When should I apply pre-emergent?
Quick answer
Apply pre-emergent before the weed you're targeting germinates. For winter grass that means autumn, before seedlings appear. For summer weeds it means late winter to early spring. A pre-emergent can't kill weeds already growing, and it also affects grass seed and new turf, so follow the label for timing, rate, watering in and grass types.
Why this happens
A pre-emergent herbicide is a barrier against germinating seeds. It works in the top layer of soil, stopping susceptible weed seeds from establishing. It does nothing to weeds that are already growing, which is why timing is everything.
Winter grass germinates in autumn, so the pre-emergent has to go down before that: late summer to early autumn in cooler zones, a little later where autumn is warmer. Summer weeds such as summer grass and crowsfoot germinate in spring as soils warm, so a separate application goes down in late winter or early spring if they’re a problem in your lawn.
Pre-emergents don’t just stop weed seeds. They can also stop grass seed germinating and affect the roots of new turf, and they can be disturbed by scarifying or coring. If you’re planning renovation or a new lawn, plan the pre-emergent around it, using the waiting periods on the label.
Each product’s label sets the rate, how to water it in, which grasses it can be used on, and how often you can apply it. The calculator below uses label rates when we have them; otherwise enter the rate from your own label.
What you may be seeing
- Winter grass coming up each autumn in the same spots
- Summer grassy weeds appearing in late spring
It depends on your grass and climate
| If you have… | What changes |
|---|---|
| Temperate and cool areas | Autumn applications for winter grass go down from late summer into early autumn, before the first seedlings appear. |
| Subtropical and warm temperate areas | Winter grass germinates a little later as autumn cools. Aim to have the pre-emergent down before then. |
| Buffalo | Use a product whose label lists buffalo. |
Rows that match the grass and climate zone you've set are highlighted. Set your climate zone
What you can do
Decide which weed you're preventingCheck first
Winter grass (autumn germination) and summer weeds (spring germination) need applications at different times of year.
Check the label suits your lawnCheck first
The label must list your grass type and the weed. Note any waiting period before sowing, turfing or renovating.
Measure your lawn
Work out your lawn area so you apply the label rate exactly. Use the Lawn Area Calculator.
Log it
Record the date and product in your Lawn Journal, and set a reminder for any follow-up application the label allows.
Apply evenlyProduct step
Granules go on with a calibrated spreader; liquids with a calibrated sprayer. Apply at the label rate.
Water in as the label saysProduct step
Most pre-emergents need watering in to work, but the label decides how much and when.
What not to do
- Applying after the weeds are already up and expecting them to die.
- Scarifying, coring or sowing seed straight after applying, which can disturb the barrier or stop the seed.
- Guessing the amount instead of measuring the lawn and using the label rate.
How much do I need?
A label rate of "20 mL per 100 m²" means: Rate 20, mL, per 100 m². Per hectare? Use 10,000 m².
How much you need
Enter your lawn area and a rate to see how much product and water you need.
Log it
Keep a record of what you applied, when, and at what rate. Your Lawn Journal works on this device without an account, and reminds you when the next step is due.
What's next
Related grass types, weeds, pests and diseases
Related questions
Related guides
Common follow-up questions
Can I apply pre-emergent once a year?
That depends on the product and the weeds. Some labels allow a second application; follow the label for re-application intervals and maximum applications.
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Sources
- Turf weed, pest and disease identification guide (poster), Bayer Environmental Science, hosted by Turf AustraliaWinter grass germinates in autumn; summer grass and crowsfoot germinate in spring; pre-emergents applied before germination. Accessed 3 October 2026.
- Herbicide mode of action table, CropLife AustraliaGroup 3 pre-emergents. Accessed 4 October 2026.