Management of Invasive Species in Restoration Projects

by Suellen May

Knapweed and common mullein.

The Problem
Human activity has moved many exotic species across previous long-standing biogeographic barriers. In the process of this introduction, whether accidental or intentional, the exotic species’ predators that keep the plant “in check” are left behind. The consequence of this activity is invasive species or weeds. Although many exotic species have proven useful, many others have disastrous effects on our natural systems. Invasive species damage natural areas by outcompeting native species and therefore altering ecosystem processes. Invasive species, such as purple loosestrife (Lythrum salicaria) can be the equivalent of biological pavement by establishing exotic monocultures that render the ecosystem useless to native wildlife. Controlling these invasive species is crucial to the success of restoring natural systems.

Handling the Problem
The first step should be a description and inventory of the land. Both native and invasive species should be identified and mapped. The most reliable and accurate data can be derived from using Global Positioning System (GPS) and Geographic Information Systems (GIS) technologies. GPS units such as the GeoExplorer or Pathfinder (Trimble Products) can provide real-time locations accurate to a single meter and label them with weed information in the field. These electronic data can be exported to a GIS such as ArcView to generate an accurate map of weed infestations. Endangered or threatened plant species should also be mapped and marked so that weed management activities such as herbicides do not injure non-target plants.

Determining the size, location, type, and density of invasive species will assist in establishing realistic goals and determining costs to achieve these goals for the restoration site. For example, a high density and distribution of a deep-rooted perennial such as Canada thistle would make complete eradication an unrealistic goal. Rather, this weed will have to be managed each year with the greatest efforts in the first couple of years. Small and recently introduced invasives can be expected to be eradicated if great care is taken to return to the exact infestation site; hence the importance of GPS/GIS technologies that pinpoint exact geographic locations while in the field.

Determining size is also important in prioritizing which invasive patches to control first. Small patches that are new infestations should be given the highest priority. The analogy of a wildfire is often used, whereby small initial infestations are likened to backcountry spotfires. These infestations are more easily controlled because they have less developed root systems, fewer stored food reserves in roots and rhizomes, and a smaller seed bank in the soil.

Location is important in developing strategies for control. One major consideration in determining tools for control is proximity to water. This will determine what type of herbicides can be used, type of equipment for mowing, and again setting priorities. Many invasives, such as leafy spurge and Canada thistle, spread primarily through waterways because their seeds float. Therefore, a patch of leafy spurge on a riverbank should be given a higher priority because of the probability of creating new infestations downstream.

After an inventory of the property is completed and land management goals have been established, the next step is to prepare the seedbed based on type of weeds, distribution, and density. A typical program may include plowing and seeding an annual hay crop in the first and second years, and replowing and seeding to a grass-legume mixture in the third year. This allows for the germination and removal of weeds from the seedbank. If accessibility or erosion is a concern, herbicides containing glyphosate (e.g., Roundup) can remove unwanted vegetation including invasives. Soil and underground plant parts should not be disturbed prior to applying Roundup or until one week following the application to allow translocation of the herbicide into underground plant parts. Glyphosate has no soil activity, and therefore native grasses can be seeded one week after application. An aquatic formulation of glyphosate-Rodeo-is also available.

Canada thistle seeding.

Follow-up Management
After the site has been seeded, invasive species will return to a lesser extent. The next step is to determine the techniques for managing these invasive species. The four categories of weed control are mechanical, cultural, biological, and chemical.

Mechanical control includes pulling, mowing and cutting. For the first year of establishment of native grasses, mowing is likely to be the only feasible method of control. Most herbicides for use on native grasses, such as Redeem, require established grasses as indicated by tillering, development of a secondary root system, and vigorous growth. Mowing can be sufficient method of control for annuals such as kochia (Kochia scoparia) and biennials such as musk thistle (Carduus nutans) and common mullein (Verbascum thapsus) but will not suffice for perennials. The mower blade should be set relatively high so as to cut the taller weeds but to not cut the shorter, slower-growing species. In general, the best approach is to mow when weeds get 2- to 3-times taller than native grass. Also, do not mow closer than 8 to 12 inches above the ground.

