Control Measures for Invasive Plants

Photo © Port of Tacoma

An effective plan to gain control of invasive plants will include a variety of tools, techniques, and timeframes to approach the issue with. A landscape with a combination of invasive trees and shrubs in the hedgerow, a groundcover taking over the garden, and perhaps some garlic mustard filling the understory of the backwoods will, for example, require a variety of control tactics implemented during different seasons. Below, we highlight effective methods of control for invasive Ground Covers, Annual, Biennial and Perennial forbs, Bushes and Shrubs, and Trees, including points for success when implementing them.

Ground covers

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Photo © Shihmei Barger 舒詩玫

Ironically, some of the most difficult invasive species to get on top of are ground covers. English ivy (Hedera helix), bishop’s goutweed (Aegopodium podagraria), Japanese pachysandra (Pachysandra terminalis), and periwinkle (vinca minor) (above) are just a few of the common perennial vines notorious for their potential to escape cultivation. They are generally tolerant of poor soils and both full shade and full sun which allows them access to a variety of growing conditions. Ground covers like this are rhizomatous and spread quickly underground with an extensive root system. Getting control of invasive plants like these means getting rid of those roots.

Removal Method

Pulling is laborious but may be the best choice for small areas or removing ivies on steep or rough terrain. Success can be achieved with the proper diligence, tools, and techniques. A few guidelines can help:

    (ALL INTERNAL RIGHTS) Periwinkle © Alden Warner

    Photo © © Alden Warner
  • Pull in spring or fall when soil is moist but still crumbles. This will allow you to dig in deeper and helps to separate the roots from the dirt, leaving the soil behind when you remove the plants.
  • Pry up a section of soil with a shovel or hand spade and break it apart, removing the roots from the mix. Leave soil loose and allow it to dry to desicate small roots left behind.
  • Start from one side and work the leading edge, a small section at a time. This helps work the soil and roots loose and keeps you organized as you move along.
  • Be sure to bag and remove all pieces as most invasive groundcovers can reestablish from a small chunk left behind.
  • Replant immediately with a desirable selection of native plants. The newly disturbed ground is prime habitat for more invasive species to colonize or for erosion and further disturbance to occur.
  • Revisit the area several times in the first two years and dig up any new or missed sprouts.
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Photo © Oregon State University

Mowing, weed whipping, or livestock grazing

Any way you slice it, removing the top greenery from ivy stops photosynthesis and forces the roots to use reserved energy to regrow more vines and leaves. Aggressive, repeated mowing, weed whipping, or livestock grazing can eventually deplete the energy reserves in the roots until they lose the ability to grow back. To get the most of your mowing efforts, chew on these tips:

  • Mow low. Get as close as you can to the soil, trimming off all leaves and stems.
  • Rake and remove all debris. This will allow the ground to dry out, putting more stress on the weakening roots. If you leave the trimmings they may provide growing benefits similar to mulch.
  • Mow often and do not allow ground covers more than a couple of inches of green growth to occur. Mow or cut often in the spring and early summer when growth is faster (up to once a week), and then one last time before winter to apply the most stress to the remaining roots.
  • Protect desired trees, shrubs, and other plants from being cut or grazed with caging, fencing, or markers to warn mowers.
  • Yes, you can hire goats to do this work for you. It is a thing, so check locally for anyone willing to “perform” this service.
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Photo © Chris Martin

Smothering

Smothering is a simple and very effective way to rid an area of difficult ground covers (and rid your storage space of all those holiday season Amazon Prime boxes!)–including lawn, but it can take several seasons to complete. Smothering works by using a cover, generally cardboard or newspaper and several inches of mulch, to effectively block light and air from getting to the plants, preventing photosynthesis and starting a decomposition process that heats the soil, killing the roots and possibly even some weed seeds. To successfully smother out invasives, be sure to cover these points:

