HERBICIDES

These are somewhat different to cutting and burning in that many kill or severely damage, as opposed to defoliate, adult herbaceous plants. This is potentially advantageous but increases the risk of damage to desired species. Herbicides are most useful for addressing weed-management problems that cannot be satisfactorily addressed by other means. In most cases, this means occasional, as opposed to routine, use. Clearly the use of herbicides in managing decorative vegetation is a contentious issue. As with burning, but more so, herbicides are typically seen as counter-intuitive to ‘caring for’ vegetation and especially ‘ecologically’ based vegetation. Herbicides are also commonly regarded to lie outside of sustainable practice, even though it is clear that lowtoxicity herbicides, such as glyphosate, are the most sustainable means of controlling large areas of unwanted vegetation and are used by England’s governmental nature conservation agency (‘English Nature’) for precisely this reason. It is entirely rational to shun herbicides that are highly toxic to humans and other animals, indeed all herbicide use in public areas is problematic and alternative techniques are preferable where this is possible. To the author, however, it seems rather confused in a highly technological society to reject extremely lowtoxicity herbicides simply on the grounds they are synthetic organic chemicals.

The most toxic herbicide by far is paraquat/diquat, sold to and widely used by amateur gardeners as ‘Weedol’. Owing to its extremely limited capacity to translocate through plants, the author has used this in experiments as a chemical defoliant in comparison to cutting and burning in managing American prairie plant communities. We are currently in the process of evaluating a low-toxicity alternative (Glufosinate Ammonium, ‘Challenge’) that would be much more acceptable in practice. When applying these contact herbicides, no attempt is made to avoid the foliage of desired species. Despite this, the only prairie species that we have observed damage from overspraying with paraquat/diquat in spring has been Rudbeckia fulgida var. deamii, although clearly it will potentially cause serious damage to evergreen species. It will eliminate all annual weeds, and will give effective control of creeping buttercup and creeping bentgrass (Agrostis stolonifera).

Herbaceous plant communities and species that are completely winter dormant, i. e. have no overwintering green buds or leaf rosettes, can be oversprayed with the non­selective herbicide glyphosate in winter. This has proved, for example, to be extremely effective in eliminating Agrostis stolonifera from sown swards of North American prairie grasses. Dunnett and Hitchmough are currently evaluating the effect of winter overspraying with glyphosate on a wide variety of winter-dormant herbaceous plant species.

Many of the most problematic colonists of forbrich meadow or prairie-like vegetations are perennial grasses, such as creeping bent, couch grass, Elymus repens and Yorkshire Fog, Holcus lanatus. All of these species can be selectively controlled without damaging the forbs by overspraying with grassspecific herbicides, such as sethoxydim

(Checkmate®) as previously discussed in the section ‘Establishment by sowing in situ

(Table 6.18).

Table 6.18. Weed-management techniques for specific herbaceous plant communities

Moist meadows

Mow very closely and remove cuttings once between late autumn and spring (depending on the phenology of the species present). Cut as hay and remove cut material between June and October (depending on the target species present). A second mowing in either autumn or spring may be desirable on highly fertile soils. With problem species not controlled by this regime, for example docks, cut out below ground or paint with appropriate translocated herbicide

Dry

meadows/steppe

Mow and remove cuttings in late winter to early spring. Timing depends on the species present. Winter colonisation by weedy annuals may be controlled by burning in early spring. Where colonisation by weedy grasses is a problem, the spot application of glyphosate or an overall application of a graminicide (avoiding Stipa and other desirable grasses) may be used between March and July. Cutting as hay in late summer may sometimes be useful on productive soils

Wet meadows

Where these contain tall late flowering-developing species, mow in spring, then cut as hay between autumn and early spring and remove from the site. Where species develop and flower earlier, they can be treated as for moist meadows

Prairie

Burn in March to April and each year if possible. The standing debris from the previous year can be either strimmed off and removed before this, or burnt in situ, although this is potentially hazardous where there is a lot of dry fuel and where propane-fuelled burners are to be used. More infrequent burning is satisfactory if weed invasion is limited. Where the invasion of stoloniferous grasses becomes a significant problem, spot apply glyphosate in early spring or employ overall application of graminicide prior to the prairie grasses emerging from the soil. Clump-forming problem species, such as docks, can be cut out below ground level

Annual

meadows

Cultivate once between autumn and spring. The precise timing will depend on the species present in the soil seed bank from the previous year in relation to what is desired. In many cases, annual oversowing at reduced rates is essential. Where there is a substantial invasion of rhizomatous grasses, overspraying preor post-cultivation with glyphosate may be essential. Alternatively, a specific graminicide can be used, or the planting rotated to a new site