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Alien Invader Plants
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| Eradicating Alien Plantsby Jean Moore, Wildlife & Environment Society of SA, 100 Brand Road, Durban 4001.There are several methods of eradicating or effectively controlling alien plants, all of which have their advantages and disadvantages; situations where they are appropriate and situations where they are not. The main weed control methods are discussed below. In practice, a combination of methods (integrated control) is often used:
A)
Manual/Mechanical:
Some form of force is used to control the target weeds e.g. uprooting (including hand-pulling), slashing, mowing or felling, ring barking (removing every trace of bark and cambium in a 30cm wide band around the stem at a height of about 50 cm) or strip-barking (stripping off all the bark from about waist height to below the surface of the soil). A highly efficient method is to lasso the plant and uproot using a winch, car tow bar or a tractor. Advantages include: little training/ supervision needed; simple tools required and only target species treated; with care, (e.g. close holes, press down loose soil and spread grass/leaf litter over the exposed area) the environment is unharmed; ideal for gardens. Disadvantages include: physically demanding; slow and costly for large areas or dense infestations where it is seldom completely successful and requires repeated follow-up operations; any soil disturbance can promote germination of undesirable weed seeds and can lead to soil erosion on slopes; in dense infestations indigenous/desirable species are often mistakenly destroyed. The plants are killed or suppressed through the action of chemicals (called herbicides), which are applied directly to the target plant or to the soil close to it. Herbicides are either selective (more toxic to some plants than others e.g. a selective broadleaf herbicide will kill broadleaf plants but not grasses), or non-selective (usually toxic to all plants). They are also either contact herbicides which cause localised injury to the leaves where they come in contact with the plant (usually used to kill small annual plants) or translocated (systemic) herbicides which move in the plant to where growth is taking place (usually used on invader plants because most are perennials). Herbicides can be used successfully in most situations, but the need for proper precautions can be limiting. Herbicides registered for use on a problem plant (name of plant on label on container) should always be used, but there are many plants with no registered herbicides. Try Garlon 4 at an appropriate dose on a few of the plants in the first instance, but the agro-chemical company will not guarantee the result. The wetter Actipron should be added to Garlon 4 when it is sprayed on leaves and stems. Similarly, the herbicides containing glyphosate, e.g. Roundup, Mamba, Clearout, Tumbleweed “will control most annual and perennial weeds in non-crop areas” and can be tried on the softer/less woody plants mentioned in this booklet. If you find an effective treatment for a new weed tell WESSA Alien Busters so they can pass the information on to the company concerned. Advantages include: sometimes the only effective method; used correctly, can be the most cost-effective; quicker than mechanical control; if used according to label recommendations, herbicides pose little or no threat to the environment! Disadvantages include: specialised equipment and training of operators is essential; plants must be “healthy” and weather conditions suitable; aerial, foliar and soil application can all affect surrounding plants; misuse of herbicides is possible and environmental contamination, damage to desirable plants etc could result; mechanical preparation of the plants may be necessary; herbicides are expensive. (Please use common sense and follow instructions on the labels!) There are six ways of applying a herbicide: 1) Foliar application - to the leaves and stems. 2) Basal Stem application - to stems of standing trees. 3a)
Partial
Frill/ 4) Stump application - to the cut surface of freshly cut stumps. 5) Stalk Immersion - the correctly mixed herbicide and water, in a suitable container, is attached to the cut stem or stems. 6) Soil application - to the soil around the plant so the herbicide is absorbed through the roots - a very specialised method only recommended for use by “experts”. 