Mature crabgrass plants drop their seeds in the soil from late summer to fall. The seeds remain dormant in the soil while the mature plants die. So, when exactly do crabgrass seeds germinate?
Crabgrass seeds germinate in the spring when soil temperature starts to warm up above 55°F. Seed germination usually coincides with the blooming of forsythia plants and may continue until late summer. Put down crabgrass preventer early in the spring just before the seeds germinate to control the weed.
If you’re timing to put down crabgrass pre-emergent such as Scotts Crabgrass Preventer, take soil temperature readings from the onset of spring to determine when crabgrass will begin to germinate.
When does crabgrass germinate?
Crabgrass begins to germinate in the spring when soil temperatures start to rise above 55°F consistently for at least 3-5 days. Some seeds will continue to germinate throughout spring to late summer. Plants that germinated in the spring go to seed in late summer when vegetative growth slows down.
If you’re tracking soil temperature to make sure your crabgrass pre-emergent is well-timed, you’ll need to observe for a consistent soil temperature above 55°F at a soil depth of 4 inches.
But why is it important to know when crabgrass begins to germinate?
Unlike most weeds that can be controlled with the application of a pre-emergent, the timing for crabgrass germination is really important. A crabgrass preventer MUST be applied just before germination of its seeds in order to control crabgrass from spring to summer.
- A pre-emergent herbicide applied too early can disintegrate into the soil, making it less effective.
- Pre-emergent herbicides, technically, do not prevent germination, but stop root development, forcing the newly germinated weed to die off before sprouting out of the soil.
In this sense, if you apply the crabgrass preventer too early or too late, it may not be effective at stopping crabgrass from germinating.
At what temperature does crabgrass germinate?
Crabgrass germinates when the average daily soil temperature is between 55°F and 70°F for 3 to 5 days at a depth of 2-4 inches. The soil should also be moist to facilitate the germination and root development of at least 80% of crabgrass seeds.
You’ll notice crabgrass germinating along concrete driveways first before sprouting in the rest of the yard because of the warmth retained by concrete right at the onset of spring’s sunny weather. This should be a sign that you need to put down a pre-emergent herbicide to stop the weed from germinating.
Does crabgrass germinate in the fall?
Crabgrass does not germinate in the fall because soil temperatures start to drop. Being the onset of the cold months and frost, seeds remain dormant during fall until when the weather becomes warm again starting early spring when germination begins.
Typically, frost kills crabgrass plants in the fall, leaving behind bare spots in your lawn. This is what causes a thin lawn. It is recommended that you start overseeding about 45 days before the first frost in your region if your lawn is infested with weeds like crabgrass.
Overseeding will help fill in the bare spots and make your lawn thicker and able to compete against weeds come spring when they start to germinate.
Crabgrass is an annual weed that reproduces by seeds and branches very well. A mature plant will have purple seedheads, with a single plant capable of producing up to 150,000 seeds. This is why crabgrass can easily take over a lawn if not controlled at germination early in the spring.
How do you keep crabgrass from germinating?
Crabgrass is an aggressive weed that grows really fast and can outcompete most turfgrasses if not suppressed with a crabgrass killer that’s safe for lawns.
Here are 3 different ways to keep crabgrass from germinating in your lawn:
Apply a crabgrass preventer
The best way to keep crabgrass from germinating is to use a pre-emergent herbicide such as Scotts Crabgrass Preventer. It works by stopping the germination process of the weed, forcing it to die before sprouting out of the soil.
I wrote this complete guide on when to apply a crabgrass preventer to stop seeds from germinating.
Be careful with your selection of a pre-emergent because it can stop you from planting grass. Some herbicides will affect the germination rate of grass seed.
In addition, some herbicides such as Tenacity Turf Herbicide not only stop crabgrass germination but can kill the weed post-emergence.
Mow high to create shade on soil
Crabgrass likes to grow in areas that receives enough direct sunlight. You can prevent the seeds from germinating by mowing your turfgrass high (just cutting the 1/3 of the leaf blades). This works by keeping the soil cooler because it is well shaded.
You might also want to mulch with grass clippings as you mow, except if you’re mowing a lawn that’s full of weeds which may need bagging.
When young crabgrass seedlings don’t get enough sunlight close to the soil surface, they will die post-germination.
Keep your lawn thick
Bare spots in the lawn invite crabgrass to germinate and spread quickly as opposed to a thick turf that can outcompete crabgrass and force it to die.
To keep crabgrass from germinating, seed bare spots in early spring and in the fall to make sure your lawn is dense enough to prevent weeds from germinating easily.
You might want to remove crabgrass with a weeder if the invasion is not extensive to prevent the weed from going to seed. Once uprooted, fill in the bare spots with new grass seed to reduce chances of weeds establishing.
References + Resources
- University of Massachusetts Extension: Biology and Management of Crabgrass
- Purdue University Turfgrass Science: When Will Crabgrass Germinate?
- Dr. Jatinda S. Aulakh and Ms. Rose T. Hiskes, Connecticut Agricultural Experiment Station: Crabgrass Control in Home Lawns
- Janet Hurley, Texas A&M Agrilife Extension, School of Integrated Pest Management: Crabgrass Germination Advice for Homeowners
See also: Baking Soda Homemade Crabgrass Killer