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CONTROLLING ANNUAL GRASSY WEEDS IN YOUR LAWN

GARDEN TIPS – Written by Marianne C. Ophardt
WSU Extension Faculty
for the Tri-City Herald, Kennewick, WA Published February 28, 2016

It would be wonderful if new weed-free lawns could stay that way forever. Right? However, weeds “get their foot in the door” when lawns are subjected to stress from the environment or poor management practices, including watering, mowing, and fertilization. However, weeds will eventually show up in lawns as they age, even in ones that are well maintained.

Much has been accomplished in the field of weed science for the control of broadleaf lawn weeds, like dandelions and clover. Whether chemicals are utilized or removal by persistent pulling or digging is employed, most broadleaf weeds in lawns can be controlled relatively easily.

Grassy weeds are a different story. It is difficult to pluck out most grassy weeds from a lawn interspersed with regular lawn grass. Using chemicals to control grassy weeds is difficult because many chemicals that will kill the grassy weeds will also kill lawn grass. Before you can begin to consider using chemicals to control grassy weeds in your lawn, you need to understand how grasses grow.

There are two main types of grassy weeds, annual and perennial. Annual grasses die and come up again from seed every year. Crabgrass and annual bluegrass are the two most common annual grasses that cause problems in our area lawns. Excessive watering, frequent shallow watering, and consistently mowing a lawn too short are practices that make it easier for crabgrass to get started in a lawn. Excessive watering and compacted soil are conditions favorable to annual bluegrass. Correcting these problems and making a lawn as healthy and dense as possible with proper maintenance makes it more difficult for both of these annual grasses to persist in a lawn.

Chemicals are available that can provide help in managing these two weedy annual grasses in lawns. Preemergent herbicides chemically prevent seed germination and are applied before the seed of the annual grasses have the opportunity to germinate and grow.

Crabgrass seed germinates in the spring and preemergent herbicides or “crabgrass preventers” are only effective if the application is made prior to seed germination. The right timing for an application of a crabgrass preemergent herbicide is when the soil temperature at a depth of one inch is greater than 55 degrees for at least a week. This typically occurs when the yellow-flowering forsythia bush has been in full bloom for a week or two.

While some annual bluegrass seed germinates in the spring, most of the seed germinates in early to mid-fall, growing rapidly during mild winter and early spring weather, flowering in the spring and summer and producing lots of seed. Preemergent herbicides applied before crabgrass germinates will not persist long enough to prevent most of the seed from germinating. However, preemergent herbicides applied in the spring for preventing crabgrass will discourage early germinating annual bluegrass seed. For effective control of annual bluegrass, a preemergent herbicide should be applied in mid-August.

There are also postemergent herbicides available to kill seedlings of crabgrass and annual bluegrass if a preemergent materials are not applied at the right time. However, these chemicals are only effective if the plants are relatively young and small. These materials have the potential to injure your lawn grass if not applied correctly, so be sure to read the label and follow the directions before use.

OH NO THERE’S CRABGRASS IN MY LAWN

written by
Marianne C. Ophardt
WSU Extension Faculty
for the Tri-City Herald, Kennewick, WA
published- AUGUST 29, 2014

OH NO THERE’S CRABGRASS IN MY LAWN

This has been a banner year for crabgrass in home lawns, including mine. About a month ago I noticed patches of this bright green annual grass sticking out like sore thumbs all over the front lawn. Oh no!

We had not had a big problem with crabgrass in the past so we did not apply a pre-emergent herbicide in the spring to prevent its germination. However, even folks who did apply ‘crabgrass preventers’ at the recommended time are finding this weedy grass in their lawns this year.

Before we talk about what to do about it, let’s review. There are actually two different species of crabgrass found in our area, smooth crabgrass (Digitaria ischaemum) and hairy crabgrass (Digitaria sanguinalis). Native to Eurasia, they do exceptionally well here, germinating from seeds in the spring, growing rapidly during the heat of summer, flowering, setting seed, and then dying out with frost in the fall. They produce copious amounts of seed capable of germinating the following spring or in future springs.

Smooth crabgrass is most common in area lawns. When first noticed in lawns, it is light green. As the plant grows, it turns a darker dull green and forms flattened clumps. Its seeds are produced from August to September on flower stalks with finger-like spikes, resembling the those of Bermuda grass.

