WILDLIFE MANAGEMENT - ALIEN PLANT INVADERS AND THEIR CONTROL

Alien plant invaders and their control
D N Johnson
Alien plants are widespread in KwaZulu-Natal, both in cultivation and the wild. The majority do not pose a threat but a few, by virtue of their aggressive qualities, have the capacity to invade natural habitats and overwhelm some,  or even all of the indigenous vegetation. Aliens usually have much greater potential for invasion than indigenous plants because they have not had time to settle into equilibrium in their new community. They may find growing conditions superior to those under which they evolved in their native land or there may be less pressure from herbivores or competitors.

These invaders come in many shapes and sizes. They may be trees, shrubs, small herbaceous plants or waterweeds but they have in common the ability to spread and reproduce rapidly and to resist all but the most determined control attempts. Alien invaders thus constitute one of the greatest single threats to conservation in KwaZulu-Natal.

Some of the invaders arrived by accident but the majority of introductions were deliberate, for reasons, which appeared good at the time. Part of the problem is that there is no set formula for recognizing potential invaders. Recognition occurs after enroachment has started. The foci of many infestations have been intentional plantings in gardens and ponds and more than half of KwaZulu-Natal’s problem-invaders were introduced as ornamentals. Some of these species are still being sold by nurseries and planted, despite legislation to the contrary. It is everyone’s duty to spread awareness of the dangers posed by invasive plants.

The important invaders
Chromolaena (Chromolaena odorata) is a spindly shrub native to South America. It is firmly established at the coast and in the warmer parts of KwaZulu Natal. Chromolaena invades disturbed ground and road edges where dense stands form and spread rapidly, excluding all other vegetation.

Lantana (Lantana camara) is a prickly creeper native to America. It is widespread in KwaZulu-Natal excepting the cold areas. Lantana invades forests edges and can smother large trees. It is poisonous and fatalities to children and cattle have occurred.

Pereskia (Pereskia aculeata) is a very spiny woody climber from Mexico. It can over-run the tallest forest and is especially dangerous because the smallest fragment of a plant, including a single leaf, is capable of rooting.

American bramble (Rubus cuneifolius) is a prickly scrambler, which appears to be a hybrid between an introduced, and a local species. It is widespread in the mist belt and highland areas where it invades open veld and forest. It forms dense stands, which exclude any form of land use.

Sesbania (Sesbania punicea) is a shrub from South America that is now spreading rapidly through KwaZulu-Natal where it thrives on riverbanks and on waste ground. Sesbania forms dense stands, excluding natural vegetation. The seeds are poisonous to both domestic animals and wildlife.

Syringa (Melia azedarach) comes from tropical Asia. It is fast growing and, with the assistance of fruit eating birds, spreads rapidly into almost any habitat. It can dominate a woody community particularly alongside rivers.

Bugweed (Solanum mauritianum) is a weedy tree native to tropical Asia. It is widespread in KwaZulu-Natal particularly in wetter parts of the Midlands where it colonises disturbed ground. In doing so it prevents natural veld regeneration. Bugweed also harbours the fruit fly, which is an important orchard pest.

Prickly pear (Opuntia ficus-indica) is native to Mexico. It readily escapes into dry rocky areas and is firmly established in the bushveld and on krantzes. Eventually it crowds out all other vegetation. Any broken fragment is capable of regeneration.

Water hyacinth (Eichhornia crassipes) is an attractive waterweed from tropical America. It reproduces at tremendous speed and rapidly clogs waterways in warm areas, particularly if the water is slow moving and enriched with dissolved fertilizer. It is easily spread to new areas by boats and by illegal dumping.

Kariba weed (Salvinia molesta) is a sterile hybrid fern originating from Brazil. Kariba weed becomes a pest when it clogs waterways and is easily spread in much the same way as water hyacinth. There are many other aliens, which are undesirable at least in certain areas. These include Mauritius thorn (Caesalpinia decapetala), St John’s Wort (Hypericum perforatum), Parrot’s Feather (Myriophyllum spp.) Inkberry (Cestrum laevigatum), peanut cassia (Cassia didymobotrya) Jacaranda (Jacaranda mimosifolia), Oleander (Nerium oleander) and Caster Oil (Ricinus communis). Some commercially valuable species can be invasive notably guava (Psidium guajava), Black wattle (Acacia mearnsii), Blackwood (Acacia melanoxylon) and Gum (Eucalyptus grandis).

Control methods
The aliens described here, and indeed many others, are invasive because they are difficult to control. Successful control measures have been developed but in many cases only after detailed research and good results can only be expected if attention is paid to detail. For example, some methods are specific to a particular weed or growth form.

Hand pulling
Hand pulling is effective where infestations are small and the invaders shallow rooted. Chromolaena is easily uprooted and so are seedlings of bugweed, wattle, syringa, and sesbania, particularly if the soil is damp. A tractor and chain may be the most suitable way of dealing with saplings.