1) Foliar Application - to the leaves and stems: The correctly mixed herbicide is sprayed onto the rapidly growing leaves and stems (foliage) of the target plant, usually to the “point of run-off” i.e. when the herbicide mix is about to run off the leaves. Some herbicides require the use of a “sticking agent” or “wetter” to ensure efficient results. Ideally, enough foliage must be present so that the plant “catches” sufficient herbicide to kill it. When spraying plants previously cut down, regrowth or coppice should generally have reached a height of between 50 and 100cm for effective control, but labels may specify requirements. Advantages include: an easy and versatile technique; can be used on small or large areas, where conditions are rough or where there are small obstacles; ideal for follow-up work e.g. use “spot-spraying”. Disadvantages include: plants often require preparation making it a two-part operation; application rates are unlikely to be accurate as they are influenced by terrain, plant density, the weather (e.g. drift) and the operator (e.g. fatigue); environmental conditions (e.g. rain) and the state of the plant (e.g. stressed due to heat, drought etc, dusty, eaten by insects etc) affect herbicide efficacy; large amounts of clean water must be transported to the site; frequent refilling of apparatus slows rate of work (about 2 man-days per hectare!) Back to ways of applying herbicides 2) Basal Stem - to the stems of standing trees: For plants with thin bark or stems up to 20cm diameter. Apply mixture of herbicide and Diesel to all bark with a paintbrush or sprayer from ground level to at least 0,25m. Spraying uses 3x more mixture but is less tiring and faster than painting. All bark pieces left in crevices after strip barking should be killed by thorough spraying/painting. In multi-stemmed plants, each stem must be treated separately. Back to ways of applying herbicides 3a) Partial/Total Frill - to holes or cuts (frills) made in the stem: Rings of evenly spaced downward cuts are made in the bark with a hand axe at a convenient height (the lower the better). Herbicide mixed with water is applied to each cut with a suitable applicator, e.g. syringe or hand-held sprayer. Large cuts (5cm long) hold about 1ml initially if cut is level. Care is needed, so work is slow. Back to ways of applying herbicides Only for Cactus family at present. Make holes in the lower part of the stem with a pole (e.g. broomstick handle) with a spike on the end. About 2ml of water-soluble herbicide solution is put in each hole. (Approximately 4 holes for a <2m plant with a maximum of 12 holes for large plants). Advantages
of the 3 types of stem application include: a simple, target specific method which requires little preparation or
training; ideal for tall trees that can remain standing (e.g. in a conservation
area. Remember that a dried-out
tree is far more difficult to fell than a living one, so ensure correct initial
planning decisions are made); not dependent on weather conditions or the status
of the plant as the diesel is water repellant and work can continue in light
rain, and although possibly less effective, work can be continued when plant is
stressed e.g. in winter. Back to ways of applying herbicides Cut the plant as close to the ground as is practicable. Use a tool that will give a clean cut on the size of stem. Use a saw for diameters greater than 50mm. The ideal stump is short, with a level cut-surface, none of the bark torn away from the wood, and no exposed roots. Herbicide should be applied as soon as possible, within 12 hours. There are two types of stump application: cut-stump and total-stump. For cut-stump, mix herbicide with diesel or water (see label) and apply to the outer ring of the freshly cut surface, close to the bark. For small stumps with diameters less than 50mm, treat all cut surfaces. For total-stump, mix herbicide with diesel and apply to the cut surface, the sides of the stump and any exposed roots. Advantages include: the most effective method which should be considered whenever practical, especially in sensitive areas, because herbicide is placed “into” the target plant; only a small amount of herbicide is used per plant; a one-pass operation, there is usually no need to return to the same plants. Disadvantages include; clear felling/cutting down is tiring work; progress is slow because falling branches need to be avoided and cleared to one side; removal and disposal of large amounts of wood may be difficult; the probable use of diesel as a carrier for the herbicide makes it expensive; removal of the canopy will stimulate many seeds to germinate; some herbicides are long-acting (or have residual action - see pg 8) and may be absorbed by non-target species planted later. Back to ways of applying herbicides No herbicides registered for this method but worth trying on all climbing species where vegetative recolonisation is possible such as Madeira Vine and Barbados Gooseberry. Also Mauritius Thorn and Balloon Vine. The main stem or stems of the plant or creeper are cut at a convenient height. The roots are dug out or treated with herbicide. ®Garlon 4 in water in a plastic bottle is pushed over the base of the cut stems and tied in place with packers tape. A bucket can be used for large stems but would need to be checked and possibly refilled after a few days. Tie a danger tape around the bottle or bucket. Advantages
include:
Very labour- and cost-effective; the herbicide is sucked up and transported
throughout the entire plant, reaching stems and seeds high up in the canopy of
trees, killing the stems and leaves and possibly affecting the germination of
