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How to Get the Best Results from Lawn Weed Killers

Posted in Garden Advice on October 05, 2018

How to Apply Lawn Weed Killer 

Broadleaf plants are often regarded as weeds in lawns and there are weed killers formulated to kill the broadleaf weeds without harm to the lawn grass.

These include:

These are some tips on how to get the best results when using such products:


  • The lawn weed killers work best when the weeds are growing actively. This will generally be spring and autumn but will be dependent on where in New Zealand you are and the growing conditions.
  • Apply in low wind conditions (less than 15 km per hour) and when rain is not expected in the next few hours.
  • Apply when the lawn is dry; wet weeds will encourage spray weed killer to run off into the ground where it less effective.
  • Do not apply to lawns under stress in hot weather or drought conditions.
  • Do not apply to young lawns less than 6 months old unless the product gives specific instructions e.g. LawnPro Turfclean can be applied to lawns 2-6 months old at half rate (5 ml/L).


  • Liquid products are best applied using a good garden sprayer or Lawnboy with a moderately coarse setting of the spray nozzles. You should aim for the largest droplet size that will give good coverage of all the weed leaf surface.
  • Mark out the lawn so you can ensure spraying is evenly applied and no areas are missed.
  • The addition of Dye & Stick to spray weed killer helps ensure even coverage, no waste, and no missed patches, plus improving the efficacy of the weed killers.

How Often

  • Weeds are likely to take 2-3 weeks to die.
  • Most lawn weed killers will specify a period before a further application can be made. If this is not stated on the label wait at least 1 month before re-application or application of another lawn weed killer.


  • If the lawn needs to be mowed before applying lawn weed killers, do it 2-3 days before and raise the mower height so as not to take off the leaves of the weeds you want to get rid of.
  • After application, try not to mow for 2 weeks. If the lawn needs to be mowed before then, raise the mower height a notch or two so that the treated foliage is still being acted on.
    Note: We recommend disposing of the first clippings after spraying, then clippings can be composted, but must be composted for 6 months before use as mulch.

Whole Lawn or Spot Treatment

  • If the weeds are sparse or limited to certain areas of the lawn it is not necessary to spray the whole lawn; you can spot spray the weeds or only spray the affected areas.
  • Coarse grasses can also be a weed in fine turf lawns. The broadleaf weed killers will have little effect on these and spot treatment with Weed Weapon Invade Gel is the best option. Apply the gel to the central growing crowns of the coarse grass.

After Care

  • Help your lawn recover after treatment by applying LawnPro Lawn Thickener. Do this after the weeds have died. It is best to wait 4 weeks after using weed killers to sow seed.
  • Some weeds are indicators that the lawn needs some attention; e.g. moss and hydrocotyle grow in acidic soil, often where there is shade and poor drainage. In this case add lawn fertiliser like LawnPro Green & Grow or LawnPro Natural Boost to raise the pH and make the soil less acidic, fork or plug the area to improve drainage and cut back or thin any shading trees or shrubs to allow more light to the area. 
  • You should check the thatch (dead grass) layer under the grass that it is not too thick and in need of raking out (scarifying). See here for more information.

This should help you get the best results from LawnPro lawn weed killers.


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