Applying pre-emergent herbicide

A pre-emergent herbicide creates a treated layer in the top of the soil that stops susceptible weed seeds from establishing. It doesn’t kill weeds that are already up, and it doesn’t last forever, so when and how you apply it decides whether it works. Done well, it’s the most effective way to stop winter grass, the weed behind more “my lawn is full of weeds” questions than any other.

How pre-emergents work

Weeds like winter grass are annuals: each plant lives for one season and drops seed into the lawn before it dies. Those seeds sit in the soil until conditions suit them, then germinate. A pre-emergent targets that moment. The treated layer stops the germinating seedling from establishing, so the weed never appears.

Three things follow from that:

• Timing is everything. The product has to be in place before germination starts.
• Coverage must be even. Gaps in the treated layer become patches of weeds.
• The layer must stay undisturbed. Coring, scarifying and heavy raking break it up.

Step 1: Identify the weed

A pre-emergent only works on weeds that grow from seed, and only on those its label lists. Before you buy anything, confirm what you’re dealing with. Winter grass is a soft, light-green grass weed that appears in autumn and winter and seeds in spring. Bindii is a low, feathery broadleaf that forms prickly burrs in spring. Perennial weeds that come back from bulbs, tubers or runners, such as onion weed and nutgrass, are not controlled by pre-emergents.

Use the Weed Identifier if you’re not sure.

Step 2: Choose the right product

Check the label for:

• your grass type, listed as a situation where the product can be used. This matters most on buffalo, which is sensitive to some herbicides.
• the weeds you want to stop, in the weeds controlled.
• the rate, watering-in directions, re-application interval and maximum applications per season.
• waiting periods before sowing seed, laying turf or renovating.
• warnings about newly laid turf, stressed lawns or particular varieties.

Many common lawn pre-emergents belong to herbicide Group 3, for example prodiamine, pendimethalin and dithiopyr. CropLife Australia publishes mode-of-action groups so that users can rotate between groups; relying on the same group year after year can favour weeds that are resistant to it. We show products only when their label data confirms the grass and the weed.

You can check that a product is registered, and see its label, on the APVMA’s PubCRIS register.

Step 3: Time it

Apply before the target weed germinates:

• Winter grass: autumn, before seedlings appear. In temperate and cool areas that usually means late summer into early autumn; in subtropical and warm temperate areas, a little later as autumn cools.
• Summer weeds: late winter to early spring, before soils warm.

If you’re unsure, the Lawn Calendar for your climate zone shows the windows, and you can add a reminder to your Lawn Journal so you don’t miss it next year. Applying too late is the most common reason a pre-emergent “doesn’t work”.

These are typical windows only; local conditions and the season vary. [CONFIRM: reviewer to confirm pre-emergent windows by climate zone]

What a pre-emergent won’t do

• It won’t kill weeds that are already growing. Dig them out or use a selective post-emergent whose label lists them and your grass.
• It won’t stop perennial weeds that regrow from bulbs, tubers or runners, such as onion weed, nutgrass and clover.
• It won’t fix the reason weeds got in. Thin, patchy lawns let light reach the soil, which helps weed seeds germinate. A thick lawn mown at the right height is still your best long-term defence.
• It won’t last indefinitely. Each label states how long control can be expected and whether, and when, it can be re-applied.

A year-round weed plan

A pre-emergent works best as one part of a plan:

1. Autumn: pre-emergent for winter grass, timed for your zone.
2. Winter: dig out or spot treat bindii and any winter grass that got through, before either sets seed.
3. Spring: feed the lawn once it’s growing so it thickens up and shades out weed seedlings.
4. Summer: mow at the right height, water deeply and less often, and deal with summer weeds while they’re small.

Log each step in your Lawn Journal. After a year or two you’ll see the weed pressure fall.

Step 4: Plan around renovation and new grass

Because the treated layer must stay intact and can stop grass seed and affect new roots, plan the year so these jobs don’t clash:

• Renovate first. Finish any scarifying, coring or topdressing before you apply.
• Don’t sow seed into a treated lawn until the label’s waiting period has passed.
• Check the label before using it on new turf. Labels often set a minimum time after laying.

If you’re planning a big renovation in spring, an autumn pre-emergent may still fit, but check the label’s interval first.

Step 5: Prepare the lawn

Mow a day or two before. Rake up leaves, clippings and debris so the product reaches the soil surface rather than sitting on top. Make sure the lawn isn’t stressed: a lawn suffering in heat or drought is more likely to be damaged, and some labels warn against applying in those conditions.

Step 6: Measure and calibrate

Work out your lawn area with the Lawn Area Calculator, then use the Product Application Calculator to convert the label rate to an amount for your lawn.

• Granules: calibrate your spreader with the actual product over a measured test area, as the spreader guide describes.
• Liquids: calibrate your sprayer with water so you know you’re applying the label’s water volume, as the sprayer calibration guide describes.

Calibration is what turns a label rate into an even layer across the lawn. Without it, you’re guessing.

Step 7: Apply evenly

Use the patterns in the spreader and sprayer guides:

• edges first, then parallel passes with a slight overlap;
• or set the equipment to half the rate and cover the lawn twice, the second time at right angles to the first.

Close the spreader or release the trigger whenever you stop or turn. Keep product off paths, driveways and gutters, and sweep up any strays and put them back on the lawn. Granules left on hard surfaces wash into stormwater drains.

Step 8: Water in as the label says

Most pre-emergents need watering in to move them off the leaves and into the top of the soil, where they work. Apply the amount of water the label specifies, or time the application before light rain if the label allows that. Avoid heavy rain, which can wash product off the lawn and into drains.

If you can’t water in when the label requires it, wait and apply on a day when you can.

Safety

Wear the protective equipment the label lists while you handle and apply the product. Keep children and pets off the lawn while you work and until the label’s re-entry conditions are met. Store the product in its original container, locked away, and read the spraying safely guide for more detail. In a poisoning emergency, call the Poisons Information Centre on 13 11 26.

After applying

• Keep people and pets off for as long as the label directs.
• Mow normally once it’s watered in, unless the label says otherwise.
• Don’t scarify, core or sow seed until the label’s waiting period has passed.
• Log the application and set a reminder for any follow-up the label allows.
• Watch for any weeds that do come up. A few may germinate in gaps or along edges; dig them out before they seed.

How well did it work?

Judge the result in late winter. A lawn with a well-applied pre-emergent should have far fewer winter grass plants than the year before. Weeds in strips or patches usually point to gaps in coverage, so check your spreader pattern. Weeds evenly scattered across the lawn suggest the application went on after germination had started. Note what you see in your Lawn Journal so next year’s timing improves.

Common mistakes

• Applying after the weeds are up. Pre-emergents work on germinating seeds, not established plants.
• Not checking the label lists your grass, especially on buffalo, which is sensitive to some herbicides.
• Skipping the watering-in the label asks for.
• Coring, scarifying or heavy raking while the pre-emergent is working, which can disturb the treated layer.
• Applying where you plan to seed or lay turf soon. Check the label for waiting periods.
• Uneven spreading, which leaves untreated strips for weeds to fill.