Reseeding is a costly investment so it’s crucial we do everything we can to give the new reseed the best chance to establish successfully. Controlling weeds with agricultural herbicide or a clover-safe post-emergence spray gives the greatest opportunity for creating a clean, weed-free sward in the longer term.
The correct time for controlling weeds
Timing is critical to any weed control treatment. It is important you have active growth and the reseed is not under any stress such as drought.
Generally speaking, the best time is five to six weeks after sowing, around the three-leaf stage and when seedling weeds such as docks are around the size of a €2 coin.
Spraying too early can mean the weed seeds haven’t germinated, but if too late, when roots have had time to develop, the spray may not kill the weeds so successfully.
When clover is present, a clover-safe spray should be used and the plant should have a trifoliate leaf – see below for advice on emergency use.
Weeds causing the greatest problems
Chickweed, docks, and thistles are the most common problem weeds when reseeding grassland and even a seemingly low population can impact grassland productivity.
Chickweed is small but aggressive and a particular threat to autumn reseeds when grass seed can take a little longer to establish.
Although an annual which dies back after flowering, chickweed can leave unproductive bare patches in grassland open to colonisation by other weeds. This tends to be more of a problem in autumn reseeds as grazing can provide an effective means of control in reseeds sown earlier in the year.
Larger perennial weeds such as docks and thistles are particularly hard to eradicate once embedded, hence control being so important at the young seedling stage.
Unchecked, dock plants produce thousands of seeds a year and the roots can penetrate to at least a metre deep. An infestation of just 5% can reduce grass yield by the same amount.
Creeping thistles do as their name suggests and spread roots underground – commonly by 1-2 m per year, but can be up to 6 m in a single growing season. Like docks, spear thistles produce thousands of seeds which disperse easily and remain viable for several years.
Other weeds seen on-farm, such as fumitory, fat hen and deadnettle, are less of a threat. As annuals, these weeds only live for one year. Most commonly in smaller numbers and are usually controlled by grazing and cutting or die naturally over the winter. If more widespread, however, it may be necessary to spray them.
Limited use of clover-safe post-emergence spray
Always adhere to label recommendations and use a herbicide approved for use when reseeding grassland. If clover is present, it is critical to use a clover-safe herbicide, closely following the recommendations for its use, but the choice of available products is limited.
The Department of Agriculture (DAFM) has once again granted emergency use (EUA) this year for a limited number of clover-safe post-emergence sprays such as CloverMax and DB Plus. These can only be used until the end of September 2022. Please ask your merchant or a qualified advisor for advice.
Top tips for controlling weeds in a grass reseed
Check weeds are small and actively growing – e.g. dock leaves are around the size of a €2 coin, and grass is at the three-leaf stage
- Choose a herbicide approved for use on reseeds, always following the manufacturer’s recommendations, using the correct dosage and water rate as detailed on the product label
- If your sward includes clover, use a clover-safe product as allowed by the EUA
- For multi-species swards containing herbs, no herbicides can be used
- Only spray in good conditions, delaying the application if rain is forecast or it is particularly dry, cold or windy
- Remember to keep records for cross-compliance purposes
Protecting reseeds from weed infestations by using a clover-safe post-emergence spray is a vital part of grassland management. Improving sward palatability and productivity from the very start, by keeping an eye on the weed burden of your reseeds, gives you the best return on investment.
For more information on grassland management and reseeding this year, go to the Germinal Knowledge Hub.
Reseeding grassland guide
This reseeding guide covers soil health, reseeding methods, timings and more.