The lawn weed, crabgrass (Digitaria spp) is a warm-season annual weed, which means it reproduces by seed. People often ask me how to kill crabgrass. The short answer: applying preemergent herbicides at the right time is the best way to eradicate this troublesome plant.
Preemergent Herbicides for Killing Crabgrass: When to Apply
To get rid of crabgrass (see pictures at right), it helps to know its life cycle. When spring soil temps (at a depth of 2"-3") reach 55-60 degrees Fahrenheit, the first crabgrass (also spelled "crab grass") seed will germinate. From mid-summer to fall, crabgrass produces seed. The crabgrass plants (but not the seeds) are killed by frosts in autumn.
Preemergent herbicides (also spelled "pre-emergent") come in either granular or liquid form and kill crabgrass seedlings as they germinate. Think of preemergent herbicides as forming an invisible shield across the soil surface that stops emerging crabgrass dead in its tracks. This shield image will serve as a reminder not to practice core aeration on lawns after applying preemergent herbicides, since doing so would only "puncture" the shield. Aerate lawns beforehand, instead.
As their name suggests, preemergent herbicides kill crabgrass at a specific time: before its seedlings emerge. For success in getting rid of crabgrass in this manner, timing is of the essence. Apply preemergent herbicides before germination, but not too far ahead. Crabgrass germination coincides approximately with the blooming of the lilac bushes. Thus the old saying that preemergent herbicides should be applied sometime between the time the forsythia bushes (which precede the lilacs by a few weeks) stop blooming and the lilac bushes begin blooming.
Types of Preemergent Herbicides for Killing Crabgrass
There are many different types of preemergent herbicides for killing crabgrass. "Weed and feed" products often contain preemergent herbicides, although some question whether their concentration is strong enough to be effective. I will focus on 2 preemergent herbicides: Dimension and Tupersan.
Dimension (active ingredient, dithiopyr) is safe to use on most lawn grasses (check label first) and provides long-lasting coverage -- an important consideration, since not all crabgrass seed germinates at once. Thus Dimension will kill later-germinating crabgrass, too. Dimension also displays some effectiveness as a postemergent herbicide.
Tupersan (active ingredient, siduron) is worth mentioning because, unlike other preemergent herbicides, it will not damage germinating lawn grass seed. In fact, its active ingredient is often combined with starter fertilizers. Alternatively, for newly-seeded lawns, wait until after 3 mowings (or 3 months, to be on the safe side) before applying preemergent herbicides.
Using Preemergent Herbicides for Killing Crabgrass: DOs and DON'Ts
- Irrigate afterwards: water activates preemergent herbicides.
- Re-apply preemergent herbicides, if you question your product's coverage. Because crabgrass seedlings do not all germinate at once, re-application can kill some of the later-germinating crabgrass.
- Follow label directions and apply the proper rate. Measure the lawn area and calibrate your spreader carefully.
- Dethatch or aerate the lawn after applying preemergent herbicides.
- Apply preemergent herbicides on new sod.
There are also postemergent herbicides for killing crabgrass well after it has germinated (e.g., Acclaim Extra), but they are effective only at killing young plants. Because these young plants are small, they are very difficult to detect in a lawn -- rendering postemergent herbicides far less useful than preemergent herbicides for killing crabgrass.
Crabgrass Control: Organically
"But do I really need chemicals for crabgrass control?" you ask. The answer is, No. While the best way to kill crabgrass is with preemergent herbicides, the best way to control it is by having healthy green grass. Here are some ways to promote lawn health -- at the expense of crabgrass:
- Fertilize (compost is fine) more heavily in autumn than spring. By autumn, frosts will have already killed any crabgrass.
- Don't let bare spots remain uncovered for long, else opportunistic crabgrass will take root. In the fall, fill in those bare spots by overseeding.
- When irrigating the lawn, water more deeply and less frequently. Crabgrass is a notoriously shallow weed.
- One of the most important tips on mowing lawns that I can offer as it relates to weed control is to "mow "high." This means leaving the lawn grass at a height of 2 1/2"-3". Doing so will allow the lawn grass to "protect its own turf" better, depriving crabgrass seeds of the light they need to germinate.
Final Notes on Crabgrass Control
A great organic "weed and feed" product is corn gluten. An organic preemergent herbicide, corn gluten will suppress crabgrass germination, while fertilizing your lawn.
Don't forget good old weeding as a method of crabgrass control. Hand-pulling small patches of crabgrass before it goes to seed makes eminent sense. To facilitate weeding, water the lawn first (weeds are more easily extricated from wet soil).