6 Manual methods of killing woody plants

6.1 Physical methods

Uprooting. The uprooting of seedlings is commonly used as it is so quick. Detached root fragments of some species can resprout (e.g. Cinchona succirubra), whereas some cannot (e.g. Pittosporum undulatum). Uprooted seedlings of some species survive if left in contact with the soil, but some do not. As with all methods of control, it is very important to carry out simple tests to see how each species in a particular area or habitat responds to control techniques. Sometimes control programmes have uprooted larger plants with picks or winches. In the Galapagos Islands picks were used to uproot Cinchona succirubra but this was abandoned in preference to herbicides once the techniques had been developed (Macdonald et al. 1988).

Cutting. In Zambia, arboricides were used to kill several woody species and the treatment of stumps was almost always more effective than the treatment of girdled trees (Piearce 1980). In the U.S.A., repeated cutting (at least once a year) of the invasive Lonicera maackii, without the application of herbicides, killed plants in the forest but not where they were growing in gaps (Luken & Mattimiro 1991).

Girdling. Wiant and Walker (1961) reported variable success after 2 to 3 years with complete girdles 25 mm wide and 45 mm deep, even when no callousing occurred. They suggested that girdling alone may not suffice for the diffuse-porous species if a rapid death is required. In the southern Cape forests Olea capensis and Canthium obovatum were killed as effectively by ring-barking as by the application of 3% 2,4,5-T in diesoline, the most effective herbicide tried (Geldenhuys 1982).

Bark stripping. Stripping all the bark off trees, either the basal portion or below cutting height, is effective in killing Maesopsis eminii in Tanzania (N. Geddes pers. comm. 1993) and Pittosporum undulatum in Jamaica. Mechanical. Sometimes alien trees have been removed mechanically. For example, in Australia two bulldozers dragging a chain from two different directions have been used to destroy monotypic woodland stands of Prosopis juliflora (Meadly 1962). Up to 40 ha can be cleared a day (Rentz 1993) but sucker regrowth needs to be controlled by herbicide. This method may be effective in some regions but it can not be recommended more widely, because of the risk of re-invasion of alien plants, as well as other environmental damage, particularly soil erosion.

6.2 Chemical methods

6.2.1 Herbicides.

We have not attempted a review of possible herbicides as many are available and the turnover of products is high. The following sources of information are useful, though none deal specifically with tropical trees:

Anon. 1987. Australian weed control handbook, 8th edn. Inkata Press, Melbourne.

Anon. 1989. Herbicide handbook of the Weed Science Society of America, 6th edn. Weed Science Society of America.

Cronk, Q.C.B. & Fuller, J.L. 1995. Invasive plants: the threat to natural ecosystems worldwide. Chapman & Hall, London.

Grossbard, E. & Atkinson, D. (Eds). 1984. The herbicide Glyphosate. Butterworths, London.

Labrasa et. al. 1984 Weed management for developing countries. FAO publication.

Vermeulen, J.B. & Grobler, H. 1987. A guide to the use of herbicides. Dept. of Agriculture and Water Supply, Rep. of South Africa, 10th Edition.

Willoughby, I. & Dewar, J. 1995. The use of herbicides in the forest, 4th edn. Field Book 8. HMSO, London.

If many species are being controlled in the same area it is quite likely that the optimum herbicide for each species will not be the same, so to avoid great complexity, a smaller number of herbicides may be desirable, accepting slightly lower mortality rates.

6.2.2 Method of application.

The following methods of applying herbicides have been used, some with limited effectiveness.

Application to cut stumps. This is often the most effective method of killing woody plants. In the Hawaii Volcanoes National Park, in 13 out of 15 species, application of herbicide to cut stumps has been the most effective method of applying herbicide and also minimises waste and damage to native plants.

Foliar application. Translocatable herbicides can be readily absorbed by foliage (though sometimes a surfactant, a chemical that increases the amount of herbicide absorbed, needs to be added if the leaves are thick and waxy). This method is only practical on smaller plants or on sprouts from cut stumps in heavily invaded areas.

Injection. This can be carried out with a specially designed tool such as a Hypohatchet, which is usually effective though the tool is quite expensive.

