Although not normally undertaken for sowings of native wildflower meadow species, some common native species are highly palatable to slugs (Hanley et al. 1995; Scheidel and Brueheide 1999), and on sites with dense mollusc populations, their establishment may be greatly reduced. The author’s research on non-native species suggests that slugs have a major impact on the establishment of palatable species, and that a single application of metaldehyde-containing pellets will at least double the number of seedlings that establish. In this particular experiment, additional applications did not further improve seedling survival. Decisions on controlling slugs at germination need to balance possible adverse non-target effects with the likely failure of palatable species.
This is more complicated than in planted vegetation due to confusion as to which seedlings are weeds and which are sown. There is also the physical problem of uprooting sown species as weed species are removed, and crushing sown species when moving through sown areas. Because of these difficulties, it is often necessary to adopt unconventional approaches to weed management in the first year. The basis of this is to try to minimise weed germination by a combination of the previou ly discussed techniques, stale seedbed techniques, use of soils with low-weed seed banks and the use of mulches., etc., as previously discussed. Weeds compete
The same experiment in year three, showing how plots with no grass control are essentially devoid of sown forbs
for light, water and nutrients, however, in the first year of a sowing, the most critical of these factors is often light. Dense shade cast by weeds causes the elimination of lightdemanding sown species.
Weeds also have an adverse effect in that, when present in high densities, they provide habitat for slugs and this exacerbates the loss of palatable species. The impact of weeds on sown species varies considerably depending upon combinations of these factors in relation to the site.
The most effective weed-management technique we have investigated is a 50 mm deep-coarse sand mulch spread over the soil surface. Sand mulching has an additional long-term benefit in that it appears to reduce slug predation in spring as plants emerge from the soil. The problems of sand mulching reducing seedling establishment have been discussed under the section ‘Weed control’.
Where it is not possible or desirable to use sand mulching, and sowings are made onto a weed seed rich topsoil, weed management will often rely on the mowing of sown vegetation to a height of approximately 50-100 mm (depending on the growth habit of the sown species). As most weeds grow faster than the sown species, weeds are more defoliated, improving the establishment chances of the former. Mowing needs to commence before the weeds get too tall, i. e. at 100-150 mm, typically in May or June. If left too late, the large volumes of cut trash left may shade-out sown seedlings. Typically, mowing can continue at fortnightly to monthly intervals. The effectiveness of mowing depends on the weeds present. It is most successful with annual dicots. Even with these, however, mowing changes the form of many erect weeds; they branch close to the ground casting denser shade than they might otherwise do.
Mowing is least effective on vigorous perennial grasses, as it encourages these to tiller and smother adjacent seedlings. Grasses can, however, be removed from sowings of forbs, by overspraying with selective grass herbicides. In Britain, sethoxydim (Checkmate®) had off-label approval for use on ornamental herbaceous plants (Whitehead 2002). We have used it on seedlings of a wide variety of species in our experiments with no obvious damage to forbs. It does not damage narrow-leafed fescues. The effect of this type of control on both the density and diversity of sown forbs on productive, weedy sites is very great (Figures 6.11 and 6.12).
Nurse crops are sometimes recommended to improve the establishment of sown species, often using annual or short-lived species, such as Lolium multiflorum. The benefits of such nurse crops are extremely dubious. Pywell et al. (2002) recorded a decrease in the number of grass weeds following the use of a L. multiflorum nurse crop, but no corresponding benefit in the establishment of sown meadow forbs. This is because vigorous nurse crops pose similar competitive pressures to non-sown weed species. Slower growing, less-aggressive nurse crops, for example annuals such as Linum grandiflorum, may confer some benefits to sown species, however it is difficult to see how they can effectively compete with weed species but not with sown species. The most valuable role of nurse crops of colourful annuals is probably to provide interest in year one, although this may be at the expense of the long-term success. In one of the author’s prairie vegetation experiments, the inclusion of the fastgrowing biennial Coreopsis lanceolata seemed successful in the first year when this species dominated the sowing. By year two, many had died out but, by then, they had also eliminated adjacent perennial prairie species. These gaps gradually colonised with weed species, leading to the collapse of the prairie community.
Where trained and diligent labour is available, it is possible to physically remove weeds in the first year of sowings. Owing to identification problems and the risk of uprooting small sown seedlings, this is often combined with mowing early in the season. By midsummer it is much easier to distinguish between weeds and sown species. Weeds can often be controlled with minimal surface disturbance by either dabbing them with a 50% solution of glyphosate through a mini wick wiper, or by using a very sharp, thin – bladed knife (we use serrated fishfilleting knives) to cut the weeds below ground by rotating the knife in the soil. The latter is also effective for controlling seedling docks (Rumex spp.), providing the tap root is cut more than 5 cm below soil level. String lines are placed across the sowings to provide a 1-2 m wide corridor to work through. Inevitably, some sown species are crushed but this has to be seen in a cost-benefit framework. Annual weeds need to be removed before they flower and set seed.
Weed control in the first year is crucial, especially in forb-only vegetation. By maximising establishment and growth, sown species are given the opportunity to dominate in the second year. If one enters the second growing season in the absence of intensive maintenance with low densities of sown species, and high densities of weeds, the prognosis is not good. The number of plants of sown species to provide a dense cover varies according to the size of individual plants, their growth habit and site productivity. Our research suggests that typically between 50 and 100 plants/m2 are a minimum value.
Establishment by planting