Applying Pre-Emergent Weed Killer and Grass at the Same Time

Applying Pre-Emergent Weed Killer and Grass at the Same Time

When trying to achieve a perfect, weed-free lawn, it can become a struggle. You may be wondering if you can take shortcuts by using both pre-emergent and weed killer at the same time. However, there is a debate on whether these treatments should be applied separately for maximum effectiveness. While some experts caution against mixing these treatments, others encourage it. Therefore, it can be confusing to decide what to do. The decision to use both treatments at the same time may depend on the types of weeds you are dealing with and the specific products you choose.

It is crucial to remember that using herbicides correctly is a legal matter. The Center for Invasive Species and Ecosystem Health highlights the importance of following herbicide labels. It is illegal to use these products in a way that does not align with their instructions. Therefore, before even considering mixing or applying any products, it is essential to carefully read and understand how both pre-emergent and weed killers work.

Understanding how pre-emergents and weed killers work

person pouring herbicide


Before deciding to apply pre-emergent and weed killer simultaneously, it’s important to understand the roles these products play and how they interact with your lawn. So, exactly what is a pre-emergent herbicide? Well, these are herbicides designed to prevent weed seeds from germinating, effectively stopping new weeds before they start. They can be selective, targeting only specific types of weeds while leaving the grass unharmed, or non-selective, killing any plant they come into contact with. Pre-emergent herbicides need to be applied at specific times to be effective, typically in early spring before weed seeds begin to germinate. Timing is crucial; applying too early or too late can render the product ineffective. Additionally, these herbicides require proper watering to be activated and form the barrier that prevents weed germination.

On the other hand, weed killers, also known as post-emergent herbicides, target weeds that have already sprouted and are actively growing in your lawn. Just like pre-emergents, they can also be selective. However, they can be applied whenever weeds are actively growing. These products often require direct contact with the weed to be effective, and their efficacy can vary based on the weed’s growth stage and the weather conditions. Some post-emergent herbicides might harm your grass if not used correctly, making it essential to choose a product that is safe for the type of grass in your lawn.

What are the alternative solutions?

person spraying herbicide


Instead of juggling multiple herbicide types, you might find it smarter to go with a single, versatile option like dithiopyr, commonly known under the trade name Dimension 2EW Herbicide. This product offers broad-spectrum control, acting both as a pre-and post-emergent solution for weed management. You can pour it directly into the soil or integrate it into your watering routine. What sets Dimension 2EW apart is its ability to tackle crabgrass before it even breaks the surface, along with its effectiveness against a wide array of nearly 50 other grassy and broadleaf weeds. However, there’s a crucial consideration to keep in mind: Dimension 2EW demands precise timing. Its potency means that it can harm desirable grasses if their roots haven’t fully taken hold. This underscores the importance of applying it at just the right moment to ensure you’re combating weeds without jeopardizing the health of your lawn. Navigating this timing can be tricky, but getting it right means you benefit from comprehensive weed control with a single product.

If you’d rather opt for the simultaneous application of both pre-emergent and weed killer, identify the weeds in your lawn to choose the most effective products. Using a selective post-emergent herbicide that targets the specific weeds you’re dealing with can reduce the risk to your grass. Read and follow the label instructions carefully for both products. Mixing incompatible products can damage your lawn or reduce their effectiveness.

Dr Heidi Parkes

By Dr Heidi Parkes

Senior Information Extension Officer QLD Dept of Agriculture & Fisheries.