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Why Does Crabgrass Turn Purple?

The crabgrass plant is a common lawn weed that is notorious for ruining gardeners’ hard work and for being difficult to get rid of. One of the reasons it’s so hard to get rid of the crabgrass is that the plant itself is hardy and very adaptable — it can quickly spread its seeds to other parts of the lawn, re-infesting the area.

But one of the most curious facts about the crabgrass plant is that it can turn purple, and this doesn’t happen every single year. 

So why does crabgrass turn purple? It turns out that the purple coloration is as a result of maturity, cold weather or lack of soil nutrients.

Purple crabgrass is a common problem in the urban landscape. 

It’s often found in lawns and gardens throughout the city, and it chokes out native vegetation, and can lead to expensive repairs. 

Unfortunately, purple crabgrass is sometimes confused with other weeds, and it’s very difficult to recognize. 

But you can deal with purple crabgrass before it gets out of hand by learning what you can about this weed and how you can keep it under control.

Why crabgrass turns purple

I’ve always loved the idea of having a backyard full of wild flowers and grasses. 

Most of the time, that dream is just that—a dream. 

When I was a child, my parents would have us mow the yard every week so that we could get the grassy look most people wanted. 

However, crabgrass would always creep into the lawn.

While crabgrass is known for being the weed that’s most likely to invade your lawn, that’s not the only way it can cause problems. 

You’ll also notice that when it starts to appear, that it looks a little different. 

Instead of the typical greenish-brown, it’s got a purple hue.

This color is related to:

  1. Maturity
  1. Cold weather
  1. Soil nutrient deficiency

1. Maturity

The color of crabgrass can change over time. When it first starts to grow in the spring, crabgrass is a light green color. 

As it matures, it changes to a purple color.

Crabgrass is a grass that many homeowners are familiar with. It’s known for its ability to grow in even the poorest of soils and is often considered a nuisance. 

But what are the characteristics that make it so troublesome? Crabgrass is a common weed found in lawns, gardens, and other grassy areas. 

It’s a highly invasive plant that will spread rapidly and can be difficult to get rid of. Its leaves are covered with stiff hairs that can irritate skin. 

Crabgrass has been studied extensively. Its biology is known in great detail. 

The most important information about crabgrass is that it can be propagated easily by seed. 

Seed is spread by wind, water, and animals. Crabgrass spreads by the roots as well.

It will grow even more quickly in warm weather. It thrives in warm, moist soil, so it’s often found in lawns that are watered frequently.

It doesn’t take long for crabgrass to grow as it can develop a root system as quickly as three weeks. 

For crabgrass to reach full maturity it can take up to three months.

Crabgrass is often mistaken for other grasses, including:

Bentgrass – Bentgrass has a bluish cast to the leaves. Crabgrass has a green cast.

Bermudagrass – Bermudagrass leaves are smaller than crabgrass leaves.

Fescue – Fescue have thick leaf blades compared to those of crabgrass.

Orchardgrass – Orchardgrass leaves have a blue tinge.

Wildrye – Wildrye leaves are more broad than those of crabgrass.

In addition to the physical differences, crabgrass can be distinguished from other grasses by its seedling leaf sheath. 

The leaf sheath of a crabgrass seedling has long, stiff hairs that can be easily felt.

2. Cold weather

If you’re a fan of the sport of golf, then you know that once the temperature begins to drop, you have to put away your clubs and wait for the weather to warm up before you can play again. 

But you may not know that the cold weather isn’t just bad for your ability to play, it’s bad for your lawn as well.

The cold weather causes the crabgrass in your lawn to turn purple. Crabgrass is an annual grass plant that is prevalent in lawns in the US. 

In the summer, it’s a bright green color, but when the temperature drops, it turns a dark purplish color.

Some people find the change strange, since the plant is still green on leaves. However, the color change is due to a chemical change in the plant’s cells. 

When the temperature drops to about fifty degrees, the plant begins to produce anthocyanin, a chemical compound that gives the plant a purple hue.

As the temperature continues to drop, the crabgrass will die. 

However, that doesn’t mean you won’t have to deal with crabgrass in your lawn next season. The crabgrass seeds will have been dispersed, and they’ll grow again next year.

3. Soil nutrients deficiency

For those who have yards, it’s no secret that crabgrass is a pesky plant to manage. It’s a type of weed that can quickly take over a lawn, and it thrives in areas that are rich in nutrients. 

Unfortunately, many of the areas where crabgrass grows are rich in nutrients, and it’s not always straightforward to keep it in check.

Crabgrass is already a very pretty green, so why does it need to change color? The reason why crabgrass turns purple has to do with the nutrient deficiency that causes crabgrass to grow green in the first place.

