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General Principles for Controlling Nonnative Invasive Plants
The best defense against nonnative plant takeovers is constant surveillance of right-of-ways, streambanks, and internal roads and trails followed by effective control measures at the first appearance of new arrivals. Early detection and treatment will minimize efforts and costs that come with treating well-established plants or full-blown infestations. More effort is required for successful eradication of established infestations, but it still can be accomplished with proper treatments, although costs may be prohibitive. In severe cases, large-scale conversion of existing infestations is the only solution, involving eradication procedures that incorporate integrated management treatments and reestablishment of native plants. Fortunately, in southern forests native plants in the soil seed bank or plants from surrounding areas will naturally reestablish once the invaders are eliminated.
If an infestation is spotted or already occurs, then proper and aggressive eradication measures should be undertaken to avoid the inevitable spread. Continued treatment and retreatments will be necessary. Most nonnative invasive plants are perennials, having extensive tough roots and runners. This means that effective herbicide applications offer the best means of containment or eradication, because herbicides can kill roots and do so without baring soil for reinvasion or erosion. To be successful with herbicide treatments:
Selective Herbicide Application Methods
Although treating extensive inaccessible infestations may require broadcast treatments of herbicide sprays or pellets by helicopter or tractor-mounted application systems, the best approach is usually selective applications of herbicides to target nonnative plants while avoiding or minimizing application to desirable plants. The selective methods described are directed foliar sprays, stem injection, cut-treat, basal sprays, and soil spots.
Directed Foliar Sprays
Directed foliar sprays are herbicide-water sprays aimed at target plant foliage to cover all leaves to the point of run off, usually applied with a backpack sprayer (use low pressure, drift retardants, and spray shields to avoid drift). Herbicide application by directed foliar spray is the most cost-effective method for treating most types of invasive plant species. With this method, herbicides are thoroughly mixed in water, often with a non-ionic surfactant, and applied to the foliage and growing tips of woody plants or to completely cover herbaceous plants. Foliar sprays are usually most effective when applied from midsummer to late fall, although spring and winter applications have use on specific plants and situations. Selective treatment is possible because the applicator directs the spray towards target plants and away from desirable plants. The addition of a water-soluble dye can assist in tracking treatment and detecting spray drift on desirable plants. Although dyes are messy and short-lived as a visible marker, they are helpful in training and tracking critical applications. Another safeguard is to only use foliar active herbicides, because directed sprays of soil-active herbicides can damage or kill surrounding plants when their roots are within the treatment zone. Never use herbicides with soil activity to treat invasive plants under desirable trees or shrubs.
Directed sprays are usually applied with a backpack sprayer and a spray wand equipped with a full cone, flat fan, or adjustable cone spray tip. These tips and spraying pressures of 20 to 30 pounds per square inch can ensure productivity with only a few fine droplets that may drift to surrounding plants. To safeguard surrounding plants from damage by spray drift, suspend applications during windy conditions. A spray shield that attaches to the end of the wand can further minimize drift. Adding a drift retardant to the spray mixture can eliminate drift although effectiveness may be diminished.
Plants up to 6 feet tall can be treated with this equipment, while the addition of a commercially available wand extension can slightly increase height capabilities. To treat plants up to about 18 feet tall, use higher spray pressures with a straight-stream or narrow flat fan tip.
Directed foliar sprays are also applied using wands on hoses attached to spraying systems mounted on all-terrain vehicles, trucks, or tractors. Also, a spray gun with a narrow flat fan tip can replace a wand for some applications. Another useful alternative for treating different sized woody plants is a spray gun with a swivel that holds two tips—narrow and wide-angled—that can be quickly changed during application.
