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The Ultimate Guide to Crabgrass

The other day I heard my neighbor say his grass had “crabs.” Oh my gosh really, I thought to myself as I looked around for some young ears. What my neighbor was saying, in his own facetious way, was that his lawn was infested with crabgrass.

As I stood there watching how this misunderstanding was playing out, I thought to myself I really needed to clear things up a bit. So without being a viewed as a nosey neighbor I bent down to examine my grass and declare that I had found crabgrass growing in my lush lawn. As the sound of clarification reached the parents ears, the shock of such an outburst quickly dissipated.

Before I knew it, I was surrounded by my neighbors inspecting my lawn and filling the air with so many questions that needed to be answered. While I was tempted to pull out my bull horn to get their attention, I decided to write down their questions and answer them in a more formal way that could become a reference for everyone who has or will get “crabs” or I mean “crabgrass” in their lawn.

How do I kill crabgrass?

Now there is a difference between killing and preventing crabgrass. When it comes to managing the weed, the goal is to kill the plant while preventing new from growing. Although easier said than done, simply preventing the plant from producing seeds will eliminate mature crabgrass later in the growing season.

Just Pull It Up

When it comes to killing crabgrass, there are a few approaches one can use. The first one is actually the easiest and it entails just pulling the plant up. Its roots are very shallow, which make pulling the plant up easy especially if the soil is moist. While this is the easiest, it is not always the most practical. Another approach is to mow the weed down to keep it from reaching its life cycle where it produces seed. This technique will work but not all crabgrass species can be mowed due to low growth height. Also, it sometimes grows where it is not conducive to mowing.

Bury It With Mulch

A non-chemical or mechanical approach to killing crabgrass is to bury it with mulch. This method works best in flower beds or gardens. You simply just cover the plant with mulch three to five inches deep. Killing crabgrass by burying it does a couple of things. It can stop the life cycle of the plant, which means no seeds. If seeds have been produced and the plant is still alive, the burying will kill the plant and prevent the seeds from germinating due to the fact that they are now too deep for proper germination.

Herbicidal Eradication

The last approach, and most effective is chemical based. Crabgrass herbicides are divided into post emergent and preemergent options. Post emergent herbicides will kill crabgrass after it has begun to establish in your lawn. Ideally, using a preemergent herbicide to help prevent it from growing is the best approach. 

Tennacity is the best option for pre and post emergent control of crabgrass in centipede grass lawns. In Bermuda grass lawns, Dimension is a great post emergent, while QuinKill Max will offer preemergent control. 

What is crabgrass?

Crabgrass is a low growing annual that belongs to the Digitaria genus. This means that it completes an entire life cycle in one year, then dies. However, this doesn’t make it any easier to control in your lawn. Globally there are about 250 varieties of the species that can be found in tropical and subtropical habitats.

You may be wondering how on earth crabgrass got to the United States. Well, this annoying lawn weed traveled from Europe either by mistake on clothing or on purpose. Beyond this introduction, crabgrass is also recommended as a field crop. Yes, I said field crop.

The reason for this is that the former crop, now weed is a warm season grass that can be raised as forage for several different animals.

It is healthy, high protein forage. This is such a valuable forage that many universities have handouts for farmers that tell them how to plant and care for this lawn enemy, which can complicate the situation when it comes to control. These same universities have handouts on how to control this weed when it gets into your lawn.

Why is crabgrass so hard to kill?

There are several reasons why it is so hard to kill. But before we move on to these reasons, let’s take a look at the life cycle of this plant.

Crabgrass is an annual plant that emerges in the early summer. It will grow through the hottest part of the summer and into the fall. In many cases, you will even see crabgrass that is still green after a hard frost. In nature, what stops the weed is the fact that it is an annual, which means after a year the plant will die. Now keep in mind that the goal of the plant is to produce an abundance of seeds for the next generation. Crabgrass excels in this, which keeps it popping up year after year.