Drilled holes. Drilling 8-15 mm diameter holes through the bark and cambium of the trunk at 4-6 cm apart about the trunk, below the lowest living branch and 20-50 cm above ground level and filling with undiluted glyphosate was effective at killing many species in Australia. It is essential that holes are placed vertically below any main branches, otherwise they may not be killed. For large trees more than 40 cm DBH up to 20 holes are needed. They should be immediately filled with a suitable herbicide using a Velpar gun (Gillespie 1991). Both injection and drilling methods have the advantage of minimal leakage of herbicide into the environment but are time-consuming.

Branch filling. Some branches are cut and the herbicide applied through a tube attached to the stump of the branch. This was not very effective against Cinchona succirubra with a DBH > 10 cm in the Galapagos Islands (Macdonald et al. 1988).

Frill girdling. It is faster than drilling but it was not as effective in one study in Australia (Gillespie 1991). Girdling with the herbicide Tordon (2,4-D and picloram) was more effective at killing Pittosporum undulatum than cutting and the application of Tordon to the cut stump.

Bark painting. This was effective against small (DBH < 10 cm) C. succirubra trees (Macdonald et al. 1988), though presumably bark thickness and characteristics strongly influence the effectiveness of this method.

6.2.3 Dosage.

This will depend on many factors, but more need not necessarily mean better. Concentrated doses of herbicide, when applied to frilled girdles at the start of the growing season, can kill the developing cambium cells, blocking the translocation of the poison and therefore reducing its effectiveness (Geldenhuys 1982).

6.3 Cost effectiveness of manual control

Manual control can be expensive and is rarely able to achieve complete eradication of a species. This method is effective when the population size of the invader is small and the population has limited or no means of vegetative propagation. The costs of removing a species are highly variable. Some examples below give an idea of the resources needed.

· In the U.S.A. in the early 1980s the cost of removing Pueraria lobata was US$563 ha-1 whereas the removal of exotic mangrove at Kaloko-Honokohau National Historic Park in Hawaii cost US$123,500 ha-1 (Hester 1991).

· Control of Psidium cattlenianum, which was at the density of 500 plants ha-1 in Kipahulu Valley, Haleakala National Park, Hawaii, took 7.5 man days ha-1 if the area was accessible and level. Control started at the highest altitude of invasion as it may not have been practical to control denser infestations lower down and people carry seeds upslope (Anderson et al. 1992).

· In the Hawaii Volcanoes National Park as a whole, control of widespread species, as well as localised species, was estimated to require about 20 full time workers for initial control stages (Tunison & Misaki 1992). Control efforts have been highly effective on 26 of the 41 less widespread species. Nine of these have apparently been eradicated from the park and seven others have been reduced to seedling stages (Tunison & Zimmer 1992). Control of Psidium guajava, which was at the density of 3000 plants ha-1 in the Hawaii Volcanoes National Park, required 30 man days ha-1 (Anderson et al. 1992).

· On Santa Cruz Island in the Galapagos Islands control against Cinchona succirubra started in 1971 and has needed 6 men employed for 6 months of each year (Macdonald et al. 1988). Control has been concentrated in a 1000 hectare Intensive Control Area and has managed to prevent the C. succirubra population increasing within it, but seed input from plants outside is increasing.

· Control of alien plants in the Cape of Good Hope Nature Reserve, South Africa, was initiated in 1943 and has continued ever since. In recent years, 14 men have been employed full time with 32 men working on following up weeding, costing US$100,000 a year (excluding transport and equipment depreciation) (Macdonald et al. 1988).

The effectiveness of some methods of manually killing plants depends on seasonality, and this obviously becomes increasingly important away from the equator. Translocatable herbicides such as glyphosate may be completely ineffective during dormant times of the year.

6.4 Environment and worker impact of manual control

Herbicides can harm non-target plants and the wider environment. For example, after the injection of Tordon (2,4-D and picloram) into the rhizome of a herb, Hedychium gardneranum, in Puerto Rico, all native plants of three common species were either killed or showed signs of damage within a one metre radius (Santos et al. 1992). Herbicides differ in the extent to which they harm plants, are immobilised and persist in soil. It is particularly important not to damage the ability of native vegetation to re-colonise a site after the removal of all alien woody plants using herbicides. Glyphosate-based herbicides such as Roundup are often claimed to be relatively non-persistent and benign on the environment.

In the tropics safety equipment is often difficult to obtain, is uncomfortable to wear in tropical heat (Negreros-Castillo & Hall 1994) and it can be very difficult to insist that people wear full protective overalls, so the low danger to humans of glyphosate-based herbicides is another major factor in their favour.

The impact on recruitment of other species of weeds is mentioned above.