So why such a bad outbreak in home lawns this year? I suspect it is due to the weather. In lawns where ‘crabgrass preventer’ was applied at the recommended timing, the chemical control may have partially dissipated by the time the weather started to warm and the crabgrass germinated. Because of this year’s long spring, some of these products may have needed to be reapplied to extend the period of control.

Our long, cool spring was almost immediately followed by extended very hot weather. A warm-season grass, crabgrass loves the heat, but our cool-season lawn grasses virtually stop growing in hot weather. Plus, lawn grasses can be further stressed by poor cultural practices, like mowing, watering, and fertilization.

So what can we do now? Healthy dense turf is one of the best ways to combat crabgrass next year. Scalping your lawn (taking off more than 1/3 of the leaf blades at one time) or regularly mowing too close weakens turf. Mow your lawn no shorter than 2.5 inches, mowing frequently enough so you don’t scalp the lawn.

Water your lawn deeply when you irrigate. Avoid frequent daily light irrigations which encourages shallow-rooted grass, the development of thatch, and promotes crabgrass germination and growth.

Fertilize your lawn at the right times to promote dense, healthy turf. Apply 4 pounds of nitrogen per 1000 square feet every year, applying one pound on September 1, November 1, May 1, and June 15. On sandy soils, consider using fertilizers with a percentage of nitrogen in a slow-release form.

If you have a crabgrass problem now, be sure to apply a crabgrass preventer next spring when the soil temperature (at a depth of one inch) is greater than 55 degrees Fahrenheit consistently for a week. That is usually about the time when the yellow flowering forsythia has been in bloom for several weeks. It is also important to apply the ‘preventer’ evenly across the lawn, taking care not skip areas.

Next spring is a ways off, so keep this information handy until then.

Published: 8/29/2014 11:33 AM

GOOD LAWN CARE DISCOURAGES CRABGRASS

written by
Marianne C. Ophardt
WSU Extension Faculty
for the Tri-City Herald, Kennewick, WA
published April 18, 2014

GOOD LAWN CARE DISCOURAGES CRABGRASS

At this time of year, many homeowners are asking about crabgrass control and basic lawn care. Is it too late to apply “crabgrass preventer?” When should I fertilize? How often should I be irrigating?

Let me take a little time to address these queries, but first we need to determine if the offending grass is truly crabgrass? Many homeowners think they have a crabgrass problem, but what they actually have is a Bermuda grass problem.

Crabgrass is an annual that comes up from seed every year and dies with frost in the fall. Bermuda grass is a perennial that comes back from its tough, wiry trailing stems and rhizomes (underground stems) every year. It’s leaves are blue-gray in color, where as crabgrass leaves are green. While not similar in appearance, these two grasses are often confused with each other because they have similar seed heads.

Best Management Practices for a Healthy Lawn: Your first line of defense against crabgrass should be encouraging healthy, dense turf using good cultural practices. This includes mowing, fertilizing, and irrigating properly. Mow bluegrass and bluegrass mix lawns at the WSU recommended height of 2 to 2.5 inches. A dense, tall turf shades germinating crabgrass seedlings and deters their growth.

Equally important in controlling crabgrass is fertilizing your lawn at the correct times of year to promote both root and side growth (called tillering) instead of top growth. The most important time of year to fertilize your lawn is in the fall. WSU recommends making a fertilizer application in early September and again after the last mowing (around mid-November) but before the soil freezes.

If you apply fertilizer in the fall as recommended, then you should wait until early May to fertilize again. Fertilizing earlier in the spring tends to encourage top growth at the expense of root and side growth, resulting in weaker turf and more frequent mowing. The best times to fertilize lawns in our region are: November 15, May 1, June 15 and September 1, applying 1 pound of nitrogen per 1000 square feet of lawn area.

When irrigating lawns, the tendency in our region is to water daily starting early in the season for 15 minutes each day. This daily, light irrigation leads to weaker, shallow-rooted, thin turf and makes it easy for crabgrass to germinate and grow. You will have to water more frequently during the hottest part of summer, but during the relatively cool weather of spring and fall you should definitely not be watering daily. Adjust your timers to water less frequently, but more deeply to promote deeper grass roots.