Stem injection (including hack-and-squirt) involves herbicide concentrate or herbicide-water mixtures applied into downward incision cuts spaced around woody stems made by an ax, hatchet, machete, brush ax, or tree injector. Tree injection, including the hack-and-squirt technique, is a selective method of controlling larger trees and shrubs (more than 2 inches in diameter) with minimum damage to surrounding plants. It requires cuplike downward incisions spaced around the stem with a measured amount of herbicide applied into each of the incisions. Special tree injectors are available to perform this operation, or a narrow-bit ax, hatchet, or machete along with a spray bottle can be used in sequence to perform the hack-and-squirt method. Completely frilling the stem with edge-to-edge cuts or injections is required for very large stems or difficult-to-control species. The herbicide should remain in the injection cut to avoid wasting herbicide on the bark and to prevent damage of surrounding plants. All injected herbicides can be transferred to untreated plants by root grafts and uptake of root exudates. Herbicides with soil activity can damage nearby plants when washed from incisions into the soil by unexpected rainfall soon after application. Avoid injection treatments if rainfall is predicted within 48 hours.
Tree injection treatments are most effective when applied in late winter and throughout the summer. Heavy spring sap flow can wash herbicide from incision cuts, making this an ineffective period.
Cut-treat involves herbicide concentrates or herbicide-water mixtures applied to the outer circumference of freshly cut stumps or the entire top surface of cut stems, applied with a backpack sprayer, spray bottle, wick, or paint brush. Freshly cut stems and stumps of woody stems, including canes and bamboo, can be treated with herbicide mixtures to prevent resprouting and to kill roots. Cutting is usually by chainsaw or brush saw, but can be accomplished by handsaws or cutting blades. To minimize deactivation of the herbicide, remove sawdust from stumps before treatment. Treat stems and stumps as quickly as possible after cutting with a backpack sprayer or utility spray bottle for spray applications or a wick applicator, lab wash bottle, or paintbrush for small stems. Add a non-ionic surfactant to the mix to aid in penetration, if permitted by the label.
For stumps over 3 inches in diameter, completely wet the outer edge with the herbicide or herbicide mixture. Completely wet the tops of smaller stumps and all cut stems in a clump. Apply a basal spray mixture of herbicide, oil, and penetrant to stumps that have remained untreated for over 2 hours.
The most effective time for the stump spray method is late winter and summer. Although winter treatments are slightly less effective than growing season applications, the absence of foliage on cut stems and branches produces some offsetting gains in application efficiency.
Basal sprays are herbicide-oil-penetrant mixtures sprayed or daubed onto the lower portion of woody stems, usually applied with a backpack sprayer or wick applicator. Full basal treatments require that the lower 12 to 20 inches of target woody stems be completely wetted on all sides with an oil-based spray mixture. Application is to smooth juvenile bark. Full basal sprays are usually effective in controlling woody stems less than about 6 inches in diameter or larger diameters of susceptible species, before bark becomes thick, corky, and furrowed. The appropriate equipment for this treatment is a backpack sprayer with a wand or spray gun fitted with a narrow-angle flat fan, cone, or adjustable tip. A wick applicator can also be used. Herbicides that are soluble in oil (mainly Garlon 4) are mixed with a commercially available basal oil, diesel fuel, or kerosene often adding a special penetrant. Some herbicides, such as Pathfinder II and Pathway, are sold ready-to-use with these ingredients.
A modified method, streamline basal sprays, is effective for many woody species up to 2 inches in diameter, as well as trees and shrubs up to 6 inches in diameter if the species is susceptible. Equipment for this treatment is a backpack sprayer with a spray gun and a low-flow straight-stream or narrow-angle spray tip. To prevent waste, maintain pressure below 30 pounds per square inch with a pressure regulator. At this pressure, an effective reach of 9 feet is possible while bark splash is minimized. For treating stems less than 2 inches in diameter, apply the stream of spray up-and-down single stems for about 6 to 8 inches, or apply across multiple stems creating 2- to 3-inch-wide bands. This same multiple-band treatment can be effective on larger stems. Direct the spray stream to smooth juvenile bark at a point about 4 to 18 inches from the ground. Stems that are thick barked or near 3 inches in diameter require treatment on all sides.
