Tips for applying a selective herbicide to a lawn
The advice on this page is for a liquid weed killer or herbicide that is applied through a sprayer, watering can or aerosol.
Most of the information on this page is not relevant to granular herbicides, as found in granular weed and feed products.
- If you are going to treat the weeds on the lawn, make sure you have the correct product. Do a little research to identify the weeds, then choose a product that will control the weeds causing problems. Only apply a selective weed killer to the lawn, NEVER treat the lawn with a total weed killer. A total weed killer will kill everything including the grass.
- A selective weed killer will be more effective if it is applied at the correct time of the year. This is usually between April and September when the grass is actively growing. May or June is often the optimum time as growth will be at its strongest during these months. There is also less chance of a drought as it is still relatively early in the season.
- If possible apply a nitrogen based fertiliser about a week before you treat the weeds, as this will encourage strong growth.
- Avoid mowing the lawn 3 days prior to the weed killer application, then don’t mow again for at least 3 days following the application. Leaving the lawn for 3 days following mowing will encourage a large leaf on the target weed. This gives a larger surface area for the chemical to attach itself to making it more effective. Leaving the lawn for another 3 days following the application ensures that the chemical travels through the whole of the weed and that it is not removed from the weed by removing the leaf with the mower.
- Spray during the correct weather conditions. Pick a dry day when there is no rain forecast. Rainfall following the application will only wash the chemical off the weeds thus reducing the effectiveness. Ensure the lawn is dry when you spray. Spraying onto a dry leaf ensures the weed killer will stick to the leaf and increase the effectiveness. If you spray onto a wet leaf then some of the chemical may run off.
When to avoid treating weeds
- Don’t spray during periods of drought. Not only can this cause damage the lawn as it places more stress on an already stressed turf, it is liable to be somewhat ineffective as growth is likely to be weak.
- Avoid spraying during periods of excessive heat as this can scorch the lawn, wait until it is a little cooler.
- Never spray in frosty conditions.
- Don’t spray in windy conditions as this can cause the spray to drift and contaminate other non-target plants and fish ponds.