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 and controlling 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. In each circumstance there will be a choice of approaches depending upon whether you use chemicals (herbicides) or not.
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 (ie 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.