Applications are usually in late winter and early spring, when leaves do not hinder spraying the stem. Summer applications are effective but more difficult. Avoid ester herbicide formulations on hot days to prevent vapor drift injury to nontarget plants.
Soil spots are Velpar L herbicide applied as metered amounts to the soil surface around target woody stems or in a grid pattern for treating many stems in an area; they are usually applied with a spot gun or with a backpack sprayer equipped with a straight-stream nozzle. Spots of soil-active herbicide (mainly Velpar L) are applied to the soil surface in grid patterns or around target woody stems. This method requires exact amounts and prescribed spacings that are specified on the herbicide label or label supplements. It is only effective on specific nonnative plant species and usually only when applied in spring and early summer. Equipment is a special spot gun, utility spray bottle, or a backpack sprayer with a spray gun equipped with a straight-stream spray tip.
Selecting an Effective Herbicide
Only herbicides registered by the U.S. Environmental Protection Agency for forestry use and noncroplands in the Southern States will be discussed here, although herbicides for other “land use areas,” such as right-of-ways, pastures, and rangelands, may be just as effective or may contain the same active ingredient. The herbicides that will be identified by trade name (and common active-ingredient name) are:
Because nonnative invasive plants are usually difficult to control, selecting the most effective herbicide(s) is important. Often herbicides that have both soil and foliar activity are most effective with the least number of applications. However, applying herbicides with soil activity can damage desirable plants when their roots are present within the treatment zone or when herbicides move downhill to untreated areas following heavy rainfall. Garlon herbicides are mainly foliar active, but they have some soil activity at high rates or when mixed with oils. Garlon 4 and Vanquish can volatilize at high temperatures and their residues can move by air currents to affect surrounding plants; therefore, avoid application on days when temperatures exceed 80° F. If possible, also avoid applications when rainfall is anticipated within 2 days, unless soil activation is needed, and during periods of severe drought as effectiveness can be reduced during these times.
When possible, use selective herbicides that target specific nonnative species, such as Transline that controls mainly legumes and composites, and minimize damage to surrounding desirable plants even though they receive herbicide contact. Minimizing damage to desirable cohorts can also be achieved by making applications when the cohorts are dormant. For example, apply basal sprays to the bark of invasives in late winter before most other plants emerge, or foliar spray evergreen or semievergreen invasives after surrounding plants have entered dormancy. Remember that desirable woody plants can be damaged through transfer of herbicides by root exudates following stem injection and cut-treat treatments or when soil-active herbicides wash off treated stems. Damage to surrounding native plants can be minimized with care and forethought during planning and application.
Read and thoroughly understand the herbicide label and its prohibitions before and during use. Many herbicides require the addition of a non-ionic surfactant to the spray tank. Always use clean water in a herbicide mixture and mix spray solutions thoroughly before applying. Do not mix in the sprayer but in a bucket with a stirring stick—stirring for several minutes or more—before transferring to the sprayer. Water that is highly basic (pH greater than 6) and contains high amounts of calcium and magnesium interferes with glyphosate herbicide effectiveness, requiring the addition of ammonium sulfate or appropriate additives. When changing from a water-based mix to an oil-based mix in a backpack sprayer, thoroughly evacuate the water from the pump and run a small amount of oil through the pumping system before filling with the oil-based mix, otherwise, a white sludge will clog the sprayer. And, always wear personal protective equipment prescribed on the label and in supplementary materials.
