Re-evaluating the role of herbicides in contemporary urban horticulture
International Symposium on Urban Tree Health, ISHS Acta Horticulturae, Paris France 1 September 1999, © Karen Foley 2016
Keywords: Environmental protection, land management, simazine, glyphosate. [*]
Herbicides provide an inexpensive way of controlling weeds in woody vegetation and result in increased plant vigour and survival rates. They are also a potential threat to the environment and, in many areas, the development of resistant weed biotypes has reduced their value.
Although common salt was recommended as a herbicide in
The question is often asked: ‘Why use herbicides in landscape management?’. If adequate financial resources were available along with suitable alternative methods of suppressing weeds, chemical control would be unnecessary. However, there is a trend for resources to decline and, despite active research on mulches and other alternative control measures, these do not yet provide an effective, economic substitute for herbicides in many situations.
The efficacy and safety of herbicides are greatly influenced by soil and
climate These vary greatly between countries as does the legislation controlling
their use. All herbicides are banned in urban situations in
On the positive side, herbicides have given land managers a cheap and
effective means of weed control during a period when sections of the public
continue to demand high standards of maintenance in public spaces. When used
properly, herbicides have resulted in healthier trees and shrubs and have
greatly reduced maintenance costs. Compared with fruit growers, amenity land
managers were initially slow to appreciate the value and relative low cost of
herbicides, but many turned to them enthusiastically in the 1980s and early
1990s and their use in
On the negative side the public has become increasing concerned about possible adverse effects of pesticides on human health, wildlife and the environment. In addition, the efficacy of some of important herbicides has declined as a result of the build-up of resistant biotypes.
2. Advantages of herbicides
2.1 Cheap and effective weed control
There is much evidence in the literature that herbicides are generally more effective and less expensive than other methods of weed control (e.g. Robinson, 1962). The advantages of herbicides are greatest in temperate areas with cool, moist summers and mild winters which encourage year-round weed growth. Moist soil conditions and high humidity also reduce the value of cultivation as a means of controlling weeds and speed up the microbial degradation of soil-acting herbicides in the surface soil layers.
2.2 Improved tree growth
Much research has shown that trees and other ornamental plants respond well to the use of herbicides (Davison, 1983). Studies on the root system of young trees help to explain the increased growth and vigour where herbicides are used. Roots are primarily located close to the soil surface and, contrary to popular opinion, absorbing roots essential for water and mineral uptake, are predominantly found well beyond the dripline (Schnelle et al,1989). Gilman (1988) states that 35% of the trees’ root system is often located greater than twice the distance from the trunk to the branch dripline. While root spread has been underestimated, root depth has been greatly exaggerated. Shallow root systems are normal for trees in most temperate situations. Normally little or no root growth extends below 1.2 metres (Watson, 1985).
Surface root growth is normal in free draining soils but is particularly noticeable in urban soils that are waterlogged or compacted. Such soils are deficient in oxygen, as water normally holds less than 1/10,000 as much oxygen as air (Perry 1982). Tree growth in herbicide-treated plots is invariably better than that in cultivated, weedy or closely mown plots and sometimes better than that in mulched plots (Davies, 1987).
From a purely technical viewpoint, the use of herbicides is not merely a substitute for hoeing or cultivation, it is a much superior system of soil management and gives enhanced tree growth and plant survival for a number of reasons. As weeds are killed by soil-acting herbicides at the germinating stage, their adverse effects are eliminated at an early stage (Davies, 1987). These harmful effects include:- competition for water, nutrients and light; interference of weeds with tree growth by the release of toxins; modification of soil and air temperatures and the harbouring of pests. The major benefits of herbicides used as recommended is that, with the exception of some mulches, they have less harmful effects on the tree root system than other methods of soil management. However, where management skills are low and where non-selective herbicides, such as imazapyr, are applied over the root zone of trees, severe damage can result (Ivens, 1996).
