What is pre-emergent herbicide?

What is pre-emergent herbicide?

Properly applied pre-emergent herbicides form the backbone of any weed control program for your lawn. Pre-emergent herbicides work extremely well if you if you get your application timing right.

Pre-emergent herbicides work by forming a layer in the soil that a germinating weed has to grow through. The pre-emergent barrier then interrupts the growth process in some way to prevent the seed from getting all the way through the seed germination stage. For that to work, it has to be in the soil at the right time and at the right temperature to beat the weed seed germinating.

Once that weed is up, the pre-emergent will not stop the weed from growing!

Exactly when your soil gets to a temperature when the weed seeds germinate will depend on your local climate, and what the weather is like in the particular season. Early spring? Late spring? Autumn? Maybe, maybe, and maybe.

For example, with Crowsfoot which is one of the world’s most common weeds, a pre-emergent can be applied as early as September in Central Queensland.


However, in the southern states, for example Victoria, soil temperature can be up to six weeks later.

Knowing your localised soil temperature is as simple as pushing a simple soil temperature gauge into your soil. However, the rule of thumb is that as it starts to warm in spring, the temperatures are getting to a place where weeds will germinate. And one day late is too late!!!

soil check for preemergent

Choosing the right pre-emergent is also an important part of a successful program. Barricade or Spartan are two very good liquid pre-emergent herbicides that will give up to six months pre-emergence control.

A granular version of the liquid pre-emergent has been released called Onset. It comes in a 20kg bag or 5kg bucket this product also offers up to six month protection.

Applied at the right time you will see year-round control with two well timed applications. That said, there are a number of options to choose from depending on the type of activities you will be undertaking in your annual lawn care program.

With that sort of longevity of control, some will choose to go out early so they do not miss weed germination. But, in certain circumstances the pre-emergent you use will depend on the time of year, and the plans you have made in your annual lawn care program.

Here are some things to consider before choosing a pre-emergent herbicide:

Which Pre-emergent controls the weeds I have?

It is important that you know the weeds you are trying to manage, especially those weeds that are really tough to manage. Some pre-emergent herbicides control broadleaf weeds well but are ineffective against the grasses. Others will manage the grasses and some – but not all – broadleaf weeds.

What Pre-emergent shall I choose?

Choosing the right pre-emergent to suit your situation is important. Some pre-emergent herbicides can slow down the root development of new lawns. In this situation you would look for a pre-emergent that does not hinder your new turf such as Oxafert or Echelon Duo in a 20kg bag or 5kg bucket.

Once your lawn is established, a popular choice in Australia is Barricade pre-emergent herbicide, which tackles about 30 different broadleaf and grassy weeds, including Crowsfoot and Wintergrass.

Pay particular attention to the label of the pre-emergent product if you intend to over sow with rye. If you pick a variety that controls grass weeds, it will stop you growing any new desirable grass seed as well. Avoid using Barricade, Spartan or Onset is you plan to over sow.

Whichever product you select, it is vital your application technique ensures the product is thoroughly and evenly applied to gain the best weed prevention. You will need to know how many square meters of lawn you have to apply the right amount. A pre-emergent must cover your target area completely and be watered in, to serve as a barrier against weed growth.

And lastly, remember that a healthy lawn is much more likely to out compete the weeds. Follow one of the annual Lawn Care Programs we have on our website, and you will have a greater chance of having a healthy weed free home lawn.