Other Treatments for an Integrated Approach
Overgrazing is a way to reduce the vigor of palatable invasive plants like kudzu, but this rarely yields eradication and may spread seeds (as with tropical soda apple). Mechanical treatments and prescribed burning can assist eradication measures, but are limited in effectiveness. Prescribed burning cannot control rootcrowns or rhizomes of perennial plants and usually only deadens small aboveground shoots, providing only temporary aboveground control. In a similar way, cutting woody plants (by chainsaw and brush saw felling or brush mowing) and mowing vines and herbs without killing roots remove only aboveground plant parts. Mechanical root raking and disking can actually intensify and spread infestations of invasive plants with runners by chopping them into resprouting segments and transporting them on the equipment. Fireplows can also spread invasive plant rhizomes and roots. However, root raking, piling, brush mowing, or burning may be the only way to start controlling dense infestations of multiple woody invasive plants. Small infestations may respond to hand pulling, grubbing with a stout hoe, or shrub pulling with newly introduced devices. Hand pulling or grubbing may be the quickest and easiest way to halt invaders when first spotted and stop them from gaining a foothold. String trimmers can reduce infestation densities and injure thick waxy leaves to improve herbicide uptake and effectiveness.
Although ineffective by themselves to achieve eradication, both mechanical and burning treatments can give added kill of herbicide-weakened plants and have a place in an integrated pest management program. The stumps and stems of nonnative trees, shrubs, and bamboos can be treated with herbicides immediately after cutting to kill roots. Resprouts of trees, shrubs, and vines that are topkilled by burning or brush mowing can be more easily treated with foliar sprays, often the most cost-effective way to use herbicides. Herbicide applications should be delayed after burning, disking, or mowing to permit adequate resprouting of target plants and, thereby maximizing herbicide uptake and effectiveness. Prescribed burning can also destroy invasive plant seeds (and bulbils of air yams) and often stimulate germination for efficient herbicide control treatments. Burning can prepare the site for effective herbicide applications by clearing debris and revealing application hazards, such as old wells and pits. Disking and root raking, if applied correctly, can dislodge herbicide-damaged woody roots and large runners, leaving them to dry and rot. With mechanical and burning treatments, take precautions, such as burning in late winter or spring leaf-out, to minimize the period of bare soil. The most effective time for controlling woody invasive plants and their germinants with fire is after plants have initiated growth in spring.
An eradication program for infestations of invasive plants usually requires several years of treatment and many more years of surveillance to check for rhizome sprouts, root sprouts, seed germination, or new invasions. Following these steps in a planned manner and with persistence is the only successful strategy to safeguard land access, productivity, native plants, and suitable habitats for wildlife.
The Rehabilitation Phase
Rehabilitation is the most important final phase of an integrated invasive plant eradication and reclamation program. The rehabilitation phase requires establishment and/or release of fast-growing native plants that can outcompete and outlast any surviving nonnative plants while stabilizing and protecting the soil. If the soil seed bank remains intact, native plant communities may naturally reinitiate succession after eradication of nonnative plants. Light-seeded native species are usually present in the seed bank while heavier seeded plants will gradually be deposited on a site by birds and other animals. In recent years, native plant seed and seedlings have become increasingly available for rehabilitation sowing and planting, but a limited number of species and absence of well-developed establishment procedures often hinder use. Tree nurseries operated by State forestry agencies are a good source of many species of native trees and shrubs. Often it is necessary to establish fast-growing tree species during the later control phase to hinder reestablishment of shade intolerant nonnative invasive plants. Reestablishing native grasses and forbs is equally important. These species are available from commercial nurseries specializing in native plants, utilizing local sources when possible. Native plant seeds will require proper treatments to assure timely germination. Seedling native plants can be also collected and transplanted from suitable field sites. Their establishment will be more challenging than the commonly available nonnative plants so often used for soil stabilization and wildlife food plots. Constant surveillance, maintaining forest vigor with minimal disturbance, treatment of new unwanted arrivals, and finally rehabilitation following eradication are critical to preventing and controlling invasions on a specific site.
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The Bugwood Network, USDA Forest Service & USDA APHIS PPQ.
The University of Georgia - Warnell School of Forest Resources and
College of Agricultural and Environmental Sciences - Dept. of Entomology
Last updated on Wednesday, November 05, 2003 at 01:37 PM
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