Weed Control in the aftercare of established sowings
Weed control avoiding chemicals
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. Chemical control of cow parsley is actually very difficult; however good control can be achieved by digging out roots at flowering, followed ideally by digging any recovering re-growth.
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.
Aftercare: using herbicides
Because most wild flower sowings include a wide diversity of species groups (including both broadleaved and grass species for example), it is not safe to apply any herbicide as an overall treatment to control weeds once a seed mixture has been sown. There are unfortunately no herbicides that are selective to the degree that they will only kill docks or only kill thistles: they will all damage or kill any related wild flower species. New sowings with young seedlings are particularly sensitive to herbicide damage.
Application methods Problem perennial weeds in established grassland can be controlled by carefully targeted applications of a suitable herbicide. Herbicides can be applied in a targeted way by spot spraying with a knapsack (at low pressure to avoid spray drift), or weed-wiping (smearing chemical onto the target weeds with a wick applicator or paint brush). Tractor mounted weed-wipers work very well in situations where tall target weeds grow clear above the sown species as can happen if residual mature docks growing from buried rootstocks out-grow immature sown seedlings.
Herbicide types: Suitable herbicides for targeted applications will either be a general non-selective herbicide such as glyphosate (very carefully applied!), or a herbicide that is to some degree selective for a particular weed or weed group. Choosing a selective herbicide, such as clopyralid to target thistles, will help limit the range of species that are at risk of accidental damage. In this way selective herbicides can be useful for treating patches of weeds that are too numerous or closely spaced for individual spot treatment. Selective herbicides that remove target weeds but leave the grassland structure relatively intact are usually a better option than spraying out patches with a total herbicide. Even if you re-sow the bare ground left after use of a total herbicide, the opportunity for target weeds to re-establish will quickly return you to square one, whereas after selective removal, the gaps left by dying weeds can be efficiently filled by surviving sown components. The other practical advantage of selective control of dense problem patches (which long-term maybe the least promising parts of a site) is that it frees time to deal more carefully with the occasional weeds in other more promising parts of a sowing.