Cuttings
Nearly all invaders will coppice if cut once, but repeated cutting during the growing season causes depletion of root reserves eventually resulting in death. If terrain permits the second and subsequent cuts can be done with a mower. Such treatment favours grass which will then be able to outcompete the weeds and assist in their elimination. Both chromolaena and bramble can be removed by frequent cutting. Cutting can kill Pereskia, but it is essential that the roots are grubbed out too and that they and all cut pieces be burnt on the spot.

Ringbarking
Ringbarking of large trees can be successful but it is slow and is only preferred to felling and other treatments when the latter is impracticable. Every trace of cambium - the growing region between the bark and the hard wood - must be removed from the ring which must be at least 0.5m wide. Note that the subsequent felling of a dried out dead tree is much more difficult that felling the living tree in the first place.

Felling
Felling by itself will not eliminate an invasive tree. Coppice growth usually results and this can be more difficult to control than the original problem. Coppice can be prevented by stripping all the bark off the remaining stump, to below ground level. Alternatively, felling can be carried out in autumn, the timber stacked over the stumps to dry over winter and the whole lot burnt in spring.

Chemical control
Many of the herbicides used in alien control are applied as leaf sprays, which only kills the plant after being translocated to the roots. For this reason, they are not fast acting and indeed it is essential not to kill the foliage before the herbicide has been transported into the plant. Accordingly spray solutions should always be made up as instructed by the manufacturer: strong solutions will probably be less effective, and certainly more expensive. Efficiency of translocation to the roots is further improved if the herbicide is applied to a large leaf area on actively growing plants. Avoid spraying new, unexpanded growth.

In choosing a herbicide there are several points to consider. Firstly, the herbicide should be one registered for use against the weed to be eliminated. Registered herbicides have been rigorously tested and the optimum mode of use determined.
Secondly, it is important to note the level of persistence displayed by the herbicide after application. Residual herbicides preclude immediate regrowth or replanting.
Thirdly, the degree of selectivity of action of the herbicide may be critical. Some kill all plants; others have no effect on non-target species, particularly grasses.
Fourthly, the effect of the herbicide upon animal life must be considered. Since alien weed elimination is usually part of a conservation exercise it can be counter productive if all animal life is killed as well. Some herbicides are dangerous to particular groups of animals. For example, fish are generally more sensitive than mammals and some herbicides should never be used near water.

The state of the weather can greatly influence the success of chemical control. Rain, immediately after a spraying operation, can more or less nullify the treatment. Similarly, heavy dew can dilute the spray and cause it to drip off foliage and reduce its effectiveness. Dust is another factor to consider. Some herbicides deactivate upon contact with soil and have little effect upon dusty foliage. Such herbicides are best applied after rain, as soon as the foliage is dry. Avoid spraying in windy weather since spray drift onto non-target plants may occur.

Some herbicides are effective when painted onto cut tree stumps as an alternative to stripping the bark. It is however, essential that painting be done immediately after felling and that the whole of the cambium is treated.

Which herbicides are suitable for use in conservation areas?

Roundup
Roundup is registered for use against Bugweed, bramble, Lantana, prickly pear, and water hyacinth. It kills all green plants, grasses included. It is usually applied as a leaf spray at a concentration of 1-2% in clean water. It is not poisonous to animal life and deactivates upon contact with soil. Land reclamation can thus begin as soon as the target weeds have died. Roundup can also be used as a 2-4% solution to spay onto freshly cut stumps.

Garlon
Garlon is registered for use against Bugweed and chromolaena and is especially effective against bramble and chromolaena. It kills only broad-leaved plants and is harmless to grasses and animal life. It does not degrade immediately after application but its residual effects are short lived. Garlon can be used as a leaf spray or as paint for cut stumps.

Clarosan
Clarosan is registered for use against Kariba weed and water hyacinth. It is most effective when used before heavy infestations have accumulated. Its direct effects upon animal life are negligible. However, a large quantity of decaying vegetation in the water will cause deoxygenation and fish deaths can result. Where a dense infestation is present it may be advisable to remove mechanically as much as possible of the water weed prior to spraying, to avoid this side effect. Clarosan has no effect upon reeds and sedges, but will kill any broad-leaved plants at the waterside.
Further details on the use of herbicides should be obtained from the local herbicide representative or from the Weeds Research Unit at Cedara.

Control strategy

Whichever method or combination of methods is chosen there are three general principles to follow: The first is that light infestations are easier to deal with than heavy infestations. They can also get worse if ignored; heavy infestations may not. So tackle the easiest problem first.
The second is that infestation generally proceeds down hill, particularly when considering riverine vegetation. It is pointless clearing an area when a reservoir of re-infestation exists uphill or upstream. Start at the highest point and work downwards.

The third is that no control operation succeeds the first time. One or more follow-ups are essential. So when attempting to clear a large area piece-meal it is better to make the second effort a follow-up operation on the first area cleared, than to start on a second area. Cleared areas should be inspected at intervals to ensure that alien elimination is complete.

KEY POINTS
Invading alien plants are a major conservation threat.
Control measures usually require a carefully planned and implemented program.
Obtain advice on chemical control from the Weeds Research Unit or your local herbicide representative.
Follow up operations are essential for complete success.
Many invaders are still being planted illegally.
Any alien could become an invader: report new invasions immediately.