  • First, mow or weed-whip low to the ground, removing as much vegetation as possible.
    Water the area until it is well saturated. This helps to remove air from the soil.
  • Use cardboard with all tape removed for best results. Newspaper can also be used but avoid colored ink as it may be toxic to organisms in the soil. We recommend against using plastic sheeting– it inevitably gets brittle, breaks up and becomes a pollution in the soil; and, it is difficult to remove once it reaches this stage.
  • Secure the cover with stakes or rocks around the edges to keep from moving in the wind.
  • Overlap newspaper sheets, 10-15 layers thick, wetting each layer as you add, to create a dense continuous mat.
  • Cover one to two feet beyond the desired patch size as a buffer against encroaching light and air.
  • Maintain cover for four to six months or more and monitor often to ensure there are no holes or tears and the edges remain secure.
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      Photo © clo

      Spraying

      When all other options for control are just not practical, a targeted, precise, and concerted use of herbicide can quickly start to improve the habitat value of a heavily invaded site such as the patch of Japanese pachysandra (Pachysandra terminalis) shown above. To help ensure the benefits of creating habitat with chemicals will outweigh the risks, you’ll want the most effective result out of the least amount of product. Apply the following measures to minimize risk and maximize efficiency of your herbicide application.

      • Start in the spring. Before getting out the sprayer, perform a full growing season of heavy mowing (see above) to weaken the root system.
      • Stop mowing in late summer to allow new growth to reach a full leaf. This will provide the most surface area to apply the herbicide and will prevent waste and danger from overspray.
      • Apply herbicides to ground covers on a calm, hot sunny day in the early fall when perennials are actively transporting fluids to depleted root systems.
      • Even and solid coverage is key. Adjust the tank pressure and the spray cone to put a fine mist on the leaves. Too heavy of a spray will bead up and drip off, becoming useless, wasteful, and dangerous.
      • Work backwards to avoid walking through the treated area. Herbicides on clothing can contaminate living areas. A colored dye in the spray will indicate coverage.
      • Wear the recommended protective clothing and safety equipment and ABOVE ALL, READ AND FOLLOW ALL DIRECTIONS ON THE LABEL PRECISELY! Herbicide can be a deadly poison if misused.
      • Best Advice: Hire a Professional. They are likely to be better trained to use less product more effectively, have more rigorous controls for safety, and are insured against mistakes and damages.

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      Annual, Biennial, and Perennial Forbs

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      Photo © AbigailWoods

      Annual plants start from seed and will grow to maturity, flower, produce seed, and die within one year. Biennial plants are similar but will start their first year as a small, non-flowering rosette, and will produce blossoms and seed in the second year. A rosette allows a seedling a chance to establish itself in a spot that is more likely to be successful before expending the resources to flower and produce seed. A biennial rosette is not capable of reproducing in its first year, and most will not survive through to the next year, which is important to know when making decisions about putting effort into its management. Garlic Mustard (Alliaria petiolata), shown above, is a common invasive biennial plant that demonstrates this survival trait. Only a few of the rosettes in the image will survive the resource competition and environmental forces their first year–those that do will become a mature, reproductive adult the following year and can be more easily managed in then.

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      Photo © Dendroica cerulea

      Annual and biennial plants will generally only reproduce from seed and, while perennials can also reproduce through various forms of vegetative means, preventing invasive plants, like the Japanese stiltgrass (Microstegium vimineum) shown here, from producing seed is a primary measure for control. Plants with the potential to be invasive tend to naturally produce copious amounts of seeds which can remain viable in the soil for decades. Over time, though, this seed bank can either be suppressed, as vegetation debris and soil accumulate on top, or depleted, through repeated germination and culling, depending on the control measure being used. Either way, stopping the accumulation and deposition of seeds in the environment is critical to long term control and successful restoration. To rob the seed bank of its monotonous reserves and invest in diversity, these insider tips can give you better ground to stand on.

      Part of the Plan

      To be successful with invasive species control, it is important to understand the conditions and recovery capabilities of the environment that is being restored. Without a workable plan to replace the vegetation, the work of invasive species removal often results in new invasive plants recolonizing the area. The invasion may have altered the conditions in the soil, through allelopathy or changes in soil structure, limiting the ability of some native species to grow there afterwardsopen_in_new.