Chemical Crabgrass Control: There are a number of home garden products often called “crabgrass preventers” that contain herbicides that are applied in the spring prior to crabgrass seeds germinating to prevent them from sprouting and growing. The general time to apply these products is when the soil temperature reaches about 62 degrees at a depth of 1 to 2 inches or about two weeks after forsythia blooms start to drop.

There are also some home garden products (containing dithiopyr or fenoxaprop) that will kill very young crabgrass seedlings after the seeds germinate, although their effectiveness tends not to be as reliable as good turf management and the use of “preventer” products.

The lawn care season is starting. Homeowners who use recommended “best management practices” will have healthier lawns and fewer problems with crabgrass.

Published: 4/18/2014 11:45 AM

CRABGRASS PREVENTERS NOW?

written by
Marianne C. Ophardt
WSU Extension Faculty
for the Tri-City Herald, Kennewick, WA
Published 3/22/2013

At this time of year I’m often asked about when to apply ‘crabgrass preventers’ or pre-emergent herbicides. These herbicides are applied to a lawn before the majority of crabgrass seeds germinate. Is it time yet? Based on phenology, the right time for applying crabgrass preventer is after forsythia (the bright yellow early spring flowering shrub) is in full bloom and before the flowers start to wither.

What is phenology? The USA National Phenology Network (NPN) says that phenology ‘refers to key seasonal changes in plants and animals from year to year – such as flowering, emergence of insects and migration of birds – especially their timing and relationship with weather and climate.’ Phenology can relate one event with another, like the forsythia’s bloom with crabgrass germination or full bloom of black locust with the emergence of bronze birch borer.

Some groups, like the Aldo Leopold Foundation and the USA NPN, and various universities have undertaken phenology studies. These studies are helping to document the effects of climate change on natural events, particularly vulnerable species like sockeye salmon in the Northwest.

Back to crabgrass… using forsythia as a phenological cue to when to apply crabgrass preventers is not always reliable, especially since there are different varieties of forsythia, some blooming earlier than others. A more accurate way to determine crabgrass germination is using soil temperature or accumulated growing degree days.

Growing degree days (GDDs) are a measurement of the accumulation of heat over time. They are calculated by taking the average daily temperature minus a base temperature. Models for bloom time, germination, or insect emergence often use a base temperature of 50 degrees, but some use 40 or 60 degree bases. For example, on a day when the maximum temperature is 70 degrees and the minimum temperature is 40 degrees, the average temperature is 55 degrees, subtract 50 degrees and you have 5 GDDs.

Now don’t groan about me getting too technical. WSU has you covered with an application that does all this for you and provides you with the minimum and maximum temperatures for each day, plus totals the accumulated GDDs. Just go to http://weather.wsu.edu/mobile/ on your phone or computer and pick the WSU weather station site closest to you by entering your zip code. Once you select your site you can retrieve all sorts of weather related information including current weather conditions, soil temperature, and GDDs. Amazing!

Using this I can tell you that as of Wednesday at the CBC weather station, the accumulated GDDs (using a 50 degree base) is 38.62 and the soil temperature is 49 degrees. Research indicates that the majority of crabgrass germinates when the accumulated GDDs reach 200 or when the soil temperature reaches 73 degrees, although some of the seeds will start to germinate at 57 to 64 degrees. The best time to apply the crabgrass preventers is before these key times.

The other thing you need to know is that the pre-emergent herbicides contained in many crabgrass preventer products has changed over time, even when though the label name has stayed the same. Many of the newer materials provide control over a longer period of time so that applying them too early is not as great of a concern as it once was. One of these ingredients, dithiopyr, provides both pre-emergent and post-emergent control up to 4 weeks after crabgrass germinates. The label recommends applying dithiopyr when soil temperatures reach 50 degrees and that’s right about now so add this to your chores for this weekend.