If nutrients are the cornerstone of a healthy lawn, then crabgrass is the key to a thin, weak lawn. 

This weed thrives in lawns that are rich in nutrients, especially nitrogen. When your lawn has an excess of nitrogen, phosphorus and calcium, you’ll see crabgrass in abundance.

The reason why crabgrass turns purple has to do with the low levels of phosphorus in the soil. If there’s not enough phosphorus in the soil, the plant will use up its reserves until the nutrient runs out. 

When this happens, the plant will turn to anthocyanin in order to continue growing.

This chemical is the same pigment that’s found in blueberries and eggplants. 

The color change is not permanent, however. Once the soil has the proper levels of phosphorus, the plant will return to its original green color.

Phosphorus is a major nutrient in lawns, and it promotes root growth. 

This nutrient is why grasses appear to be greener in the spring and summer. The roots have grown and are now able to take in more nutrients.

So the next time you see crabgrass in your lawn, don’t be afraid. It’s actually a sign that you have a healthy lawn. 

This weed thrives in lawns that are rich in nutrients, after all.

4. Spray chemical

The reasoning behind this is precisely because of the chemical reaction and the addition of copper sulfate. Copper sulfate is a chemical that kills crabgrass. 

When properly combined with other herbicides, it causes the grass to die.

When you use a chemical to kill crabgrass, you’re essentially killing the roots. 

The roots are where the crabgrass stores the nutrients that allow it to grow and spread. When you attack the roots, you starve the plant of nutrients and eventually you’ll eliminate it.

Unfortunately, not all spray chemicals are the same. 

Iron sulfate, for example, will not do a superb job on crabgrass.

Some spray chemicals only affect the top of the grass, which is why you see crabgrass turning purple. 

The spray chemical either doesn’t affect the roots or it does a poor job of attacking the roots. Since the roots are still alive, the crabgrass can still get nutrients and continue to grow. 

Because the chemicals are mixed with water, some of the potency is lost. 

When the chemicals are mixed with the water, they can get diluted. The dilution can make the chemicals less efficient at destroying the crabgrass.

Also, as the chemicals are sprayed, some of it is lost in the air. The wind can blow some of the chemicals away and away from the crabgrass. This means that the crabgrass will only be affected by a portion of the chemical.

In short, the less effective spray chemicals aren’t good at destroying the crabgrass. 

As a result, the crabgrass will remain unaffected and it will turn a different color.

How to get rid of purple crabgrass from your yard

If you’ve been plagued by crabgrass in your backyard, then you know it’s no laughing matter. 

It’s a pain to remove, and it’s even worse to watch it slowly spread across your yard. 

But it’s not something you have to live with—here are a few tips on how to get rid of purple crabgrass from your yard.

There are a few different ways to remove crabgrass from your yard. 

One is to use a chemical herbicide, which is a quick, easy, and effective way to get rid of crabgrass. 

Another is to use a non-chemical herbicide, which is a bit slower and more work, but is just as effective. The cheapest and easiest way to get rid of crabgrass is to pull it out by hand or with a hoe. 

Whatever method you choose, it’s best to start early in the season, before the crabgrass has a chance to spread.

Chemical herbicide

If you choose to use a chemical herbicide, be sure to read the label carefully. 

You’ll want to know how to apply it correctly, how to store it, and what to do if you get it on your skin. 

Most herbicides can be applied with a spray bottle, a pump sprayer, or a hose-end sprayer. 

It’s best to apply the herbicide in the evening, when it doesn’t get hot and evaporate quickly, and when it’s not going to be rained on right away. 

If you do get the herbicide on your skin, wash it off immediately with soap and water.

The best chemical herbicides for crabgrass are ones that contain 2,4-D. 

Some of these products come in concentrate and ready-to-use formulas. 

The ready-to-use products can be sprayed over large areas, whereas the concentrate products are usually diluted with water and sprayed over smaller areas.


If you choose to use a non-chemical weed control, you have a few options. 

The cheapest and easiest is to use a hoe or a rake to physically remove the crabgrass. 

Another option is to use a natural herbicide that contains corn gluten

Corn gluten is a byproduct of corn processing, and it’s used as a pre-emergent herbicide. 

Essentially, it keeps crabgrass from growing if it’s applied before the crabgrass starts growing. 

Zebedee Nambaleo
Zebedee Nambaleo

Zebedee is the founder of RealEstate Ke. He creates content by carefully examining and analyzing the real estate market, home improvement resources, and government data. His analysis is based on the principle of supplying high-quality, relevant, and in-depth information to his audience. By evaluating the current conditions and predicting future trends, he provides his audience with invaluable insights that allow them to make better decisions.