Pre Emergent for Spring: Stop Weeds Before They Start

Every spring the same thing happens. The weather warms up, your lawn starts growing again, and right on cue, weeds appear everywhere. The part most people miss is that this battle is won weeks before you ever see a single weed. That is exactly what a pre emergent does. It is a herbicide you put down before weed seeds sprout, so they never get the chance to take hold. Get your pre emergent timing right in late winter and early spring and you save yourself a whole summer of pulling weeds by hand.

A pre emergent is a weed control product that works underground, before weeds ever break through the surface. Rather than killing a weed you can see, a pre emergent creates a thin barrier in the top layer of soil. When weed seeds try to germinate and push up through that layer, it stops the young roots and shoots developing, so the seedling dies before it reaches daylight.

That is the key thing to understand. A pre emergent does not kill established weeds that are already growing. It targets seeds as they germinate, which is why timing matters so much. Put it down too late, after the weeds are already up, and it will not do much at all.

Most of the weeds that ruin an Aussie lawn over summer are warm season annuals. Think summer grass, crabgrass, crowsfoot and creeping oxalis. These weeds wait out the cold months as seeds in the soil, then germinate in a rush as soil temperatures climb through spring.

That germination window is your one big chance. A pre emergent laid down before the soil warms up sits there ready, so the moment those seeds try to sprout, it shuts them down. Miss the window and you are back to spraying and pulling weeds all summer.

As a rough guide, apply your pre emergent as soil temperatures climb toward 15 to 18 degrees. In most parts of Australia that lands in late winter to early spring, so from around August into September depending on your climate. In warmer northern regions you may need to go earlier.

TIP: Soil temperature matters more than the calendar. If you have a cheap soil thermometer, start checking in the morning. Once soil temps trend up toward 15 degrees, it is time to get your pre emergent down.

Applying a pre emergent is not complicated, but a few things make a big difference to how well it works.

• Mow first. Give the lawn a mow and a light rake a day or two before, so it reaches the soil rather than sitting on clippings.

• Apply evenly. Whether you use a granular pre emergent through a spreader or a liquid through a sprayer, even coverage is everything. Any gaps are where weeds sneak through.

• Water it in. Most pre emergent products need watering in to move the active ingredient into the top layer of soil where the barrier forms. Check the label, but around 5mm of water soon after applying is a common guide.

• Sequence your renovation. If you are aerating or renovating, do it first then apply, so you finish on a clean, even surface. If you do aerate afterwards, research shows it has little effect on a pre emergent, so there is no need to leave the lawn untouched.

A pre emergent like Spartan or Barricade does its best work on a healthy, evenly watered lawn. If your soil is dry and patchy, water will not carry it evenly into the surface and you get weak spots in the barrier.

A wetting agent helps here. On sandy or water repellent soils, a wetting agent like Pure Wetter helps water and your pre emergent soak in evenly rather than running off or pooling. That even moisture is exactly what it needs to form a consistent barrier across the whole lawn.

It is also worth feeding the lawn through spring so it grows thick and dense. A thick lawn is your second line of defence, because any weed seed that slips past the pre emergent struggles to find the light and space to establish.

A few simple mistakes catch people out every spring.

• Applying too late. The most common one. Once weeds are visible a pre emergent will not touch them. You have to be in before germination.

• Skipping aeration out of fear. A common myth is that aerating or renovating after applying ruins a pre emergent. Research shows a core aeration has minimal impact on weed control, especially on a healthy lawn, so aerate when it suits. The tidiest order is simply to aerate first, then apply.

• Uneven coverage. Rushing the spread leaves gaps. Slow down and overlap your passes.

• Not watering in. Left dry on the surface a pre emergent may not activate properly. Water it in as the label directs.

Weeds are so much easier to stop than to remove. A pre emergent laid down at the right time in spring does the hard work for you, quietly stopping thousands of weed seeds before they ever surface. Get the timing right, water it in and keep the lawn healthy, and you will spend far less of your summer on your knees pulling weeds.

Want a simple plan that tells you exactly when to apply your pre emergent, feed and mow through the year? Download our Free Lawn Calendar. It lays out what to do and when, with the right products matched to each season.