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      Photo © djvass

      In the picture above, invasive garlic mustard (Alliaria petiolata) is being pulled by hand. This labor intensive method can be initially satisfying but tends to create bare and “disturbed” areas from soil upheaval. This may bring buried seeds from past years to the surface and provide a perfect opportunity for the invasive plant to persist. Many seasons of pulling may be needed to deplete the seed bank –a viable but time consuming method. Consider the lifecycle of the invasive and develop a tactic that will hinder its reproduction. In the case above, a method of removing the flowering tops of the garlic mustard will stop the seed production and avoid bringing old seeds in the soil to the surface. If it is early in the season you may have to return to cut off new flowering growth, while cutting late in the season will require collecting and disposing of the seed pods which are capable of maturing and becoming viable, even after being cut off. As the existing seed bank is buried deeper by the accumulating debris, its ability to dominate, or even persist, is severely limited.

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      Photo © Anthony Sokolik

      In a dense invasion, this timing and technique is likely to work. In a more diverse environment, similar to the one on the left, with fewer invasive plants to control, it may be more practical to simply pull and bag the garlic mustard, leaving the native vegetation to fill in. Your plan to control invasive plants should include a mix of these measures, appropriate for the site, the level of concern and control desired, and will consider a path to replacing the invasive species with desired vegetation.

      Photo © © Erika Nortemann/TNC

      Mechanical Removal

      Pulling plants means getting to the roots of the problem. It is not uncommon for plants with invasive traits to have defenses against animal grazing. When the plant’s roots are left in the ground by mowing or poor weeding it mimics grazing–where an animal simply removes the above ground foliage. Many perennial plants are adapted to this and will quickly resprout, sometimes regrowing multiple branches from the crown or any remaining stem, potentially producing multiple times the flowers and seeds, and making it even more difficult to remove at the next attempt. Get a good hold on invasive annual and perennial forbs with the following tips:

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        Photo © Neeta Lind
      • Pull shortly after rain or after watering the area with a hose or sprinkler to loosen the soil and make the job easier. Allow the soil to drain until it is moist but not muddy so that it easily falls off the roots. This keeps the tools and gloves clean too.
      • Learn the identification of the invasive plant and get to know its habits. Is it an annual or perennial? When does it seed and how often? What type of root system does it have?
      • Taproots can resprout from several inches below the soil. Often a well aimed shovel, though, can sever the taproot deep enough to kill it. Then, simply pry up with the shovel to break the root’s connection to the soil and it will pull right out.
      • Annuals with shallow fibrous roots can be removed by pulling by hand after prying up with a shovel from underneath to loosen the ground.
      • The fibrous root system of perennial plants can be several times the size of the aboveground plant, this is bamboo shown here, and may require more digging to get as much of the root system as possible. If you can return to the site, you can allow resprouts to reveal the position of any surviving roots. They will likely be easier to pull then and it could save a lot of digging and sorting through the ground.

      Photo © © Michelle Kalantari/TNC

      Mowing or weed whipping

      With annual and biennial invasive plants, mowing or weed whipping before the plants produce viable seeds can be the most efficient way to clear a large area. The invasion occurs, in part, from the copious amount of seeds these plants produce. Stopping that part of the reproduction cycle is the first step in whipping the area back into shape.

      • Time your trimming. Wait until plants are full grown to deplete the root’s reserves, reducing their ability to regrow and flower the same season.
      • Consider pollinators. Cut when plants are either just about to flower or just after the blossoms have fallen off but before seed heads form.
      • Flag desired plants. Mow above or around desired plants and allow them to flower and produce seed.
      • Leave trimmings and fall leaves to build up the soil and suppress the existing seed bank.
      • Transplant other desired plants into the area in the fall to get a jump on dominance in the spring.
      • Take other wildlife considerations into account when mowing large areas.

      Smothering

      As with ground covers, smothering can be just as effective when clearing an area of annual vegetation and is recommended over mowing for perennial vegetation. Also called sheet mulching, smothering works by covering the site with cardboard and several inches of mulch. This blocks air and light from the plants and prevents growth and reproduction. Smothering invasive annuals and perennials is similar to ground covers and lawn. Again, be sure to cover these points:

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      Photo © Matt Simon
      • First, mow or weed-whip low to the ground, removing as much vegetation as possible.
      • Water the area until well saturated. This helps to remove air from the soil.
      • Use cardboard with all tape removed or blank newsprint. We recommend against using plastic sheeting–it is difficult but necessary to remove as it inevitably gets brittle, breaks up and becomes a pollution in the soil.
      • Secure covering with stakes or rocks around the edges to keep from moving in the wind.
      • Cover 1 to 2 feet beyond the desired patch size as a buffer against encroaching light and air.
      • Maintain cover for 4-6 months or more and monitor often to ensure there are no holes or tears and the edges remain secure.

