Crabgrass weeds with seed

How to Prevent and Kill Crabgrass

What Is Crabgrass and Why Is It Bad?

Crabgrass is an unattractive annual weed that enters your lawn (usually through bare spots). Its coarse texture and variation in color makes it stick out and prevents you from achieving a uniform looking lawn. It is a problem weed in both cool season and warm season lawns.

When Does Crabgrass Germinate?

Crabgrass can germinate whenever soil temperatures are ideal. Once soil temperatures hit 55 degrees crabgrass seeds begin to germinate. Peak germination is around 65 degrees, and once soil temps are in the high 70’s most crabgrass seeds that are in the soil should already be germinated. When soil temps are in the 80’s it’s too hot for crabgrass and most other broadleaf weeds to germinate.

During the summer and early fall, crabgrass will drop its seeds. The seeds remain in the soil over the winter and germinate the following season as soil temps warm up after the winter.

The summer months is when you’re likely to see crabgrass thrive. This is what confuses people into thinking that new seed is germinating but that’s not the case. At this point, what you’re really seeing is the already germinated crabgrass becoming more mature as it flourishes during these hot months and outcompetes the rest of your lawn, especially for those with cool season grasses.

What Does Crabgrass Look Like?

Crabgrass has to be the most misidentified weed there is. Pretty much anytime there is a grass like weed with wide leaf blades people call it crabgrass. This is a serious problem though since there are many other undesirable grass types that resemble crabgrass but won’t react to the same treatments.

Tall clumping fescue for example might look like crabgrass at first glance but it is very different and requires different control methods.

Crabgrass is usually light green in color and stays low to the ground, usually landing underneath mower blades depending on height of cut. The only part of the weed that grows tall is the stalk which carries tiny flowers that eventually turn to seed.

For a more in depth look on identifying crabgrass check out: Organo-Lawn

Soil Temps And Crabgrass

Crabgrass starts to germinate when your soil temperature hits 55 degrees. The best thing you can do is find out when your soil temperature hits 50 degrees and apply your crabgrass pre-emergent then. If you apply when soil temps are at 50, by the time it hits 55 the pre-emergent will already be in your soil.

You’re probably wondering how you’re supposed to know when your soil temperature is 50 degrees. Luckily, there is a great site called GreenCast Online.

All you have to do is go to the site and enter your zip code. Look at the 24 hr. soil temperature average and when it hits 50 degrees it’s time to throw down your first application.

About 5-6 weeks later you want to apply another round of pre-emergents as soil temps hit 65 degrees.

Once soil temps go above 80 degrees crabgrass germination comes to a halt. Adding a pre-emergent at this time won’t be very effective. I should point out that at the end of summer as soil temps fall back down to 70 degrees there are fall germinating broadleaf weeds that can pop up. That’s why I recommend a third application of pre-emergents heading into fall.

For a better understanding of soil temperature and how it should be used in your lawn care program check out: The Importance of Soil Temperatures in Your Lawn

Prevent Crabgrass With Pre-Emergent Chemicals

Of all the lawn care products out there, pre-emergent herbicides are probably the most misunderstood and misused.

Most people rush to the stores the second the weather starts to get nice in early spring, and they load up on a step 1 product that most fertilizer companies sell.

The step 1 for most of these companies contains a crabgrass pre-emergent and a fertilizer all in one. On the label you can find a very broad time frame of when you are supposed to apply.

There’s nothing wrong with applying a step 1 product to your lawn. But for it to be fully effective you have to have an understanding of soil temps.

If you’re looking for a simple crabgrass pre-emergent strategy using the step 1 products you see everywhere then apply the step 1 twice, and at the correct soil temperatures. Your first application should be when your soil temps hit 50 degrees, and your second application should be about 5-6 weeks later as soil temps hit 65. This is a fine strategy if you want to keep things simple.

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* Pro Tip: I actually recommend applying pre emergents for a third time heading into fall when soil temps are decreasing (heading back down to 70 degrees). This will prevent Poa Annua and other fall broadleaf weed seeds from germinating. So keep in mind that while all these products will help prevent crabgrass in the first two applications early in the season, they can be applied a third time when soil temps head back down to 70 degrees heading into fall to prevent fall weeds from emerging. Check out this easy program that shows this schedule here: Lawn Care Schedule Using 3 Products

If you’re looking for next level pre-emergents for crabgrass as well as other broadleaf weeds then consider choosing from the following products below and apply the same soil temperature rules:

1. Dimension (or Dithiopyr): Dimension (active ingredient Dithiopyr) is a very effective pre-emergent that is used by professionals. Dimension can be found in a couple of different products but my favorite and most accessible for homeowners is Lescos 19-0-7 Crabgrass Preventer. This is a great product if you’re looking for a pre emergent and fertilizer combo.

One other benefit of this product is it does provide some post-emergent weed control in addition to pre-emergent. So if you’re getting a late start and you’re afraid you missed the early 55 degree soil temperature go time window, then using Dithiopyr could help knock out some broadleaf weeds that have already begun to germinate.

