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 2026
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 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 weed control began with the patenting of selective auxin herbicides such as MCPA and 2,4-D in 1945. In the past 50 years much has been learned about herbicide use and its strengths and weaknesses. After half a century, it is appropriate 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 effective alternative methods of weed suppression, chemical control would be unnecessary. However, resources continue to decline and, despite active research into mulches and other alternatives, these do not yet provide an effective or economic substitute for herbicides in many situations.
The efficacy and safety of herbicides are greatly influenced by soil and climate, which vary widely between countries, as does legislation controlling their use. All herbicides are banned in urban situations in Scandinavia, but many other countries take a less severe approach. 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 at a time when the public continues to demand high standards of maintenance in public spaces. When used properly, herbicides have resulted in healthier trees and shrubs and reduced maintenance costs. Their use in Britain and Ireland remains widespread, with approximately 17 active ingredients approved for amenity use (Clay and Stephens, 1992).
On the negative side, public concern has increased regarding possible adverse effects on human health, wildlife and the environment. In addition, the efficacy of some important herbicides has declined due to the build-up of resistant weed biotypes.
2. Advantages of herbicides
2.1 Cheap and effective weed control
There is strong evidence that herbicides are generally more effective and less expensive than other weed control methods (Robinson, 1962). Their advantages are greatest in temperate regions with cool, moist summers and mild winters that encourage year-round weed growth.
2.2 Improved tree growth
Research has shown that trees and other ornamental plants respond well to herbicide use (Davison, 1983). Absorbing roots are primarily located near the soil surface and often extend well beyond the dripline (Schnelle et al., 1989; Gilman, 1988). Root depth, by contrast, has often been exaggerated, with little growth normally occurring below 1.2 metres (Watson, 1985).
Tree growth in herbicide-treated plots is consistently superior to that in cultivated, weedy or closely mown plots, and sometimes better than in mulched plots (Davies, 1987). Herbicides eliminate weed competition at an early stage and generally have fewer harmful effects on root systems than cultivation or hoeing.
3. Disadvantages of herbicides
3.1 Potential environmental damage
Herbicides vary widely in chemical composition and environmental impact. Some, such as bromacil, are highly persistent, while others, such as glyphosate, have minimal environmental effects (Grossbard et al., 1985).
During the 1960s to early 1990s, simazine was widely used in urban areas. Although it has low mammalian toxicity, it exceeded EU water residue limits during the 1980s, particularly when used at high doses on hard surfaces. [*]
3.2 Build-up of resistant biotypes
Repeated use of the same herbicide can lead to the development of resistant weed biotypes. Since the early 1980s, triazine resistance has become widespread. Prevention strategies include herbicide rotation, preventing seed shedding, and integrating physical control measures.
4. Current trends
Current trends show a move away from persistent herbicides towards rapidly degradable products, combined with increased use of non-chemical methods and integrated management approaches.
4.1 Reduction of simazine residues in water [*]
In the UK, the revocation of simazine for total weed control on non-crop land in 1993, combined with good practice programmes, resulted in significant reductions in water residues (Carter et al., 1996).
4.2 Environmentally friendly herbicides
Restrictions on triazines led to increased reliance on glyphosate, which degrades rapidly in soil and poses little risk to groundwater. However, exclusive reliance on any single herbicide is unsustainable.
4.3 Minimising herbicide use
Land managers can reduce herbicide use by defining clear landscape objectives and by encouraging public acceptance of lower weed control standards. This approach is more advanced in parts of continental Europe than in Britain and Ireland.
4.4 Species-rich grassland
There is growing interest in species-rich grassland and informal vegetation as a more sustainable alternative to intensively managed turf, particularly in Britain and Ireland.
4.5 Non-chemical control methods
Non-chemical methods such as mulches, ground covers and thermal techniques have been explored, but mulches remain the most widely adopted alternative despite their cost.
5. Conclusions
Despite growing opposition to herbicide use, public demand for tidy urban landscapes remains strong. Herbicides with proven environmental safety are therefore likely to remain part of integrated vegetation management strategies.
EU review processes will continue to shape future weed control practices. Approved herbicides will play a role, though they cannot be regarded as a universal solution.
References
- Carter, A.D. and Heather, A.I.J. (1996). Atrazine and simazine levels in surface water sources. Aspects of Applied Biology, 44, 165–170.
- Clay, D. and Stephens, D. (1992). Herbicide use in industrial and amenity areas. The Horticulturist, 1(2), 5–8.
- Davies, R.J. (1987). Trees and Weeds. Forestry Commission Handbook 2.
- Davison, J.G. (1983). Weed control in newly planted amenity trees.
- Grossbard, E. and Atkinson, D. (1985). The Herbicide Glyphosate. Butterworths.
- Timmons, F.L. (1970). A history of weed control. Weed Science, 18, 294–307.
- Watson, G. (1985). Tree size and root regeneration. Journal of Arboriculture, 11(2), 37–40.
[*] 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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