the seeds. Back to ways of applying herbicides 6) Soil Applied Herbicides (NOT recommended for general use): A very specialised method, particularly on sandy soils. There are a few herbicides that are applied to the soil at the base of the plant, but their use is restricted to where there is no chance of desirable trees nearby being affected e.g. reducing thorn bush encroachment. They are unlikely to be suitable for use in urban open spaces. Leave this method to the expert! Advantages
include: herbicide
is easily and rapidly applied and is broadly selective. Back to ways of applying herbicides Uses natural agents, particularly insects, mites and fungi or bacteria, to damage or suppress a target weed. Some are chosen for their ability to reduce existing infestations by attacking the leaves and stems of the weed, but will not be used if the target plant is useful at times (called a “conflict of interest” situation). Others do not kill plants or reduce the infestation, but prevent further spread of the weed by attacking the flower buds, flowers and seeds. These agents are used where a “conflict of interest” arises because the plant is also useful e.g. wattles are a timber crop and a weed. Successful bio-control is the ultimate in weed control as it is self-perpetuating, highly selective and has no undesirable effects on the environment, but for various reasons, complete control by biological means alone is seldom achieved. Potential bio-control agents are carefully selected and studied for their effects on target weeds in their native lands. The most suitable are imported to South African quarantine laboratories for culturing and screening for host specificity (meaning that the insect will die rather than feed on a plant other than the specific weed targeted). These tests are very stringent and can take up to 5 years to complete satisfactorily. Permission to release the organism will only be granted by the Department of Agriculture if the tests are conclusive. Once released, the Biocontrol agent is carefully monitored and its effectiveness assessed. For a number of reasons success rates vary for survival of released agents and for their ability to control the target plants away from their native land. Insect agents have been most successful on plants growing in special situations e.g. in water - Kariba Weed, Water Lettuce; dry climates - Prickly Pear, Jointed Cactus. Agents are being tested by the Plant Protection Research Institute for many of our alien invader plants. GETTING YOUR ALIEN-BUSTER TEAM STARTEDAlien invasive plants are determined to stay! They will always be a nuisance in the landscape and will continue to threaten our environment. It is up to every one of us to help control this scourge before the battle is totally lost! 1) Start by assessing the extent of the problem. The key questions are: where control should start and how to ensure all alien plants are killed? To achieve success, an integrated, on-going control programme must be carefully planned, because very few, if any, control operations succeed the first time. 2) Start by removing weeds in the least affected areas and work towards the heavier weed infestations. By doing this, you can rapidly safeguard relatively large areas of indigenous vegetation. Next identify areas where vigorous indigenous bush meets weedy areas and carefully work outwards from “Goodies” to “Baddies”. Always start at the highest point and work downwards. 3) Remove weeds carefully and try to cover exposed soil with cut vegetation or leaf litter that is free of weed seeds and that will not regrow if in contact with the soil. Press any loosened soil down lightly taking care not to damage native plants, and mulch with plant material where possible. This will help stop exotic weeds from filling the gaps left by weeding. 4) Try to prevent weeds from producing seeds or fruit by cutting back before they flower wherever possible. Carry seeds, fruit, bulbs, tubers, stems that root easily etc away from the area in plastic bags and dispose of them safely. With a plant like Pereskia, it is advisable to burn the pieces “on site” if at all possible. 5) Often, the most time- and cost-effective way of dealing with heavy infestations is to arrange for the correct use of herbicides. Ideally, use a foliar spray carefully applied to target plants thus ensuring minimum soil disturbance and so reducing the chance of invader seeds germinating in the “seed bed” created by “weeding”. Slash the plant down and return in a few months to foliar spray the regrowth. Paint/spray the cut stumps of the larger and more difficult invasives. Paint the lower stem without “disturbing” the plant of really difficult to kill species like leuceana. 6) Follow-ups are absolutely essential! Monitor cleared areas on a regular basis until the supply of viable invasive seeds is exhausted and indigenous plants are again re-establishing themselves. You might need to reintroduce and replant certain species to restore a well-balanced and healthy ecology that is once again able to support the host of creatures dependent on it to feed, breed, nest and rest! This page was last edited on 22 April, 2006 |
