First, consider whether this can be done using non-chemical means such as digging out or mowing. Where these methods are 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.

Cultural control

Cultural control is practicable where there is a limited infestation; use a fork to lift out the weed by the roots in late spring when the flowering stem has developed sufficiently to provide good purchase. It is important to prevent the seed from developing.

Repeated mowing will exhaust and largely eliminate this weed from grassed areas. 
 

Weedkiller control

Where seed is known to have fallen on grassland, use a lawn weedkiller the following spring during the earliest stages of seedling development. At the seedling stage lawn herbicides containing 2,4-D plus dicamba or mecoprop-P (eg. Westland Aftercut All-in-One Feed, Weed and Moss Killer or Vitax All in One Lawn Feed, Weed and Mosskiller) will strongly check growth, but well-established plants are resistant to these lawn weedkillers.

Well-established plants are resistant to lawn weedkillers so are best controlled using a systemic herbicide such as glyphosate (eg. Roundup Fast Action, Job done General Purpose Weedkiller or Doff Advanced Weedkiller).

Glyphosate is best applied when the cow parsley is in full growth (April-May), as the more chemical you can apply to the foliage, the more effective the chemical is. Treatment after flowering is not effective.

Unfortunately, glyphosate is non-selective, killing any green plant material it comes into contact with. However, as the cow parsley is a tall-growing plant it should be possible to spray the foliage of the weed without the spray coming into contact with the grass. Alternatively, plastic sheeting can be laid over the turf whilst spraying.

Glyphosate can take 3-6 weeks to kill weeds, depending on weather conditions. It doesn’t remain in the soil so re-sowing can commence soon after the weed has died down. If the soil is to be cultivated, ensure that the weed roots are dead before digging to prevent live root sections being spread through the soil.

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.
 

Download

Weedkillers for gardeners (Adobe Acrobat pdf document outlining weedkillers available to gardeners; see sections 1b and 4)

Links

Chemicals: using a sprayer
Chemicals: using safely and effectively
Chemicals: using spot and broad-scale
Weeds: non-chemical control