Most annual weeds such as crabgrass start to grow in the spring. During this season, the temperature of your lawn’s soil begins to warm up and provide a very good environment for germination.
The best time to apply crabgrass preventer is early spring just before the rain and not after rain, usually when soil temperature rises to about 55 °F. Put down the herbicide just before it is forecasted to rain but not on wet grass. Rain will wash the pre-emergent into the soil and stop crabgrass seeds from germinating.
Some pre-emergents such as are not very effective and can allow a few seeds to germinate even with the perfect timing of the season, month, and soil temperature. Some of the crabgrass pre-emergents I’ve found to be very effective include:
|1. Scotts Halts Crabgrass and Grassy Weeds Preventer|
|2. Syngenta Tenacity Turf Herbicide|
|3. Barricade 65 WDG Pre-Emergent Herbicide|
Keep in mind the timing for application may differ depending on the climatic zone or region you fall in. The tips I’ve illustrated below will help you get it spot-on.
When to apply crabgrass preventer
Three things will help you know it is the right time to apply a crabgrass preemergent herbicide:
- Season -- early spring
- Soil temperature rising to about 55 °F
- When the weather forecast shows it’s about to rain.
Here’s a quick infographic with the guide. I’ve explained in detail below the graphic.
Crabgrass is an annual weed. As such, it spreads by seed and not through rhizomes. You want to control crabgrass by preventing its germination with a pre-emergent, otherwise, killing full-grown weeds can be very difficult.
Timing Tips to Put Down Crabgrass Pre-emergent
There are various methods you can use to know the right time to put down pre-emergent in your lawn. While most people choose to use a weather map, the method can be erratic because the forecast is usually for the larger region and not your specific area.
The timing should, therefore, be on point, but how do you know when to apply crabgrass preventer? Here are a few pointers to help you with the timing:
Early Spring Season/Month
Crabgrass germinates from seeds that dropped in summer or fall. These seeds stay in the soil through winter and start germinating when the weather starts to get warm -- usually during spring.
To prevent crabgrass from germinating in your lawn, apply a pre-emergent early spring because this is the time when the weed will start to sprout in your lawn.
Any time between late March and early April are the perfect times to put down a preemergent herbicide such as Scotts Halts Crabgrass and Grassy Weeds Preventer, Ortho Crabgrass Preventer, etc.
If you’re late, you can still do this in early May.
Apply in early spring to prevent the germination of crabgrass, chickweed, poa annua, and other common lawn weeds before they start to spread. Apply before the 3rd or 4th mowing to protect all season long. Scott’s Turf Builder Halts Crabgrass (label).” -- Scotts
Warm Soil Temperature
Spring is when the sun starts to come out. This is when a lot of plants germinate because the heat from the sun warms-up the soil, activating enzymes that stimulate seed germination.
Crabgrass seeds start to germinate when the soil temperature reaches about 55 °F to 60 °F, usually from late spring through to mid-summer.
Soil temperature may sometimes be affected by other factors such as wind, shade in your lawn, and the general climate of where you live. So be sure to take soil temperatures on a daily basis as soon as March starts.
The best time to put down your pre-emergent is when the soil temperature is around 55°F to inhibit the root development of the germinating crabgrass seeds. This will go a long way in helping you control this tenacious grass weed for the whole season ahead.
Before or After Rain?
The best time to apply crabgrass preventer is before the rain. After it rains, the preventer will have been washed into the soil and crabgrass seeds to stop them from developing roots and germinating. If you apply after rain, water your lawn immediately after to wash the pre-emergent into the soil.
When the rain comes, it will wash the herbicide into the soil and the crabgrass seed killing it before germination.
You should be careful not to apply crabgrass preemergent herbicide too early because it will break down and become less effective.
Most good crabgrass pre-emergent products indicate that it is a good practice to water the lawn a few days after application in order to wash off the concentrate into the soil.
