Is It Advisable to Apply Pre-Emergent to Grass Late After Seeing Weeds?

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Is It Advisable to Apply Pre-Emergent to Grass Late After Seeing Weeds?

Dealing with weeds is an inevitable part of having a lawn. These parasitic plants grow quickly and suck up all the nutrients meant for your grass, making them difficult to get rid of once they’ve started growing. To combat them, many people use pre-emergent herbicides, which target the weeds while they are still seedlings. However, if you miss the application period, it’s not worth using pre-emergent on visible weeds.

Pre-emergent herbicides work by attacking germinated seeds and stopping them from developing their root and shoot systems. Although seeds are typically strong and durable, they become vulnerable during the germinated seedling phase when it’s time to start growing internally. This is when the pre-emergent strikes.

To use pre-emergent, you apply it to the soil in liquid or solid form. When you water the area, the herbicide is activated by moisture, allowing it to spread its surface area and cover the weed seedlings underground. Once the seedlings come into contact with the herbicide in their quest to take root and grow, they die. However, pre-emergent won’t work on already-emerged weeds.

Timing is the most important step

Gardener spraying herbicide

Banprik/Getty Images

Because pre-emergence herbicides only work on germinated seedlings that are about to take their next steps into independent life, timing the application is tantamount to its success. It helps to know what type of weeds you’re dealing with and understand their life cycles. Then, you can be sure which pre-emergent is best to tackle the problem. There are pre-emergent herbicides for every yard type — be it for your grass, your flower garden, or specific weed species. Be sure to follow the labels, or consult a weed specialist or botanist if you’re having trouble figuring out what to use.

The ideal time for pre-emergent application is one to two weeks before the weed seeds germinate. This typically puts the time anywhere from early spring to the summer. Once again, the type of weed you’re facing makes all the difference. Late-bloomer weed seeds such as grassy weeds will require pre-emergent application later in the spring or early summertime compared to early-bloomer seeds that germinate as soon as the ground thaws out.

One thing about pre-emergence herbicides is that they can become redundant with an increase in temperature. This is because even though it’s insoluble, the herbicide is broken down by soil microorganisms that regain activity during the warm season. During the colder months, these microbes were more or less dormant. The earlier the application, the better for your lawn.

How to apply the pre-emergent to your lawn

Spreading granular substance on lawn

Ingrid Balabanova/Shutterstock

When you’ve researched what weeds are predominant in your yard and have found the right pre-emergent to use, you need to apply it at the most opportune time. This may sound backward, but if you have a weed problem in your garden and you are using a pre-emergent, then you should want as many weed seeds to germinate as possible. If they don’t germinate, you can apply the pre-emergent from now until the next ice age — it won’t have the effect you’re looking for.

Follow the instructions on the herbicide label to the tee. If it’s in granular form, then you have to use a spreader to apply it evenly across your garden or lawn as specified. If it’s a liquid pre-emergent, pour it into a sprayer so that you can cover more ground uniformly. Make sure the ground gets a healthy dose of water afterward. If you live in a rainy zone, then rainfall would be great. If you have to settle for your sprinkler, do so.

The length of time that pre-emergent weed killer lasts is dependent on its chemical makeup, the soil type, and the activity of the soil microbes. Some pre-emergents have a longer-lasting effect than others, but they typically have a lifespan of at least a few months. Hopefully, it’s enough time to kill as many weed seedlings as they can.

Dr Heidi Parkes

By Dr Heidi Parkes

Senior Information Extension Officer QLD Dept of Agriculture & Fisheries.