If you’d like to separate your fertilizer application from your pre emergent application then I recommend using Dithiopyr 0-0-7. I love how this product is easy to purchase online and delivered right to your door.

2. Prodiamaine:

Another great crabgrass pre-emergent is Prodiamine. Prodiamine is the name of the active ingredient so it can be found in several different products. It’s commonly found in products that contain Barricade. To make things less complicated just know that Barricade and Prodiamine are the same thing. If you see a fertilizer that says ‘Fertilizer plus Barricade’ you know you’re getting Prodiamine. In fact a great fertilizer plus Barricade combo is The Andersons 19-0-6 with Barricade.

If you don’t want a fertilizer and pre emergent combo then you can buy Prodiamine 0-0-7 which is just prodiamine with a little Potassium here: Prodiamine 0-0-7. This is what I use for most of my pre emergent applications. This is also easy to purchase online and delivered right to your door.

3. Pendimethalin:

Pendimethalin is the pre-emergent you’re most likely to see in the step 1 programs such as Scotts. Honestly, there’s nothing wrong with Pendimethalin and if that’s what you choose to use for one, or your first two applications then that’s fine. But most professionals would agree that Dimension and Prodiamine are superior to the Pendimethalin.

Once again, If you’re new to lawn care or you’re getting too confused and don’t know where to begin, then keep it simple and just apply Scotts Step 1 for your first two applications. Just stick to the soil temperature guidelines (hitting 50 degrees and then again at 65).

Post Emergent Herbicides to Kill Crabgrass

While prevention is definitely the best method for managing crabgrass, it is likely you will still have some clumps of crabgrass that find a way into your turf. It’s usually in the most vulnerable locations within your lawn. This is when you want to use a post emergent:

1. Ortho Weed B Gone Plus Grabgrass Control– The best way to kill crabgrass that is already established is to spot treat using a liquid, lawn safe herbicide such as Ortho Weed B Gone Plus Crabgrass Control (active ingredient 2, 4-D). This is a selective herbicide so it will kill crabgrass without killing your lawn surrounding the crabgrass.

I recommend going out and walking your lawn once a week to spot treat new clumps of crabgrass that pop up. It will likely take 2-3 repeat applications to knock back more mature weeds.

2. Tenacity– Another great product for killing crabgrass and other weeds is a product called Tenacity (active ingredient Mesotrione). This is a lawn safe, selective herbicide that comes in a liquid concentrate that gets mixed with water in a tank sprayer. What’s great about this product is it’s safe to use when you are seeding your lawn. Most post emergents require you to wait at least 30 days after application to spread seed. Tenacity on the other hand can be applied at the same time as seeding.

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Another added bonus with Tenacity is it can be used as a pre emergent as well. Personally I’m more of a fan of granular pre emergents but once again, if you are planning on seeding within 3 months of application then spraying Tenacity as a pre emergent instead of applying the Dithiopyr or Prodiamine is definitely the way to go.

As always, read the label first before purchasing since this isn’t safe to use on some warm season grasses such as Bermuda Grass.

3. Celsius WGCelsius WG is a great post emergent herbicide that should only be used on warm season lawns. This is one of the best products for killing crabgrass as well as other grassy weeds you find in warm season lawns. It is a concentrate that gets mixed with water in a tank sprayer. If you have a serious crabgrass problem on your warm season lawn then this is definitely the product you want to use. Follow the label for mixing instructions and correct usage.

When it comes to liquid post emergent herbicides, I always like to mix in a small amount of Surfactant. Surfactant helps the herbicide stick to the leaves. With more mature weeds there can be a waxy coating on the leaves that makes it hard to get the herbicide to stick. If you’ve ever wondered why your weed killer hasn’t work it could be that the herbicide ran off of the leaves. Surfactant will solve this problem.

Manually Remove Crabgrass

If you just have some random crabgrass weeds that pop up here or there then consider manual removal. Sometimes I don’t have the patience to wait for chemicals to kick in. I prefer to get rid of it the moment I see it. The trick here is to manually remove crabgrass before it gets too big.

If it’s just a small weed simply pull it out by hand or use a really helpful weed pulling tool . If there is a more substantial patch then you have to dig it out using a shovel and that will create a bare spot in your lawn.

Removing smaller crabgrass weeds is definitely easier and better for your lawn!

Maintain a Healthy Lawn

It’s true. Nothing is better for combating lawn weeds then maintaining a healthy, full lawn. Sticking to a good lawn care program will allow your lawn to out compete crabgrass.

An established lawn with no bare spots makes it a lot harder for weeds to gain traction. If you have bare spots in your lawn then consider repairing them by top dressing.

Also, mowing tall and often is a great way to prevent crabgrass. Mowing tall will allow your lawn to block sun from reaching the low growing crabgrass. Mowing often will prevent stalks from forming flowers which eventually become seeds.