3. Disadvantages of herbicides
Herbicides vary greatly in chemical composition and in the degree of threat they pose to the environment. Some like bromacil are highly persistent (Ivens, 1996) whereas others like glyphosate have little or no environmental effect (Grossbard et al, 1985).
During the period 1960s to early 1990s, simazine was widely used in urban areas for weed control both on hard surfaces and in amenity plantings. This is a broad spectrum herbicide with low mammalian toxicity but in the 1980s it breached the EU limit (0.1Tg/l) for residues in water.
When simazine is applied to top soil as a selective herbicide at low doses, it is largely adsorbed and degraded in the surface soil layers. If heavy rain falls soon after application some may be leached into deeper soil layers. Degradation in these layers and in ground water is slower. However, lysimeter and other studies show that simazine has very little potential to leach under conditions of use recommended at present (Novartis Crop Protection, 1997).
It is widely recognised that the main reason accounting for residues of simazine and other triazines in ground and surface water was the widespread use of these herbicides at high doses on hard surfaces. Here there is little or no adsorption on inert, inorganic material and run-off into drains and natural watercourses is rapid.
Anxiety about chemical residues in the environment has increased greatly in the last decade. These fears and concern about possible litigation have led many land managers to reappraise their weed control strategies. Change has also been forced on them by the decrease in the number of approved herbicides as a result of the high cost of registration. In addition, approval has been withdrawn from more toxic and persistent herbicides.
3.2 Build-up of
Apart from their effect on the environment, another major problem with herbicides has been the build-up of herbicide-resistant biotypes where the same herbicide has been used repeatedly for a number of years. This problem was not clearly foreseen at the start of the herbicide revolution but, since the early 1980s, triazine resistance has developed in most countries where these herbicides have been used. The usefulness of a number of other herbicides, including paraquat, dichlofopmethyl and sulfonylurea types has been affected by the development of resistant biotypes. Methods of dealing with this problem include prevention of seed shedding, crop rotation, herbicide rotation, control of weed escapes and tillage practices.
Crop rotation is not relevant in an amenity situation where the ‘crops’ are usually perennial but other control measures may be appropriate in certain situations. If weeds are prevented from setting seed, resistant biotypes cannot develop. The possibility of preventing the build-up of resistant species by a policy of zero tolerance for weeds is being tested in a 1.5 hectare amenity area (Robinson, 1986). Since 1969 almost complete control of weeds has been obtained with a rotation of herbicide supplemented where necessary by hand weeding and some mulching. Although it was impossible to prevent all weeds from setting seed, the amount of seed shedding was reduced to a low level. So far resistant biotypes have not developed after repeated applications of simazine for 28 years.
The results suggest that where resistant biotypes have not yet developed, it should be possible to extend considerably the effective life of a herbicide. This could be achieved if land managers were made more aware of the threat of resistant biotypes and made greater efforts in intensively managed areas to prevent weeds from shedding seeds by the use of a rotation of herbicides supplemented by physical means such as mulching, hand hoeing and hand weeding.
4. Current trends
While the situation concerning herbicides is in a state of flux at present, some trends are evident More persistent herbicides are being replaced to a large extent by non residual chemicals that pose less of a threat to the environment. In addition action has and is being taken to minimise the residual effects of the more persistent chemicals such as simazine. The need for herbicides is being reduced by the use of alternative control measures and by a partial change in landscape type in favour of those that require little or no herbicide use.
4.1 Reduction of
simazine residues in water
A range of measures have been taken to reduce levels of simazine in ground
and surface water. In the
Since the application of simazine at low doses on woody crops is unlikely to have been responsible for the contamination of ground water, its use on a range of crops and amenity plantings is still permissible in many countries. It is likely that simazine will continue to be used at low doses on woody plants in those countries where it is legal to do so and where resistant biotypes are not a problem. In other cases, strategies based on other chemical and non-chemical methods will develop.
