A Guide to Pre-emergent Herbicides and How They Work

Written by Ian Thompson

Ian holds a Masters in Turf Management from Sydney University, has over 20 years experience working as a golf course Superintendent, turf manager, product formulation and development. We call him our resident Lawn genius.

A lush, weed-free lawn is one of our major lawn goals. Achieving this can seem like a non stop process of back breaking weed pulling but it doesn’t have to be. Not only do we have herbicides to kill existing weeds but a pre-emergent can prevent a weed (particularly grass weeds) from ever entering your lawn. Unlike post-emergent herbicides, which target existing weeds, pre-emergent herbicides act as a proactive shield, preventing weed seeds from establishing in the first place.


Pre-emergent herbicides create a protective barrier in the soil, inhibiting the growth of weed seedlings as they attempt to sprout. This preventive approach is particularly effective against annual weeds, disrupting their life cycle and reducing the need for extensive post-emergent intervention.


What are the Different Pre-emergent Herbicides?

A variety of pre-emergent herbicides are available in the market, each with its own specific strengths. Understanding the options allows homeowners to choose the most suitable product for their lawn’s unique needs. Here are some common pre-emergent herbicides:



Mode of Action: Prodiamine works by inhibiting cell division in germinating weed seeds. This means a new weed seed takes up the herbicide through it’s roots and is unable to make new roots, without the roots the seed doesn’t even make it to the surface.

Reference Products: Spartan and Barricade are well-known brands that utilise prodiamine as their active ingredient.

Target Weeds: Common annual grasses like wintergrass and some broadleaf weeds.

Application Timing: Apply before the anticipated germination period of target weeds. This pre-emergent lasts for a long time (up to 6 months), with applications recommended in early March and again in September. Do not use on new turf or cool season varieties.



Mode of Action: Oxadiazon disrupts the cell wall development of germinating weed seeds. This means the seed (or new turf) can still put down roots but the new leaves are inhibited by the chemistry and so the weeds fail and die.

Reference Product: OxStar is a notable product that harnesses the power of oxadiazon.

Target Weeds: Common annual grasses like wintergrass and some broadleaf weeds.

Application Timing: Apply before the emergence of target weed seedlings. Apply every 10-12 weeks as per label directions.


When to Use These Products on Your Lawn

You may notice some subtle but very important differences between the two modes of action and their longevity. As Barricade and Spartan lasts for 6 months it’s our go to product for normal maintenance use but when it comes to new turf where we don’t want to stop new roots from forming that’s where OxStar really shines.


Timing of pre-emergent herbicide

As you can imagine a pre-emergent does its best work before the weed comes up. So timing must be based on when those weeds are likely to germinate. For winter annual grasses like Wintergrass, an application in late-February before soil temperatures drop below 13 degrees Celsius will help prevent the weeds emerging, don’t wait until you can see them, going a few weeks early is good practice not a waste.


When it comes to preventing Summer annual grass weeds like Crab grass and crows foot it’s best to apply in early September as soils start to come up through the 13 degree window, for most areas this is September but can be earlier in warmer areas. Again, going early is good practice. With this application you can also add Acelepryn liquid, creating a tank mix to provide protection against weeds and insects, giving you piece of mind against both.


An added bonus of a pre-emergent

While pre-emergent herbicides like Spartan and Barricade are primarily designed for lawns, they can also be utilised in garden beds to prevent weed growth. Here’s how to use them effectively:

  1. Start by removing existing mulch, the herbicides will stick to this and not make it to the soil where weeds are germinating.
  2. Ensure product placement as close to soil surface as possible.
  3. Wash in immediately with minimum 3 mm rain or irrigation


By understanding pre-emergent herbicides you can add a very powerful tool to your weed control arsenal. Selecting the right pre-emergent herbicide and applying it with care will pave the way for a weed-free haven in your outdoor space, including both lawns and garden beds.

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