The Biggest Mistake Everyone Makes

Overall, the biggest mistake I see most homeowners make is applying crabgrass pre-emergent only once in early spring. Applying 2 applications 5-6 weeks apart is crucial!

Applying pre-emergent twice allows you to not have to worry about your timing so much. It can be hard to predict when crabgrass will germinate so applying 2 applications of pre-emergent takes away a lot of the guess work.

It also ensures that there is plenty of pre-emergent in your lawn over a longer period. This is important since there is a long period in spring/summer where crabgrass has the ability to germinate.

Biology and Management of Crabgrass

Crabgrass is native to Europe or Eurasia and is distributed worldwide. It was introduced into the U.S. in 1849 by the U.S. Patent Office as a potential forage crop. Now, it is found in virtually every crop or non-crop situation.

Similar Species

The two closely related species of crabgrass in the United States are large or hairy crabgrass (Digitaria sanguinalis) and small or smooth crabgrass (Digitaria ischaemum).

Life Cycle

Large crabgrass, a summer annual, is a member of the grass family. It is one of the most troublesome weeds in lawns. Crabgrass reproduces by seeds and and it has a prolific tillering or branching habit. A single plant is capable of producing 150 to 700 tillers and 150,000 seeds. Crabgrass plants are very adaptable to mowing height. Plants can produce seeds at mowing heights as low as 1/2-inch.

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Crabgrass seeds are dormant for a short period of time after they shed from plants. Seed germination is related to soil temperature. When the soil temperature at the surface reaches 55°F for four or five consecutive days, crabgrass begins to germinate. Seeds germinate best from early spring to late summer. Crabgrass continues to grow until midsummer when days become shorter. Vegetative growth slows and plants enter their reproductive stage. Purplish seed heads form until frost kills the plants. Plants that emerge early in the season and have a long period of vegetative growth are much larger and more competitive than plants that germinate late in the season.

Habitat

Crabgrass is found in almost every turf and landscape situation. It is prolific in lawns, golf courses, athletic fields, gardens, orchards, and waste places. It thrives particularly well in lawn situations.

Physiology

Once established, crabgrass tolerates both high temperatures and dry weather conditions because of its physiological makeup. Crabgrass species are often very competitive because they are C4 type (warm season) plants. C4 grasses thrive during the hot weather and abundant sunlight of the summer when cool season turfgrasses (C3 plants) are under stress.

Description

Crabgrass is very noticeable in lawns. It is a rapid growing, coarse textured yellowish-green grass that is conspicuous when found growing among fine textured, dark green cool season turfgrasses. The stems are spreading and much branched. Roots develop at nodes on the prostrate stems.

The first leaf is only about twice as long as it is wide. It is tinged light purple and has a white strip running down the center. Both sides have silky, shiny hair. Leaves are 2/5- to 1/2-inch wide and 1/3- to 1-inch long. The leaf sheaths of large crabgrass seedlings are tinged purple and are covered with long stiff hairs. The ligule is large membranous, and toothed. A ligule is a thin membrane or row of hairs at the top of the junction of the leaf sheath and the leaf blade. Auricles are absent. Auricles are the appendages projecting around the stem from both sides of the collar.

Cultural Management

The basic principle of a crabgrass management program is to prevent re-infestation by seeds. Controlling seed production for several years will help reduce the viable seed supply. Crabgrass cannot be controlled in one growing season because of the great number of viable seeds that accumulate in the soil from years of infestation. A good weed management program in lawns is one that consists of both focused cultural practices and the use of herbicides as appropriate for the control of any given species. Satisfactory control may require several seasons of conscientious adherence to a good management program.

Establishing a dense and healthy stand of turfgrass is the best way to control crabgrass and other annual weeds including grasses and broadleaf weeds. The proper mowing height and frequency, fertilization and irrigation are part of a sound weed control program and should be practiced throughout the growing season.

  • Seed in late summer for new lawns. Crabgrass and other annual grasses that germinate in late summer will be killed by frosts in October or November.
  • Mow your lawn at a 2- to 3-inch height of cut. The taller grass shades the soil and keeps soil cool. Crabgrass seeds do not germinate under cool conditions. Adjust your cutting height as appropriate for the turfgrass species.
  • Water deeply once a week and avoid frequent light irrigation.
  • Avoid summer fertilization. Crabgrass benefits more from fertilizer application under high temperatures than Kentucky bluegrass and other cool season grasses because of the physiological make-up of the plants.

Management with Herbicides

Crabgrass can be selectively controlled in turfgrass areas by judicious use of preemergence or postemergence herbicides. Timing is important for herbicide applications. The best time for preemergence application of herbicides is before or at the time that Forsythia is in full bloom, or when soil temperatures reach 55° F for four to five consecutive days. This is late April or early May in Massachusetts. Postemergence herbicides can be used when crabgrass is in the 2- to 5-leaf stage. Repeat applications may be required depending upon treatment specifics.

For a details on current herbicides registered for control of crabgrass, refer to the latest edition of the Professional Guide for IPM in Turf for Massachusetts.