4.2 Change to more
environmentally friendly herbicides
The discovery of triazine residues in water and the ban in
Although glyphosate is effective in amenity plantings, any attempt to control all weeds solely with this herbicide is likely to fail in the long term because of the ability of weeds to adapt rapidly to any one method of control used against them. Apart from the development of glyphosate-resistant biotypes, it is also certain that, if a programme based solely on spring and autumn glyphosate application is used repeatedly for several years, weeds that emerge after the spring application and seed early will become prevalent.
Other herbicides have also increased in importance, such as glufosinate-ammonium and diuron. Like glyphosate, glufosinate-ammonium breaks down rapidly in the soil and has negligible environmental impact. Diuron has comparable soil mobility characteristics to simazine. Residues have been detected in ground water (DWI, 1991) and so it would not be suitable for use at high doses for total weed control.
Experience in many countries shows that it is not possible to rely solely on one method of control. The only way forward is to adopt an integrated approach where a range of chemical, physical and other methods are used as appropriate. Within this approach, it will be necessary to rotate approved herbicides and to supplement these with non-chemical methods.
4.3 Minimising herbicide
Land managers can do much to reduce the use of herbicides by formulating
clear objectives about the type of landscape needed (intensively managed
landscape or area of rough land) and by deciding clearly in advance about
appropriate weed management standards. While weed control is necessary to reduce
inter-plant competition, it may be possible to persuade the public to accept a
somewhat lower standard of weed control. This has already occurred in
Unless substantially increased funding is available for cultural means of weed control, it will not be possible to give up the use of herbicides and maintain highly managed landscape plantings as they were previously. In these circumstances, Hitchmough (1994) recommends a gradual move towards plant communities or plant species and cultivars that either suppress weeds more effectively or towards those that can ‘carry’ weeds without great loss of aesthetic appearance. Where levels of resources for weed control are low and no herbicides are to be used, then vegetation types such as woodland, informal woodland edge type shrub plantings, hay meadow type plantings and parkland involving trees and mown grass are attractive options. Such an approach would enable Local Authorities to continue to maintain small areas of greenspace to a high horticultural standard.
4.4 Species rich
There are signs of change in
4.5 Non chemical methods
Faced with the twin problems of declining budgets and increasing public opinion against the use of herbicides, many other possible methods of weed control have been considered (Robinson,1994). These include thermal techniques, biological control, endemic herbicides, ground cover plants and mulches. All have advantages and disadvantages and none has found widespread application in amenity horticulture with the exception of mulches. Organic mulches of bark and wood chips are widely used particularly in shrub plantations. They can give good results but sometimes adverse effects can occur, especially on moisture retentive soils in wet seasons. The main disadvantage of organic mulches is their high cost although, in the future, as wood chippers become more common place, their cost could fall.
Despite growing opposition to the use of herbicides in the management of
urban greenspace, it seems likely that a large sector of the public will
continue to demand high standards of weed control and will be quick to complain
if standards slip. Herbicides with proven safe record in the environment will
continue to be used for vegetation control in a variety of situations including
nature reserve management (Marrs 1984).
The EU has begun a review of all herbicides and the results will have
significant consequences for the future of weed management strategies in urban
horticulture. Because of the significant financial pressures on land managers
and the beneficial effects of high standards of weed control on plant growth, it
seems likely that those herbicides that are approved by the EU will continue to
play a valuable role as a component in integrated vegetation management,
although they cannot be regarded as a general panacea.
[* Footnote. Since this article was written, the
herbicide, simazine, has been banned in Europe under Commission Decisions
2004/141/EC(3), 2004/248/EC(4), 2004/140/EC(5) and 2004/247/EC(6), taken within
the framework of Council Directive 91/414/EEC of 15 July 1991. This came into
effect on 26th April 2004.]
[* Footnote. Since this article was written, the herbicide, simazine, has been banned in Europe under Commission Decisions 2004/141/EC(3), 2004/248/EC(4), 2004/140/EC(5) and 2004/247/EC(6), taken within the framework of Council Directive 91/414/EEC of 15 July 1991. This came into effect on 26th April 2004.]
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