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Crabgrass Control

The scourge of summer is vulnerable in the spring


Crabgrass Control

Large Crabgrass

photo © Rasbak/Wikimedia Commons

Crabgrass can be a difficult weed to control. It usually flourishes in the late summer but is best dealt with in the spring before or during seed germination. Crabgrass can spread quickly, especially into bare areas and compacted or stressed lawns. The most common method of controlling crabgrass is the use of pre emergent herbicides in the early spring as the seeds begin to germinate. The natural way to control crabgrass is to maintain a thriving healthy lawn an overseed often.

What is crabgrass?

The two main species of crabgrass are Large Crabgrass, sometimes called Hairy Crabgrass, and Smooth Crabgrass. An annual plant capable of producing 150, 000 seeds per plant, per season, preferring dry, poorly drained, under-fertilized soil and thin turf or bare patches. Crabgrass is known for it's light green color and coarse, hairy leaf blades on thick stems expanding outwardly to a diameter of 12 inches per plant. Crabgrass is capable of outcompeting and taking over an entire lawn in a season if the conditions are right.

When Does Crabgrass Control Begin?

Timing is everything when it comes to crabgrass control. Normally, the blooming of forsythia will approximate the time of crabgrass germination and provide a window of several weeks to apply pre-emergent herbicide.

While the flowering of forsythia is a good cue, crabgrass germination actually occurs after the soil temperature has reached 55ºF for several consecutive days.

What is a pre-emergent herbicide?

A pre-emergent herbicide is a weed killer applied prior to the emergence of the weed from the soil. Chemicals like dithopyr (Dimension) and pendimethalin (Pre-M, Scotts' Halts) prevent all seeds from germinating, including grass seed. Grass seed cannot be applied for 6-12 weeks after application, depending on the specific product.

The chemical siduron (Tupersan) prevents only  the weed seeds from starting, allowing grass seed germination. It is costlier and usually used sparingly.

Corn gluten meal is an organic alternative initially used as feed on hog farms but discovered to have pre-emergent characteristics.

How does corn gluten work?

Corn gluten meal is also about 10% nitrogen by weight, meaning one hundred pounds of corn gluten contains 10 lbs of nitrogen. This organic source of nitrogen is slowly released over a 3-4 month period.

Corn gluten inhibits root formation of germinating seeds. Timing a corn gluten application is crucial for it to work properly. Corn gluten needs to be applied before weed seed germination. The seed will germinate and form a shoot, but not a root. Prior to germination, a short drying period is needed to kill the germinated, but rootless, plant. If conditions are too wet during germination, the plant will recover and form a root. If seeds have already germinated, a late application of corn gluten will only serve as fertilizer for the weeds.

Applications of corn gluten should be timed with rainfall. If it doesn't rain within 5 days of application, it needs to be watered in with 1/4 inch of water. A dry period of a day or two must follow the watering in to prevent the seedling from growing another root.

How much corn gluten is needed?

For corn gluten to be effective as an herbicide it needs to be applied at a rate of 20 lbs/1000 sq ft. This rate suppresses crabgrass seedlings and provides about 2 lb of nitrogen per 1000 sq ft. Heavy soils, extended rainy weather and hot spells may require monthly application or a repeated application in late summer. The effects of corn gluten are cumulative, meaning results improve with repeated use over time.

What's the downside of using corn gluten?

The high rates needed for corn gluten to be effective results a costlier product than conventional pre-emergent herbicides. It also means that you could be handling hundreds or even thousands of pounds of the product, depending on the size of the yard. Sprayable, liquid forms of corn gluten can make applications easier but are still costly.

The first application of corn gluten will only suppress up to 60% of the weed seeds. The initial results may be disappointing but after several application it can achieve better than 80% effectiveness.

Proper timing is crucial, applying too late can drastically reduce the effectiveness of the herbicide. This is true for chemical pre-emergents too, though there can be some post-emergent effect with some chemical products. It's important to remember that most crabgrass preventer suppresses all seed germination for up to 12 weeks, including grass seed.

Why use a pre-emergent at all?

Crabgrass is a filler weed. It will thrive in areas with thin turf or bare patches. Many organic turf growers contend that a bag of grass seed can be as effective at suppressing crabgrass as any pre-emergent herbicide. Not giving the seeds a place to germinate is the best prevention. Dense, healthy turf will naturally crowd out crabgrass, so growing more grass and filling in those thin areas and bare patches will result in minimized crabgrass pressure.

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