      Chemical Control

      Photo © jon hayes

      Often, invasive herbaceous perennials, like the black swallow-wort (Cynanchum louiseae) shown here, have root systems so vast that no amount of mowing is going to reduce the population. In fact, many perennial plants, such as Japanese knotweed (Polygonum cuspidatum), can reproduce from broken fragments lying on the ground and are often spread from roadside mowing. Digging and pulling may be too monumental a task and an application of herbicide may start to sound appealing. Due to the complexity of chemical management for each individual species, we recommend researching your local land-grant Cooperative Extension for species specific information on timing, amounts, and application techniques. Better yet, hire a professional. They are likely to be better trained to effectively use specific herbicides for specific plants, have more rigorous controls for safety, and are insured against mistakes and damages.

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      Woody Shrubs and Bushes

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      Photo © Kurt Haubrich

      Honeysuckle (Lonicera tatarica), Autumn olive (Elaeagnus umbellata), Scotch broom (Cytisus scoparius), barberry (Berberis spp.), pictured above, and privet (Ligustrum spp.), are some of the most common invasive woody shrubs. These plants have been frequently used as ornamental yard bushes or for erosion control in roadside plantings. They can quickly become difficult to manage, spreading into natural areas and pushing out the native vegetation and the wildlife that depend on it. Although daunting at first glance, with the right approach, removing these scrappy scrubs can be an efficient and rewarding way to re-establish that lost habitat.

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      Photo © Washington DNR

      Just pull it

      Pulling may be one of the most effective, direct, and immediately rewarding ways to remove invasive shrubs. The correct tools and soil conditions can improve the ease of removal and the long term results. Using a weed wrench in the spring or fall when the ground is wet, like the volunteers in the image above, can make quick work of pulling heavy rooted shrubs, small trees, and other woody stemmed invasives. Below are some tips to make pulling more effective and less back breaking.

      • Choose your weapons wisely. Weed wrenches, as well as most landscaping tools, come in different sizes for bigger or smaller jobs. The right sized tool will do most of the work for you.
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        Photo © JJohnston
      • Work your way in. Use pruners, hand saws, or chainsaws to cut away at the branches, removing all but the main stems.
      • Get a handle on it. Leave a few tall pieces of the largest central stems to use as levers for prying and pulling.
      • Use a shovel and pickaxe to get under and pry up the root crown. Cut off large side roots and dig them up separately.
      • Get as much of the root as possible. These plants can be invasive because they easily resprout from pieces of root left behind. However, if you can monitor the site often, it may be easier to let that regrowth point out any roots that do survive.

      It’s a dirty job

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      Photo © U.S. Department of Agriculture

      When working in areas with invasive species, assume there are seeds in the soil and take precautions when leaving the area so as not to carry any out with you. Seeds can get trapped in mud and picked up by boots and tools, and in vehicle tires. Cleaning these off before leaving the area can keep new populations from cropping up elsewhere. Also, bag up the roots for transport and disposal to keep from losing seeds along the way. Knowing the consequences of invasive species and the vectors that spread them, and then doing your small part to prevent it, can make a big difference in conservation efforts to control nonnative plant species and their potential to dramatically alter the environment.

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      Photo © Chris Bentley Flickr

      Cut and Paint Method

      Pulling and digging up shrub roots can leave the area fairly disturbed and can reintroduce the invasion by bringing old seeds to the surface while opening up space and light for them to germinate. Shrubs may also just be too old and too large to be removed practically. When either of these are the case, an herbicide treatment may quickly clear the area and allow new plantings to take over, replenishing habitat resources in a short time. Effective results come from proper planning and performance –to affect the plant at its weakest point and prevent unintentional consequences hit these points:

      • Apply herbicides in Fall, at the end of the growing season, when risk of contact with other plants is minimal and woody plants are actively transporting fluids to their root system.
      • First, cut the stems of the plant down to about 2 inches above the ground.
      • Then, immediately apply a thin coat of Roundup (active ingredient, glyphosate) to the outer rings of the stump, just inside the bark. This ring is the cambium layer where the only active growing takes place. Herbicide anywhere else is unnecessary.
      • For maximum effect, cut the shrubs down to about 12 inches in the spring and let new sprouts grow all summer to deplete the root system of reserves, then cut to two inches in the fall and paint.
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      Photo © With Associates

      Shearing/ Pruning

      You can keep invasive species from spreading by seed if you prune them after flowering. As mentioned above, many invasive shrubs are considered ornamental and are used in landscaping of all kinds. Once the flowers become berries, birds and other wildlife begin to consume them, eventually depositing the seed somewhere else. In settings like the scene above, berries and seeds can be washed down by rain and transported to other locations through the city’s drainage infrastructure. If you want to keep the hedges in your own habitat, add a well-timed pruning plan to your management goals. You may even offer to help with the neighbor’s shrubbery as a buffer against seeding in unwelcome places.

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      Invasive Trees

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      Photo © David Berry

      Invasive trees are good at winning the long game. Their common traits of prolific seeding and shade tolerance, demonstrated by the Tree of Heaven (Ailanthus altissima) above, allow a large percent of seedlings to establish early and persist in environments where native species may have difficulties. They often outgrow native trees and, with their early spring leaf-out and late fall leaf-drop, they have a longer growing season and tend to shade out competition. Deer and other predators select the dwindling native saplings over the invasives, giving them an even further advantage. As mature native trees are lost over time, the majority of replacements are the invasives. From managing a timber stand to weeding gardens, a combination of the following tactics can get you ahead of the game.

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      Photo © Charles Willgren

      Pulling seedlings

      This is the ideal time to remove invasive trees. Seedlings, even small saplings up to two inches in diameter, can often be pulled right out. As successional species they are often fast growers and do not put as much energy into securing roots as they do in gaining height. This two foot high Tree of Heaven (Ailanthus altissima) seedling will only have about a four to six inch tap root. Use a shovel or a garden weeder to get under it and pry up a little to loosen the soil, it should pull out fairly easy.

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      Photo © jalexartis

      Cutting and/or Painting saplings

      Larger saplings are more likely to be better rooted and digging may not be practical. A small pruning saw can quickly cut the small tree at the base. This will remove it for the time being but resprouts from the stump will likely occur. One option is to continue cutting these resprouts, easily done with hand clippers. Allow the resprouts to grow one to two feet before cutting to deplete root reserves and eventually it may give up. Stripping the bark from the stump may also help prevent resprouts. For a more aggressive and final approach, follow the steps above for cutting and painting shrubs and bushes.

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      Photo © CalgarySandy

      Felling or Girdling the Seed Producers

      Stop invading tree species at their source. If you don’t curb the seed production, the management of seedlings and saplings will continue relentlessly. In many residential settings this may be the only option, due to property rights and safety issues, but good timing and diligence can keep the job of pulling invasive seedlings light and manageable. When possible, however, invasive trees should be managed to stop their reproduction. The Norway maple (Acer platanoides), shown above, will produce more seeds than native maples that are less desirable to granivores, faster to germinate, and thus likely to be more successfulopen_in_new. In larger habitats, cutting down invasive trees will open up space and provide more resources for native trees to fill in. You can harvest the timber if practical or leave the downed logs as important habitat for wildlife.

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      Photo © Gary Cziko

      Leave it for wildlife

      Girdling is a simple and effective option to stop seed sources without the need to remove or dispose of the wood. Girdling disconnects the life support from the leaves to the roots by cutting a ring out of the bark, the phloem, and the cambium layers just under, while still allowing transport of water out of the roots. This technique will kill the roots first, limiting resprouts, and will eventually kill the top of the tree, stopping further seed production. The standing dead tree, then, creates a snag, another excellent resource for wildlifeopen_in_new.

      • Do this with an ax or a chainsaw, there are also various girdling tools you can purchase.
      • The girdle should be six to 12 inches wide and should not get too deep into the sapwood.
      • If resprouts do occur after the first year, a second girdle below the first one can be applied to finish it off. Some tree species are just more resilient.
      • Manage seedlings and saplings within the first two years to their prevent their potential rapid growth from the increased light and other available resources.

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