Published: 3/22/2013 11:01 AM

CRABGRASS CONTROL

written by Marianne C. Ophardt WSU Extension Faculty for the Tri-City Herald, Kennewick, WA Invariably at this time of year I

m asked if it

s time to put crabgrass preventer on lawns. Sometimes this query also includes a comment about how they want to make sure they apply it at the right time because it didn

t work the last time. That initiates a horticultural conundrum for me. Was it the timing that was wrong or was the wrong material used? Perhaps the crabgrass wasn

t really crabgrass, maybe it was a different type of weedy grass. It

s a puzzle worth investigating. After checking into the problem, I often discover that the problem is not with the timing or the material used to prevent crabgrass. The real problem tends to be that the offending grass is Bermuda grass or another weedy grass… not crabgrass. Both Bermuda grass and crabgrass are common in area lawns. So what

s the difference? CRABGRASS There are actually two common species of crabgrass, smooth crabgrass and hairy crabgrass. They

re both native to Eurasia, but can be found in lawns and gardens throughout this country. Of the two, smooth crabgrass is more frequently an invader of turf. Crabgrass is a low-growing, warm season, annual grass that starts every spring from seed that was produced in previous years. Once it germinates and starts growing, it likes the heat of summer and puts on its most rapid growth during the hottest part of the growing season. The newly sprouted plants are light green, but later the leaves are dark green. The crabgrass plant spreads during the growing season by rooting at the “nodes” or joints of it

s horizontal prostrate stems. It tolerates mowing down to a height of 1/4 inch and will even produce seed at this height. Crabgrass dies with a hard frost in the fall. It does not have a tough or wiry root system that persists through the winter. Probably one reason that crabgrass is often confused with Bermuda grass is that they have similar inflorescences or “seed heads”. Crabgrass has 3 to 11 finger-like spikes that originate at different points, spaced from 1/8 to 1/4 inch apart, on the end of the flowering stem. Bermuda grass has three to seven spikes that originate at the same point on the end of the flowering stem. Herbicides or “crabgrass

preventers are often used to help control a problem with crabgrass in a lawn, but cultural management techniques can help avoid relying on chemicals too heavily. Over-watered lawns or lawns watered frequently but lightly are often weaker and thinner, providing excellent conditions for crabgrass growth. Also, mowing a lawn lower than the recommended height will make the lawn more vulnerable to attack. If crabgrass is a severe problem, it usually means the turf is not dense and healthy. Proper mowing, fertilization, and watering help promote a dense turf and discourage crabgrass. Crabgrass is not very competitive and can

t germinate and grow where the vigorous lawn grass shades and crowds it out. Pre-emergent herbicides, known to many as “crabgrass preventers” are applied to prevent the seed from germinating and growing. This means that they must be applied before the seed germinates to be effective. Crabgrass seed germinates in the spring when the soil temperature is about 55 degrees Fahrenheit for three to seven consecutive days. This usually coincides with the flowering of forsythia, the shrub that blooms early in the spring with bright yellow flowers. Most lawn experts say to apply your pre-emergent herbicide or “preventer” when forsythia flowers start to wither, not when it first flowers. Why is there so much discussion about the “right” time to apply a crabgrass preventer product? It

s because nice balmy spring weather with warm sunny days is so ephemeral. Because of changes in the weather, the period of crabgrass germination can last for quite a while. Research at the University of Maryland suggests that germination and emergence of crabgrass can occur over a 10 to 12 week time span . That creates a problem, because if you have applied your preventer product too early, it may start to break down and become less effective. It doesn

t hurt the lawn if applied too early, it just might not adequately control crabgrass. When selecting a crabgrass preventer product you should check the directions for use and follow them. Most will indicate that they should be watered in after application for effective control. This is problematic if you don

t have water available yet for irrigating your lawn. You should also be aware that certain products have a longer residual than others because of the different types of active ingredients or the concentration of the ingredients. Check the label to see what length of control the manufacturer promises. BERMUDA GRASS While the flowers of Bermuda grass and crabgrass may look similar, the plants are completely different. BG was introduced into this country from Africa sometime around 1751 as a forage grass. Much later, about the 1920

s it started to be utilized as a grass for lawns and golf courses. BG is a perennial warm season grass that spreads by seeds, underground stems (rhizomes), and aboveground runners (stolons). It thrives in the heat of summer but goes dormant during the cool temperatures of late fall, winter, and early spring. It has a trailing type growth habit with leaves that form a herringbone pattern along the stem. The leaves are a blue-green in color. Crabgrass roots are fairly shallow and easy to pull, but BG roots are tough, wiry and deep. BG is perennial, regrowing from established roots every year. BG is an aggressive grass which makes it a particularly troublesome weed. However, a good healthy lawn is the best defense against an infestation of BG. Once it gets started in a lawn, it

