Pre-emergent herbicide in Australia: how and when to stop grassy weeds before they start

Some weeds you can spray once they appear and be done with. Grassy weeds are not among them. Winter grass, summer grass, crab grass and crowsfoot are difficult to control once they are up, because a selective spray that kills a grassy weed without killing your lawn is hard to find, since the weed and the lawn are both grasses. Every year people fight these weeds after they emerge and lose. The way you actually beat them is to stop them germinating in the first place, and that is what a pre-emergent does.

What a pre-emergent actually does

A pre-emergent works completely differently from a normal weed spray, and understanding that difference is the whole game. It does not kill weeds you can see. Applied to the lawn and watered in, it binds to the top layer of soil and forms a treated zone. When weed seeds in that zone germinate, their root development is blocked and the seedling dies before it ever emerges. Your established lawn is unaffected, because its roots sit well below the treated zone.

The one rule that follows from this is the rule that decides everything: the barrier has to be down before the seeds germinate. A pre-emergent applied to weeds that are already up does nothing to them. It is not a rescue product. It is a prevention product, and timing is what makes or breaks it.

The two windows that matter

Grassy weeds fall into two seasonal groups, and a pre-emergent has a window for each. This is where most people get the timing wrong.

Autumn, for winter grass. Winter grass (Poa annua) germinates as the soil cools down through about 18 degrees in autumn. To stop it, the barrier needs to be down before that, which across most of the country means March into April. Miss it, and winter grass is up all through winter and seeding by spring.

Late winter into spring, for summer grasses. Summer grass, crab grass and crowsfoot germinate as the soil warms up through about 13 degrees and climbing in spring. The barrier needs to be down before the soil reaches that point. In the warmer states, Queensland, northern New South Wales and Western Australia, that means late winter to early spring, and the window closes early. In the cooler southern states, Victoria, South Australia, Tasmania and southern New South Wales, it runs a few weeks later through spring. If you are seeing the first summer grass seedlings in the warm sunny parts of the lawn, the window has closed for the year and you have moved to a post-emergent job.

The principle underneath both windows is the same: watch soil temperature, not the calendar. A soil thermometer costs very little and takes the guesswork out. Get the barrier down before the relevant threshold and you prevent the whole season's weeds in one pass.

What to do if the weeds are already up

If you have missed the window and the grassy weeds are already growing, a pre-emergent will not help this round. The move is to knock down what is there first, then lay the pre-emergent to stop the next generation. For grassy weeds already up, that means a post-emergent suited to your grass type. For broadleaf weeds like bindii and clover, Bow and Arrow handles those, and we cover the full broadleaf list in our broadleaf weed guide. Once the existing weeds are dealt with, the pre-emergent stops you being back in the same spot next season.

Spartan, the pre-emergent we use

Spartan contains 480g/L prodiamine and is one of the most reliable pre-emergents available for home lawns. It controls the full range of grassy weeds from a single application: winter grass, summer grass, crab grass, crowsfoot, barnyard grass, African lovegrass, Parramatta grass and rat's tail fescue. One well-timed application gives up to 6 months of protection, which is enough to cover a whole germination season.

It is safe on established couch, buffalo, kikuyu, zoysia, carpet grass, Queensland blue couch, Bahia grass and seashore paspalum, so it suits almost any Australian lawn. It is unscheduled and exempt from poison scheduling.

How to apply it

The rate is 10 to 30ml per 100m2 in 2 to 5L of water, through a backpack or garden sprayer. Use the higher rate of 30ml where weed pressure is high or you want the full 6 months of protection. Use 10ml for standard situations and expect around 4 months. Do not exceed the label rate.

At 10ml a 500ml bottle covers up to 5,000m2. At 30ml it covers around 1,666m2. Either way it is several applications for a standard suburban lawn.

Two things matter for it to work:

• Apply evenly across the whole area. The barrier only works where the product lands, so any gaps in coverage are gaps weeds will find. Even, consistent spraying is essential.
• Water it in thoroughly within a few days of applying. This is what settles the prodiamine into the soil and activates the barrier. Without watering in, it cannot form the treated zone and will not work.

Avoid mowing for 2 days before and after application, and do not apply to stressed or water-deficient turf.

Once it is down and watered in, do not disturb the soil surface. Cultivating, coring or heavy scarifying after application breaks the barrier and lets weeds through. If you are renovating, do your renovation first and apply the pre-emergent after the soil has settled.

Complete the program

A pre-emergent is one layer of weed defence. The other, and the one that lasts, is a lawn dense enough that weed seeds have nowhere to establish even if some slip through. A thin, hungry lawn is an open invitation. A thick, well-fed one crowds weeds out on its own.

That density is what the Turf & Surf 5-step program builds. Run the program to keep the lawn thick through the season, and use Spartan at the right window to stop the grassy weeds the lawn cannot crowd out by itself. Together they are far more effective than either alone.

New to the program? The Starter Pack covers Soak, Vital and Surge for $150.

Frequently asked questions

When should I apply pre-emergent in Australia?
There are two windows. Autumn, around March to April, to stop winter grass. And late winter into spring to stop summer grasses like summer grass and crab grass. In warmer states the spring window is late winter to early spring, in cooler southern states it runs a little later. Watch soil temperature rather than the calendar, and always apply before the weeds germinate.

Will Spartan kill the weeds already in my lawn?
No. A pre-emergent only stops weeds germinating. It has no effect on weeds that are already up. If weeds are already growing, deal with those first, then apply Spartan to prevent the next generation.

What rate should I use?
10 to 30ml per 100m2 in 2 to 5L of water. Use 30ml for high weed pressure or when you want the full 6 months of residual. Use 10ml for standard situations, which gives around 4 months. Do not exceed 30ml per 100m2.

Do I have to water it in?
Yes. Spartan needs watering in thoroughly within a few days of applying to settle into the soil and form the barrier. Without watering in it will not work. Rain counts, so timing an application ahead of light rain works well.

Is Spartan safe on my lawn?
Yes, it is safe on established couch, buffalo, kikuyu, zoysia and several other common turf types. It is a pre-emergent though, so do not apply it to a lawn you have just seeded or are about to seed, because it stops grass seed germinating along with the weeds.

How long does one application last?
Around 4 months at the 10ml rate and up to 6 months at 30ml, from a single well-timed, properly watered-in application. In high-pressure situations or extended germination periods a second application may be needed.

Can I apply it after renovating my lawn?
Do your renovation first, let the soil settle, then apply the pre-emergent. Applying it first and then cultivating, coring or scarifying breaks the soil barrier and lets weeds through.