Weed control in preparation for sowing
Weed control without chemicals
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.
Weed control using herbicides
Non-selective herbicides as an overall treatment or as a targeted spray, are an effective way to eliminate weeds and other vegetation. Herbicides like glyphosate that are actively taken up and moved (translocated) to all parts of the plant are especially effective for perennial weeds with deep or creeping rootstocks. Herbicides do offer an invaluable way of preparing land for seeding. Thorough control of difficult weeds may require repeated applications of herbicide. Always consult the chemical manufacturers' instructions for the optimum timing and growth stage for best results. NB: nettles and white clover are not well controlled by glyphosate, other methods will be required.
Glyphosate (roundup) only affects plants with growing leaves; it is rapidly inactivated in soil and presents no hazard to un-germinated seed, even if applied on the day of sowing.
Selective herbicides for example those which kill broad leaved plants (flowers) but leave grasses unharmed, can be useful to control perennial weeds eg docks and thistles in preparation for sowing wild flowers into existing grass where no desirable flowers are present.
NB Some selective grassland herbicides remain active in the soil for months (especially products containing clopyralid) so it is important to leave a safe time interval between treatment and sowing seeds for residues to degrade - consult the chemical's instructions for guidance.
Combination treatments of herbicide and cultivation fallows (as above) often produce better results than either approach used alone.
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 spraying or 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 to 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.