s extremely difficult to eliminate even with the help of specific herbicides. Generally, the only chemical available to home gardeners for control of BG is glyphosate ( the same chemical contained in Roundup and numerous other homegarden herbicide products). Glyphosate must be applied to the BG when its actively growing. Treating when the grass is dormant (still brown and not growing) in the early spring or late fall and early winter, is ineffective. It will also usually take more than one application to provide control. CAUTION: Keep in mind that glyphosate products are not selective… they will kill all the green plants to which it

s applied. If you have patches of BG in your lawn, you will have to treat these individually. If you treat your entire lawn, your entire lawn will be killed. If you decide to renovate your lawn and kill out the good grass and the BG and then start over again, you

ll want to be patient. It can take as many as two to four applications of glyphosate applied four weeks apart to completely kill the BG. You may get even better control by tilling the soil about two weeks after your first or second application and then allowing the BG to regrow before any additional applications. This is not an easy weed to eliminate. Even if you get rid of it in your lawn, it

s likely to reappear within the next year or so, especially if there is a nearby infestation. In landscape and flower beds, you can spot treat BG with glyphosate or use other herbicides that can be used more safely around your trees, shrubs, and flowers. One of these is sethoxydim and the other is fluazifop. Look for grass-killing products that contain one of these ingredients. Be sure to check the label for specific directions on their safe use around your ornamental plants. These also require more than one application to provide some control. CRABGRASS OR BERMUDA GRASS? Hopefully, you now know the difference between the these two rather troublesome grasses. Make sure you know the difference before applying any chemical for control and keep in mind that your first defense against both is a healthy, dense turf. If you are in doubt, you can bring a sample into the Benton County WSU Extension office in Kennewick (5600E West Canal Drive) and we

ll try to identify it for you.

Published: 3/20/2004 2:29 PM

ASK WHY NOT JUST WHEN

written by

Marianne C. Ophardt

WSU Extension Faculty

for the Tri-City Herald, Kennewick, WA

When? It’s a question that many gardeners ask at this time of year. When should I apply crabgrass preventer? When is it time to fertilize my lawn? Invariably I feel very redundant answering these questions year after year, but I know that folks new to our region still need to know the answers. However, I wish the questions were initiated with a “why” instead of “when” so gardeners could learn the science behind the answers.

When should I apply crabgrass “preventer” or pre-emergent ? It would be better to ask, “Why should I wait to apply crabgrass pre-emergent until forsythia is in bloom?” Crabgrass seed germinates when soil temperatures reaches about 60 degrees Fahrenheit. That typically coincides with the blooming of the yellow flowering shrub forsythia. If you apply certain pre-emergent materials early in the spring when the soil is still cold and crabgrass has no chance of germinating, the materials dissipate and are not as effective later when the crabgrass does germinate.

Why do lawn care companies put pre-emergent control materials on earlier than you recommend? Commercial pesticide applicators have access to different materials than home gardeners purchase at the garden store. Many of the pre-emergent materials that lawn care companies use have a longer residual. Also, some newer home garden “preventer” materials contain chemicals that have a longer residual and the timing of application is not as critical. Check the label on the product you use for information on how long it is effective.

It’s important to point out, that the “preventer” materials must be applied before the crabgrass seed germinates. They prevent seed from germinating and growing. Once a crabgrass seedling starts to grow, the preventer materials won’t work.

When is it time to fertilize? More insightfully, “Why do you recommend waiting to fertilize my lawn until late April or early May? ” I know how hard it is to wait. Spring is here and everyone wants a green lawn. Me too. A brown lawn that is only green in the spots where dogs “fertilized” it during the winter is frustrating. Nevertheless, I have to discipline myself to be patient, because I know fertilizer applied this early will promote the lush green top growth that I want to see, but that this growth will be at the expense of the roots, turf density, and overall lawn health. I want a nice thick lawn that’s able to keep out invading weeds like crabgrass and dandelions.

At our house we’ll try to wait as long as possible before we apply any additional fertilizer to the lawn this spring. We did apply a good lawn fertilizer in early November that included some slow release nitrogen. That nitrogen should become available once the soil temperatures warm up. Hopefully, we can be patient until then. If we hadn’t fertilized our lawn last fall, we might need to move up fertilization to the middle of April.