Cultural control methods manage weeds by establishing desirable plant species. Soil cultivation (cutting through and turning over the soil), seeding, fertilization and irrigation are all tools for cultural control. Although fertilization and irrigation may not be likely solutions for restoration sites in natural communities, selection of the appropriate competitive species should be exploited. For example, Russian knapweed (Acroptilon repens) is a deep-rooted perennial that is highly competitive and readily establishes monocultures. In addition, this plant is allelopathic, producing chemicals that inhibit the growth of other plants. Therefore, Russian knapweed control includes not only herbicides to kill existing plants, but also a competitive species to be planted to prevent reinvasion.

In situations where a site is dominated by an allelopathic weed, species selection should be determined based on the response to the allelopathic chemicals. For example, research from Colorado State University indicates that western wheatgrass (Agropyron smihii) germination may be suppressed by the aqueous extracts of Russian knapweed. However, smooth brome (Bromus inermis) germination was not inhibited. Although smooth brome might not be the desirable grass species for a native community, it could act as a cover crop while the allelochemicals from Russian knapweed biodegrade. It is important to understand the characteristics of the invasives and plants to be seeded as well as the interactions between the two to determine the likely success of restoration.

Biological control methods include the introduction of a weed’s natural predator, such as the leafy spurge flea beetle (Aphthona ssp.), for the control of leafy spurge. The flea beetles control leafy spurge by feeding on the foliage during the adult stage of the insect and feeding on the roots during the larval stage. In their native country, both leafy spurge and flea beetles are kept in balance by nature’s predator-prey relationship.

The presence of both insect and plant prevent one or the other’s population from growing out of control. For example, an increase in leafy spurge populations results in a larger food supply for the flea beetle. Flea beetle populations rise due to the increase in food. As insect populations begin to grow larger, food supplies (i.e. leafy spurge) dwindle, and the insect population declines. The smaller insect population gives rise to more leafy spurge...and the cycle continues.

Flea beetles are host-specific; they will only eat leafy spurge, not desirable vegetation. The disadvantage of this cycle is that the weed is never eradicated. Therefore, biological control is not appropriate for small, manageable infestations in accessible areas.

Generally, the introduction of insects for the control of weeds, such as leafy spurge, takes approximately 3-8 years to see a significant change in the vegetation complex. However, once populations have readily established, insects can be collected with a net and redistributed to other infestations. Contact your state department of agriculture if you are interested in biological control.

Herbicides play a limited role in restoration projects during the first couple of years after seeding. Grasses need to be well established, as evidenced by tillering and a secondary root system, to prevent injury by chemicals. In addition, herbicides recommended for invasives common in natural systems will injure all broadleaf plants. Therefore, if an intensive chemical control program is anticipated, it is best to seed grasses and then plant forbs when weeds are controlled.

Another option to prevent injury to desirable vegetation is to use glyphosate applied with wiper technology to control invasives. This technique works well in wetlands dominated by Canada thistle. The presence of water in a wetland prevents the use of many herbicides that would be able to provide a deep root-kill of this perennial. Therefore, using a herbicide such as Rodeo prior to revegetation and following up with Rodeo applications via a weed wick or wiper provides suitable control, although it is highly labor-intensive.

The management of invasives in restoration programs requires a well-planned, organized program ideally by integrating methods of control. The first phase includes a description and inventory of the property. Establishing goals and objectives, based on the inventory, is the second phase. Next, weed control techniques should be determined based on factors such as the invasive species’ characteristics and restoration goals. Patience is also critical; many land managers are discouraged to see invasives return after initial control efforts and reseeding. However, if a systematic program is followed based on the ecology of invasives and natives, sites can be relatively free of invasive species in 3-5 years. L&W

For more information, contact Suellen May, weed specialist, Larimer County Natural Resources, Fort Collins, CO, (907)498-5768, fax(970)498-5780, mays@co.larimer.co.us.