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Herbicide resistant biotypes
Mulches
Urban Horticulture (1)
Echium Pininana
Nursery Stock
Eucalyptus Species
Weed Control
Use of Herbicides
Role of Herbicides
Juania australis
Cold Hardiness
Plant Hardiness
Climatic Zone Plants

Re-evaluating the role of herbicides in contemporary urban horticulture

International Symposium on Urban Tree Health, ISHS Acta Horticulturae, Paris France 1 September 1999, © Muriel Robinson 2008

Keywords: Environmental protection, land management, simazine, glyphosate.

Abstract

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.

In Britain and Ireland , land managers are increasingly caught in the dilemma of having to cut costs at a time when public opinion against the use of pesticides is increasing. Some managers no longer use herbicides while others remain dependent on them. There is a general trend away from the use of soil-acting, residual herbicides towards rapidly biodegradable translocated types. There is also increasing interest in the use of non chemical methods of control, such as mulches, and in plant associations that need little or no herbicide use. At the same time, more detailed information on the behaviour of herbicides in soil, air and water is resulting in safer practices and lower residues in the environment. Accordingly as financial resources available to managers decline further, herbicides are likely to continue to play a valuable role as a component in integrated vegetation management although they cannot be regarded as a general panacea.

1.   Introduction

Although common salt was recommended as a herbicide in Germany in 1854 (Timmons, 1970), the major revolution in herbicides and weed control practice began with the patenting of the selective auxin type herbicides MCPA and 2,4-D in 1945. In the past 50 years much has been learned about the use of herbicides and about their strengths and weaknesses. After 50 years, it is an appropriate time to re-evaluate their role in urban horticulture.

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 Scandinavia but many other countries take a less severe attitude towards their use. This paper deals mainly with the situation in Britain and Ireland .

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 Britain and Ireland is still widespread. A large number of approved commercial products are available for amenity use, comprising about 17 active ingredients (Clay and Stephens, 1992).

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

3.1   Potential environmental damage

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 resistant biotypes

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 UK the use of simazine for total weed control on non-crop land, such as railways and industrial areas, was revoked in 1993. A ‘Good farming practice programme’ was introduced to reduce substantially residues in ground and surface water and to retain the uses with highest benefits and minimal risk to the environment. Recommendations include the avoidance of application in cold climates and in the autumn when degradation is slow (Seiler 1996). As with the closely related herbicide atrazine, these restriction have resulted in falling residue levels. Detected concentrations of simazine in surface water in the UK show that concentrations reduced considerably from September 1993 which coincided with the revocation of uses on non-cropped land. Concentrations in 1994 and 1995 were generally below 0.1mg per litre (Carter et al, 1996).

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 Britain on their use in amenity horticulture resulted in a marked switch to non-residual herbicides, particularly glyphosate. This herbicide is rapidly biodegraded in soil to harmless compounds and is unlikely to contaminate ground water (Grossbard et al, 1985). Some woody ornamental species show a degree of tolerance of glyphosate at certain times and more testing of this and other herbicides will benefit the landscape industry.

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 use

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 Scandinavian countries, Germany and the Netherlands but not to any extent in Britain and Ireland . Hitchmough (1994) suggests that it may be the very strong amateur gardening ethos in Britain that is responsible for the demand for ‘tidiness’ in all contrived greenspace vegetation. In Ireland, more complaints are received by Fingal (Co. Dublin) Council from the public about ‘untidiness’ in plantings (i.e. rank weed growth) than about the use of herbicides (Lynch, 1997).

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 grassland

In Britain and Ireland , in particular, great importance has been placed in the past on grass swards that are uniformly green throughout most of the year. Specialised sports turf must behave in a predictable way and managers of such facilities work hard to achieve weed-free, level areas composed only of grass. Large sections of the public still perceive that this is also the ideal condition for public and domestic grass swards even though this system of management requires large inputs of herbicide, fertiliser and mechanical energy. The strong pressure to maintain ‘good’ levels of grass maintenance in Britain and Ireland contrasts with the positive lobby in parts of Europe for informal vegetation.

There are signs of change in Britain in this area partly because of the need for more environmentally friendly landscapes and the rapid loss of semi-natural, species-rich grassland as a result of agricultural intensification. There is growing interest in the creation of new grassland that would have some of the desirable wildlife and amenity qualities of the grassland that has been lost. This trend has been confirmed in a survey conducted in England and Wales by Marder and Kendle (1994). Sixty two of 64 local authorities who responded indicated that they had sites where they intended to encourage wild flowers. Sixty of the authorities said that they had policies to continue the development of wild flower grassland - a marked change in landscape work relative to 10 years ago when wild flowers were seldom considered in Britain .

4.5   Non chemical methods of control

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.

5.   Conclusions

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.

References

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