Once the seeds have ripened on the flower stalk, they are carried by the wind and/or simply dropped on the ground. The seed can also be carried by animals and on peoples’ clothing.

Seeds Can Last For Years

If conditions are not right, the seeds can wait years before germinating. However, the wait is normally very short since this plant is very flexible in its growth requirements. This means that you are pretty certain that you will have a new crop to deal with every year.

To add to the mix, it can also root in the soil any where and will attach its nodes to the bare ground’s surface. What does this mean? Well, simply put where you had one plant now you have two, which can produce more seed. The seed production of crabgrass creates a snowball effect for homeowners to deal with.

Crabgrass Produces Thousands of Seeds

Now on to why it is so hard to control. Thousands of seeds are produced every season. These seeds drop onto the soil surface and wait for warmer weather. Then they will germinate. At this point, you could spray a chemical on the soil surface to keep the seeds from germinating but it does not keep the wind from blowing, which can bring more seeds to your lawn. Your friends and family can also bring their share of seeds on their clothes. To complicate the matter, if you live in a rural environment, you can also be fighting the seeds produced from forage growing in a field.

Beyond these sources of seeds, some species of crabgrass are too short in height to mow down to prevent them from producing seeds, which is one approach for hairy or tall crabgrass compared to smooth crabgrass.

Where is crabgrass found in the United States?

Thirteen species can be found throughout the United States. Smooth and large varieties are the most common. Crabgrass is also known to be a problem in cropland. Most predominately in sugarcane, cotton, and sorghum.

While not noted as a huge problem, it can be found growing in orchards, vineyards, and in ornamental crops, such as shrubs.

What does crabgrass look like?

There are several different varieties of crabgrass but the two most common you will find in your lawn is the smooth (Digitaria ischaemum), and hairy (Digitaria sanguinals).

The smooth, or sometimes called short crabgrass, reaches a mature height of around six inches. While the name may imply that the plant has no hairs along the stem or leaves, this is not true. Smooth crabgrass does have hairs. A few exist along the plant’s auricles, which are small, ear-like projections coming from the inside of the base of each leaf.

The leaf of smooth crabgrass can be as long as five inches in length with a point on the very end. The stem of this plant bends at the nodes. Later in the season, the stems can turn red.

Smooth crabgrass is native to Europe and Asia but has established itself throughout the United States.

The hairy or tall crabgrass is covered in hair, which includes stem and leaves. The leaves grow up the stem in an alternating pattern and can be 2 to 10 inches long and 2/3 inches wide. The flowers are in a whorl pattern. Each plant can have one to three whorls. The petals look like fingers coming up from the whorls with 2 to 15 spike-like clusters

This variety will pop up anywhere there is bare soil or a crack in the sidewalk. This plant loves clay or sandy soils. It can reach a mature height of three feet if it is not mowed and spreads easily on soil that has been disturbed. It’s native to Europe, Asia, and Africa.

Other species found in the U.S. include Asian and southern varieties. However, these two are difficult to distinguish from smooth and hairy crabgrass. Luckily, we control them like the previous, and distinguishing them is not necessary.

How do you prevent crabgrass?

Preventing crabgrass can be a challenge. It is important to manage your expectations and realize control will take a full year. The first step is to control any crabgrass that is currently growing in your yard. Select a post emergent herbicide from the recommendations above, based on your type of grass in your lawn. A post-emergent herbicide will eliminate growing crabgrass without hurting your lawn. These herbicides are applied to your yard during late spring and throughout the summer.

Eliminating currently growing crabgrass is just half the battle. The next step is catching it before it can establish in your lawn. This is where a preemergent herbicide comes into play. Apply a preemergent discussed above (based on lawn type) in early spring.

When it comes to applying this type of herbicide, it is better to plan the application after the second mowing of your lawn. These products will prevent growth of crabgrass and other nuisance weeds before they can take over your lawn.

It is important to read the product label of both pre and post emergent herbicides to determine the best time to apply.

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