Weed Management

A weed is often defined as “any plant growing in the wrong place”, in other words any ‘uninvited’ plant that is not a planned crop, landscape or garden plant. In the context of sowing wild seeds there is no reason to regard all unsown arrivals as weeds that need to be dealt with; considerable effort and worry can be saved by targeting only those weeds which by virtue of their competitive nature would, if unmanaged, cause problems.
There are two circumstances in which weed management will be faced. The most important is in clearing weeds in preparation for sowing. The other is dealing with (hopefully occasional) weeds that appear after sowing having perhaps escaped pre-sowing controls or invaded later. It is useful to be able to identify which ‘weeds’ or weed types are benign and can be tolerated if managed, and which are best controlled or avoided.

Annual weeds such as groundsel, knotgrass, fat hen and black-grass usually have little or no long term effect on a sowing provided that the soil is not too fertile. Most soils contain a large store of weed seeds and some annual weed growth should be expected even with good preparation. The pressure from annual weed infestations can be usefully reduced by fallowing and adopting the stale seed bed technique prior to sowing. Annual weeds that appear in perennial grassland and meadow mixture sowings will be eliminated by the end of the first season simply through repeated mowing.

Pernicious perennial weeds including well known weeds such as nettles, thistles, docks, couch grass and brambles can compromise the results of a sowing by crowding out sown species. These weeds may also devalue a scheme through their perceived association with neglect and poor management. A variety of methods can be used to control perennial weeds. It is always best to get control of problem perennial weeds before attempting to sow as selective control after sowing is difficult and laborious. Modest numbers or discrete patches of problem perennial weeds are usually manageable. Significant weed problems, or the presence of particular weeds, may however reveal a site that is difficult to change and unlikely to produce good results (eg nettles which indicate high soil fertility).

Competitive perennials are another significant group of species not usually regarded as weeds that can compromise a sowing by virtue of their strong competitive growth. Species in this group include tall oat-grass, cocksfoot, Yorkshire fog, creeping bent, ryegrass, white clover and some other legumes.

How each of these species is regarded and dealt with very much depends upon the circumstance. The tall grass, cocksfoot, for example, in moderation is a valuable component of tussocky grasslands, however, before sowing a fine grass and wild flower mixture it may be necessary to exclude it from the site (if only temporarily). Sometimes whether the species behaves as a weed or not is a function of soil and site factors. Clovers for example are more likely to cause problems on soils with high phosphorous and low nitrogen status, and tall tussocky grasses may only spread and dominate if the site is not managed (mown) rigorously.

In general, species in this group should be avoided in mixture specifications, or used with caution! Even on sites where ‘weedy’ species are ecologically appropriate they will typically appear uninvited in more than adequate numbers without needing to add seed.

Alien invaders

The presence of ‘alien’ species (i.e. species that are recent non-native introductions to the British flora) are not necessarily a cause for concern. Many, like Michaelmas daisy in urban sites, can be accommodated as part of the diversity of a site and others like woad and opium poppy echo past land use and are a part of a site’s history.

Problem introduced weed species like giant hogweed, Himalayan balsam or Japanese knotgrass however should be categorised with other pernicious perennial weeds and dealt with accordingly.

Weed control in preparation for sowing

Weeds can be controlled by mechanical removal or exhaustion. Removal by hoeing, digging out or repeat cultivation is an effective control for most annual and biennial weeds but less effective (as most gardeners know) for perennial weeds with underground rootstocks. It is possible to exhaust persistent perennial weeds by repeated removal, but it is easy to allow a weed to recover if repeat cultivation is delayed for any reason (e.g. unsuitable weather). Other strategies like fallowing, or covering the ground with plastic mulch are usually essential to success.

Fallowing 

The aim of fallowing is to leave land exposed to provide an opportunity to deal with weed problems. During the fallow period a number of strategies can be followed to clean the land depending on the soil type and weed burden. Deep ploughing will reduce perennial weeds by causing a proportion to rot, and by producing a temporary check to their vigour as they are forced to re-grow to the surface. This burying must be followed by summer fallowing to be fully effective as a weed control measure. The aim of summer fallowing is to expose and dry out the rootstock and rhizomes of established perennial weeds, and to exhaust their growth reserves. On lighter soils the roots can be dragged out with a harrow or rake as the soil dries. On heavy soils the land should be very roughly cultivated or ploughed in spring to leave the maximum surface area and depth exposed. The resulting large clods and ridges should be left to dry out completely in the spring and early summer to ‘bake out’ the perennials.

The Stale Seedbed technique

The “stale seedbed technique” can work well to reduce competition from annual weeds whose seeds will remain in the soil after clearance. This method involves preparing a seedbed then delaying sowing to allow a flush of weed seed germination from the surface layers. This flush of weeds is then killed, by surface cultivation/ hoeing, before sowing your seed mixture onto the cleaned “stale” seedbed – the surface of which now has a reduced weed seed burden. Timing and weather conditions are important for success. The soil must be moist and warm enough to encourage weed emergence. The weeds must not be given enough time to set new seed or develop new persistent root stocks. Cultivation should be shallow to avoid bringing fresh buried weed seeds to the surface.

Weed control in aftercare of established sowings

Perennial weeds in grassland may be kept in check if they are cut, grazed or grubbed out at the right time, and if the grassland is well managed.

For many perennials the optimum removal strategy is to take away as much of the plant as possible at a point in the season when it has its maximum commitment of its reserves exposed (above ground). For thistles this occurs in July when the plants have their underground stores and growth potential committed to rapid above ground growth for flowering and seeding. Thistle plants should not be tackled too soon as this will divert their effort into renewed root growth: aim to cut hard as they start to flower but before they set any seed. Cutting too late in the season allows weed seed dispersal and also time for the plants to put down root stores once again. The same principles can be applied to other weeds.

Good grassland management is important to keep control of weeds. Overgrazing leaves large gaps in which weeds like ragwort can establish. Zero grazing or mowing on the other hand allows coarse and woody weeds, brambles and scrub to develop.