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How to use Lawn Weed Killer

If there are too many weeds to spot or hand weed, follow this procedure to apply weed killer to the whole lawn using a weed killer concentrate

When to apply lawn weed killer

You will only kill weeds when they are actively growing. This means commencing sometime in April and finishing sometime in late September or early October.

I would not rush to apply lawn weed killer to the whole lawn in the early spring as there will be a lot of weeds that won’t surface until later on. In early spring just spot or hand weed. For a good weed kill over a broad range of weeds in your lawn try starting in May. If you need to repeat the treatment you still have plenty of time before cold weather makes it impractical. Read the label of your weed killer to make sure repeat treatments are allowed.

One of my favoured times for complete lawn weed control is in late August or September whilst there is still some warmth around. I would recommend this if you just need the one application in the year. The reason for this is:

  • Most weed types are still around
  • The spring seeds in your lawn will have germinated so you get those as well
  • Killing weeds in autumn is easier than killing ‘winter hardened’ weeds in spring
  • You’ll have a weed free lawn until the next round of weed seeds germinate the following spring

Conditions for using lawn weed killer

  • When moisture is available in the soil. You can harm the grass in drought conditions
  • When it is unlikely to rain for at least 6 hours
  • On dry grass. Wet grass will dilute your weedkiller
  • In calm weather – wind will blow spray even from a watering can sprinkle bar
  • Avoid frost or freezing conditions which may occur in spring
  • Avoid strong sunshine
  • In summer it is best to apply weed killer in the cooler evenings
  • Any time after fertilising. Let the feed kick in (one to two weeks) then treat the weeds to get a better weed kill
  • After mowing. Leave at least 2 days if applying liquid lawn weed killer to the whole lawn. Don’t mow for a further 3 days
  • When mowing prior to weed treatment don’t mow close. If you mow close and chop the leaves off the weeds there is nothing left to spray!

How often can you use lawn weed killers?

This all depends on the weedkiller being used. Each manufacturer is different so do read the label.

Our recommended weed killers Resolva and Weedol (Verdone) suggest only one treatment per year but if you use each one once that allows you two full lawn treatments in a year. This only applies to weed killer for the coverage of the whole lawn NOT ready to use for spot treatments.

My advice would be a maximum of 2 treatments per annum using two different weed killers with different active ingredients. Further, you are only likely to treat this often in the first and possibly second year to get rid of established weeds. Once you get in to subsequent years a treatment in September will be the most you’ll need if you do some weed control with a spot weed killer during spring and summer. If you have good lawn care practices you’ll find you won’t need a complete lawn treatment for several years. I give my lawn a full treatment about every 3 years!

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