it kills other plant by producing alleopathic chemicals that act like a herbicide and prevent any other species from taking root or growing roots into areas where it has become established.
Buckthorn is a problem because it forms dense thickets and is extremely shade tolerant. The ground beneath a thicket of buckthorn becomes bare and completely shaded to the point where the only thing that will grow under the shade of a buckthorn thicket is more buckthorn! And that’s not all! Buckthorn are pure evil! (okay, so I am getting carried away with the exclamation points) Buckthorn is a prolific seed producer and the seeds are persistent. Which means that the fruit of the buckthorn is often the only thing available in late October or early November for migratory species that are still passing through. And since the seed needs to be scarified, and the gut of a bird is the perfect place to do this, it is a win-win for buckthorn.
Bush Honeysuckle is a problem because it is one of the first plants to leaf out in the spring and one of the last to lose its leaves in the fall. These plants can dominate a site simply because they are shading out a site before other species have even begun to bud out and they have a longer growing season which means that they can convert more energy to seed production.
One thing that many of these plants have in common is their ability to grow back from severed roots. Many people will pull these plants out of the ground as a means of controlling their spread which can work very well. However, if you don’t pull the entire root system out of the ground then a new plant can grow from the remaining root. And you can’t just leave them lying on the ground. If the plant is mature enough to produce seed then it will very often produce seed even if the roots are no longer in contact with the soil. These plants are extremely hardy and don’t give up easily.
So what can be done?
The twin goals of weed management are to minimize competition and yield loss by your “crop” species and to limit reproduction of competitors so that you can still manage their population in future years. For most of us our “crop” species are our native woodland plants and the first step in any planning process is figuring out what you have, both the good and the bad. Once you know which invasive species are in your woodlot you can start to think about control methods.
Eradication!
I hear this term a lot when I hear folks talk about invasive species. While this is an admirable goal it is impractical and expensive. Remember, one of the goals of weed management is to reduce competitors so you can manage the population. You can never get rid of them entirely because wind, or birds, or fur-bearing critters will always bring new seed on to your property but you can work at managing the population and removing those plants that are producing seed. Once you get rid of them your life will get a whole lot easier.
Control Methods?
The big three are chemical, mechanical, and fire. Depending on your woodlot, the pests that are invading it, and your resources; you may want to look at using one or all of these methods to control these plants.
There are a number of good resources online that describe the best methods for controlling these plants. Because your individual situation is unique you should read up on what is available and talk with your forester if you aren’t sure how to proceed.
Chemical control can best be used to destroy plants before they start to produce seed. Basal or foliar applications can be very effective but these are most often applied as spot treatments and can kill non-target organisms if you are not careful.
Mixed chemical and mechanical systems work better on large, seed-bearing plants. Cutting the plant down and then spraying the stump with a herbicide or allowing it to re-sprout and then applying a herbicide is an excellent way to kill these plants. But, as you can see, your time and money investments are much greater. So it is better to get them while they are young.
Mechanical controls rarely kill the plant; they set them back and prevent them from producing seed but they also have their drawbacks. Simply cutting the plant down will cause it to re-sprout from the stump or possibly the roots creating a colony that may produce seed that year. Pulling them out of the ground just creates a really nice seedbed for these plants to re-establish themselves by seed and if you don’t pull the entire root system out of the ground many of these plants will simply re-sprout from the severed root. Mowing can be effective as well. But just like a manicured lawn you need to constantly, or at least consistently, mow to prevent the plants from going to seed.
Fire is an interesting option. It can be very effective in the right situation and has the added benefit of restoring native plants that are adapted to fire. However, for fire to work you have to have the right “crop” species that will benefit (regenerate) from a fire, the necessary carrying fuels on the forest floor, the proper permits, and the professional help necessary to insure the job is done right. Fire won’t solve all of your problems but it can be the right tool for prairie, pine barren, and oak savanna systems.
Restoration
It isn’t enough to get rid of the bad plants; you have to re-establish the good plants. A healthy ecosystem is the first line of defense against invasive species. Restoring the system and giving the native species the opportunity to use their competitive advantages to establish their rightful place in their niche is the best management tool there is. But this requires active management, time and money, and borrowing from J.K. Rowling “Constant Vigilance.”
Resources
You can learn more about invasive plants online at the Invasive Plants Association of Wisconsin website
An excellent book from UW Press is ” Invasive Plants of the Upper Midwest” this publication has excellent information on invasives identification and recommendations for control. You can purchase it online at University of Wisconsin Press
The Wisconsin Department of Natural Resources has a number of web pages and publications that you can use to Identify invasive plants, learn about their life cycles and see control recommendations.
Invasive Species Control Methods
Forestry Best Management Practices for Invasive Species
A Field Guide to Terrestrial Invasive Plants in Wisconsin
So, good luck! This is one more thing that you can add to your “To Do” list the next time you are in the woods. It won’t be easy but as with most things of value it will be worth the effort.