Pro tip: Follow your local weather forecast closely. As soon as the forecast shows it will rain in a few hours, start putting down a crabgrass preventer in your lawn. In case it does not rain 2 to 3 days after application, ensure that you apply water on the herbicide as per the instructions on the product’s label.
Can you apply crabgrass preventer on wet grass?
When applied on wet grass, crabgrass preemergent will most likely stick on the leaves. Some products such as Crabgrass Barrier recommend applying it on wet grass. However, you may need to water your lawn soon after putting down the herbicide.
The reason for this is that you really want the herbicide deep in the soil to prevent crabgrass from sprouting. So, if you apply on wet grass and then it fails to rain, ensure you water your lawn with a sprinkler to wash the herbicide into the soil.
How long do pre-emergents last?
Pre-emergents last 2 to 6 months, meaning they will prevent the germination of weeds in your lawn for quite a long time.
However, not all of them are the same, so be careful not to apply a pre-emergent that might deter your grass seeds from growing.
If you already have crabgrass germinating in your lawn, use a good crabgrass killer to destroy the grassy weed.
How often can you apply crabgrass preventer?
Since most weed prevention herbicides last up to 6 months, it is highly recommended that you apply twice per year.
You should allow at least 2 to 6 months between applications, and ensure that you do not apply on newly seeded lawns. Some pre-emergents can cause slow or poor germination of some grass seed species.
If you’re late, you might want to try post-emergent weed killing options. Check out my ultimate guide on the best ways to get rid of crabgrass in your lawn here.
Read Herbicide Labels
The best way to determine a good time to put down a pre-emergent herbicide for preventing weeds such as crabgrass, foxtail and goosegrass is to read the label of your preferred herbicide.
Each manufacturer recommends the best time to apply for the best result. Some products will give you season-long control of lawn weeds while others provide short-term solutions and probably spaced-out applications
“Barricade can even be applied in the fall for crabgrass control the next season. Dimension can be applied as early as March 1.” -- Kansas City
If you are not very experienced at caring for your turf, I would recommend that you try these long-term solutions for preventing and getting rid of unwanted weed grasses in your lawn.
Types of Pre-emergents
So far, we can categorize these herbicides as short and long residual preventers. Your current problem and level of experience may determine which one you should choose as a pre-emergent solution.
Short residual pre-emergent
These usually last for about 60 days after application. Though the best time to apply may depend on the weather pattern, the best time to apply crabgrass preventers with short-lived effectiveness should be around Mid-April.
Examples of short residual preventers on the market today include Scotts Halts, Team, and Tupersan Crabgrass Control.
Long-term residual preventers
If you have no idea how to time the application of a grass weed preventer, I would recommend you go with the long-term solutions.
A single application of these herbicides will rid your lawn of crabgrass throughout the season.
The best examples include Barricade and Dimension. You can apply any one of these earlier than the timing of the short-term options. The effect will last throughout the season.
Q: Can I use pre-emergent to kill existing weeds?
It is a rule of the thumb that most pre-emergent herbicides do not get rid of crabgrass and existing weeds. This goes for Scott’s Turf Builder Halts Crabgrass and Barricade – the most popular preventers.
However, Dimension can be used when crabgrass is still very young (about two leaves young).
When should I water after applying a crabgrass preventer?
If it does not rain a few days after application, it is a good practice to water your turf 2 to 3 days afterward. This will wash the herbicide off the leaves into the soil where it will prevent the germination of grass weeds.
Crabgrass Control Before Germination
Your timing needs to be perfect for the best results. Here are tips to help you with the timing.
- Take and track soil the temperature of your lawn. The best time to apply a crabgrass pre-emergent is just before the rain when the temperature is between 50 and 55°F.
- You can observe dogwoods or forsythia and use them as an indicator. In this case, start the crabgrass control measures just before the dogwoods begin to bloom.
- Michigan University Extension: Timing crabgrass pre-emergence applications
- ACSESS Digital Library: Degree-Days for Predicting Smooth Crabgrass Emergence in Cool-Season Turfgrasses