When do I apply dormant sprays to my trees? “Why should I apply dormant sprays to my trees?” would be better. There are two major types of dormant sprays, fungicides aimed at managing fungus disease and oils that target overwintering insects. The fungicides are used primarily on fruit trees that have recurring fungus infections, but other landscape trees and shrubs may also be treated if they have been attacked by a persistent fungal disease, such as powdery mildew on roses. The timing of dormant fungicides is specific to the disease being controlled and the materials being used for control. They’re generally applied as a management tool when the plant has been infected in the past. They shouldn’t be applied as preventatives because diseases can buildup resistance to the materials, making control difficult in the future.

Dormant oils are applied to trees and shrubs to control insect pests that overwinter on the tree bark. This primarily includes aphids, mites, and scale. The name “dormant oils” is misleading though. The oils are really applied at the “delayed dormant” stage. That’s just as the buds start to open. That’s also when the insects are becoming active. Applied too early when the tree or shrub is dormant, the insects are “dormant” too. Oils applied too early are ineffective.

Many of the dormant oils available to home gardeners are classified to as “horticultural” oils. They are more highly refined than the older true dormant oils and can be applied at one rate at the delayed dormant stage and at another more dilute rate after the leaves have developed. The label of any oil product should be read and carefully followed to avoid plant damage.

When can I plant my frost sensitive annuals? A more insightful question is, “Why should I wait to plant my frost sensitive plants?” Our last average date is between May 1st and May 15th in this region. Because this is an average, the last date of a killing spring frost could be in April or even June. (Yikes!)

Gardeners often like to get their plants in early and then utilize ingenious methods of frost protection such as hot caps, Wall-of-Waters, tents, and more. It makes some of us gardeners look like real procrastinators. Early planting coupled with frost protection, only provides gardeners with a slightly earlier harvest. Cold soil temperatures and chilly days do not provide warm season plants with the warmth they need for growth even if they have protection from frost. A week of cool temperatures, below 55 degrees, and cold soil will stunt plant growth. That means procrastinating and waiting for warmer soil isn’t so bad after all.

Published: 3/15/2008 2:04 PM

PREVENTING CRABGRASS

written by

Marianne C. Ophardt

WSU Extension Faculty

for the Tri-City Herald, Kennewick, WA

I’m confused. Our university experts in the past have recommended applying crabgrass pre-emergent herbicides or “preventers” when soil temperatures reach about 50 to 55 degrees and stay there for a week or so. They have pointed out that this seems to correspond with the flowering of forsythia. However, other researchers report different optimum soil temperature for germination. Varying research indicates that peak germination occurs at 50 to 75 degrees. That’s quite a range!

Some of this may be due to regional variations or due to the difference in crabgrass species, but it might also be due to germination conditions. Research at Cornell University and the University of Maryland indicates that crabgrass germinates later in dense turf when soil temperatures reach 73 degrees. They also found that crabgrass germinates earlier in thin turf with gaps between plants. Apparently this allows the soil to warm up more quickly and most of the crabgrass will germinate sooner over a shorter period of time.

While interesting, does this mean that lawn owners should change the timing of their crabgrass preventing chemical applications? Should they measure the soil temperatures before applying the chemicals. Not really. It’s still best to apply “preventers” in early spring about the time forsythia starts to drop its blooms.

Of course, everyone is tempted to apply chemicals when forsythia starts to bloom which will be very soon. This is only a problem if you use a product that contains pre-emergent chemicals that aren’t very persistent and don’t last long in the soil. WSU recommends the pre-emergent herbicides of benefin, trifluralin, and pendimethalin for crabgrass prevention.

Check the label of any product you apply to see how long it’s supposed to be effective. Those with longer persistence are more forgiving if you apply them a little early, but those with only several weeks of efficacy may not last long enough if cooler weather returns and slows the germination process. Also, be sure to read the label regarding the rate of application and any other application directions, such as the need for watering right after application. If the product requires water for activation and you have no way of watering your lawn, it will not be effective. For good control be sure to apply the material evenly at the recommended rate for good coverage.

Just last week one of our WSU Master Gardeners was trying to help a friend understand the label of a particular crabgrass control product. He called me puzzled because the product label wasn’t very informative and seemed to indicate that it was applied to crabgrass after emergence. He was unaware of home garden products available for killing crabgrass plants after they germinate or “post-emergence”. Most of these materials contain MSMA (monosodium methanearsonate.) While available to home gardeners, these organic “asenicals” must be applied early in the season when the crabgrass plant is small with only three to five leaves. They are not effective against larger, older grass plants…and that’s the time when most people notice that their preventer didn’t work.

