How to mix weed killer

How to Mix Lawn Weed Killer with Fertiliser

Here are two simple ways to use weed killer and soluble or granular lawn fertiliser that is easy for you, safe for the grass and deadly for the weeds.

Your weed killer works best when the weeds are growing well which means the lawn has been fed, there is moisture in the soil and the weather is warm.

How to Apply Weed Killer and Granular Fertiliser

  • In spring apply your Lawnsmith granular fertiliser at the desired time usually between March and May.
  • Apply your liquid weed killer late May to July

In autumn you should apply your weed killer anytime after the fertiliser has greened the lawn but before it turns cold.

Why do I need to wait?

Two reasons:

  • Weed killer works by massively increasing the growth rate of the weed so that it basically burns itself out. You can increase the speed by which this happens by watering and feeding the lawn at least a week before using the weed killer.
  • Because liquid weed killer is so strong and effective most products only allow one or two applications a year. If you apply too early in spring before all the weeds are growing well you'll have wasted an application. In addition, you shouldn't be putting lots of weed killer into the ground unnecessarily. This isn't an issue in autumn.

If you can be patient and follow this guidance you'll have such a good result that you'll more than likely eliminate 99% of the weeds and can get away with spot spraying or manually removing the weeds in the following couple of years.

How to Mix Weed Killer with Soluble Fertiliser

When using soluble fertiliser you have the opportunity to mix this with weed killer in the same container and complete two jobs in one! This is best done once the weather warms between late May and late August. This slightly contradicts the advice above regarding waiting but remember, soluble fertiliser reacts very quickly because it is adsorbed through the leaf so no time delay!

How to Calculate the Mix

  • The Lawnsmith soluble fertiliser should be applied at 4 to 6 grams per square metre. For this example we'll use 5grams.
  • We'll use Weedol as the weed killer which should be applied at 1.5mls per square metre.
  • If you're using a watering can with a sprinkle bar you need to use 25 to 50 litres water per 100 square metres of lawn.
  • If you're using a sprayer you only need 8 to 12 litres of water to cover the same area as the spray is finer and coats better.

For this example we'll use a lawn area of 150 metres square and we'll use a watering can. This means:

150 x 5 grams soluble fertiliser = 750 grams total

150 x 1.5 mls weed killer = 225mls

It's hard to know how much water to use unless you do a test run with just water. Let's say this test run used 50 litres and my watering can holds 10 litres. This means I need to make up 5 equal lots as follows:

10 litres of water

750 grams fertilizer divided by 5 = 150 grams

225 mls weed killer divided by 5 = 45 mls

Put the water in a bucket, then your soluble fertilizer and weed killer, then give it a good stir until the fertilizer has dissolved. You may end up with a few grains that won't dissolve in the bottom of the bucket so when you pour your mixture into the watering can make sure to leave these dregs in the bucket. This way you won't block the sprinkle bar. 

Now go and put it on the lawn and repeat 4 more times.