First, consider whether this can be done using non-chemical means such as digging out. Where this method is not feasible, chemical controls may need to be used.
The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.
The best way to eradicate field woodrush and prevent it coming back it to apply lime.
Apply ground chalk or ground limestone in late autumn or early winter, after mowing has ended, at 60g per sq m (2oz per sq yd). Do not use hydrated lime.
Many nitrogen fertilisers acidify the soil and are best avoided. Sulphate of ammonia is particularly acidifying. Most lawn feeds won't significantly affect pH, but where high nitrogen fertilisers are needed consider chicken manure pellets or nitro-chalk (also sold as Nitratechalk) which should be neutral in effect, rather than sulphate of amonnia.
Field woodrush is resistant to lawn weedkillers, but those containing mecoprop-P (e.g. Doff Lawn Weeder, Crowne Green Lawn Weedkiller or Westland Resolva Lawn Weedkiller Extra, Roundup Lawn Ultra or Roundup Lawn Optima) may check growth if repeated applications are made and the soil pH is raised by liming.
Inclusion of a weedkiller product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener.
Weedkillers for gardeners (Adobe Acrobat pdf document outlining weedkillers available to gardeners; see sections 1b and d)
Chemicals: using a sprayer
Chemicals: using safely and effectively
Chemicals: using spot and broad-scale weedkillers
Weeds: non-chemical control