IS IT REALLY CRABGRASS? In our region, many homeowners mistakenly think they have a problem with crabgrass, an annual that comes up from seed every year. That’s why “preventers” are applied in the spring. However, many folks actually have problems with Bermuda grass in their lawns. This tough warm-season perennial starts to grow from dormant roots, rhizomes, and stolons after the weather warms up. If you think it’s crabgrass, but it has tough wiry root and stems when you try to pull it up, it’s probably Bermuda grass. We’ll tackle it’s control another time.

Published: 3/17/2007 10:27 AM

CRABGRASS MANAGEMENT

written by

Marianne C. Ophardt

WSU Extension Faculty

for the Tri-City Herald, Kennewick, WA

The other day I was noticing some, actually quite a lot, crabgrass in the lawn near my office. There also have been a number of samples of crabgrass being dropped off at the Master Gardener plant clinics for identification… more than usual at this time of year. I wonder why? It’s probably because of that long, wet spring made conditions just right for germination over a longer period than usual.

Let’s refresh ourselves about the nature of crabgrass. It’s a low-growing, bunch-type annual grass that germinates in the spring. It has pale, apple-green leaves that are about one-quarter inch wide. When it flowers at this time of year it develops flowers that have 3 to 10 purplish finger-like branches originating from one point. As a warm-season annual crabgrass goes pretty much unnoticed by many gardeners until it takes off and starts growing like crazy with the hot weather.

As a summer annual, crabgrass seeds germinate in the spring. All the plants we are seeing in lawns and garden this summer weren’t there last year. They germinated this spring from last year’s plants or from plants that grew in previous years. Crabgrass seed persists for many years in the soil and are capable of growing when the conditions are right.

The right conditions consist of moisture, light and warmth. Crabgrass seed germinates when the soil temperature is about 60 degrees Fahrenheit for about a week of consecutive days. It will stop germinating when the soil reaches 95 degrees.

Once it germinates, crabgrass needs light and moisture to grow. It usually will not be successful in healthy dense lawns mowed at two and a half inches tall. In the lawn near my office, the crabgrass is growing along the edges next to the walk where the grass was edged and the soil surface opened up to light. Frequent, light irrigation also helps crabgrass prosper in a lawn. This provides the seeds near the top of the soil with the moisture they need to grow.

Prevention of crabgrass is the best control. At this time of year it’s really too late to control it with chemicals. Hand-pulling and removal (to prevent the development of more seeds) is the best bet. Keep in mind that a single crabgrass plant can produce 150,000 and these seeds can remain viable for up to 30 years!

Fall is also a great time to get the lawn in good shape for the coming year so it can grow well and shade out any germination crabgrass. Fertilize in early September and again in early November. These are the two most important times of the year to fertilize your lawn to encourage healthy grass and dense growth.

Pre-emergent herbicides or “crabgrass preventers” can be used in the spring to prevent the germinating seeds from growing in the spring. These should be applied when the soil temperature reaches 60 degrees. This coincides with the time that the bright yellow flowering shrub, forsythia, is in bloom. Check the label on these products to see how long the chemicals will be effective. Those with a longer residual will work best when we have a long, wet spring like we did this year. They also last long enough to help prevent other summer annual grasses that germinate later in the spring.

If you miss getting a pre-emergent on at the right time, all is not lost. There are also post-emergent crabgrass herbicides that are effective when applied when the crabgrass is still small, in the two to four-leaf stage. After that, these post-emergent materials will not be very effective. Unfortunately, most folks don’t notice crabgrass until it’s much larger, like at this time of year.

Crabgrass can also be a problem in garden and landscape beds. As you might guess, it likes bare soil areas. If crabgrass is growing in these areas, take time to pull or dig it out NOW before it goes to seed. If you moisten the soil first, it should be fairly easy to yank out. Cover the soil with mulch to shade out any plants that start to grow in the spring. A good mulch is usually adequate for providing control, but there are pre-emergent herbicides (crabgrass preventers) that can be used in garden and landscape beds, but these are a last resort. Always read the label and follow the directions for the safe use of garden chemicals.

Published: 8/26/2006 11:08 AM

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