When do I apply my Crabgrass Preventer?

April 23, 2014

A key to the successful control of annual grasses (such as crabgrass) in established turf is the correct application timing of preemergence herbicides. Preemergence herbicides should be applied by May 1st in central Iowa. Dr. Christians has noticed that this date does not vary much from year to year, after monitoring germination dates for the last 34 years.



In addition to timing: application uniformity, using recommended product rates, and the requirement of (1/2 inch) irrigation within 3-5 days of application can play a vital role in crabgrass control. 

Several products are available for effective annual grass emergence control. These products vary slightly in mode of action, length of control, specific weed efficacy, desired turfgrass seed inhibition, and early postemergence control. Benefin, benefin + trifluralin,bensulide, oxadiazon, siduron, pendimethalin, mesotrione, prodiamine, isoxaben, and dithiopyr are preemergence products available in the market today. 

Please note that some products are not labeled for certain turfgrass species. For example, oxadiazon is not recommended for use in fine fescue; however, oxadiazon provides   excellent goosegrass control in Kentucky bluegrass. Always read thoroughly and follow the label directions. Remember, the label is the law.

Dithiopyr and prodiamine have the longest window of effectiveness and can control weeds for up to 16 weeks. Dithiopyr and mesotrione offer early postemergence control when applications are made following weed emergence. Siduron and mesotrione have a unique property that allows herbicide application to seeded areas. Siduron selectively controls weedy annual grasses such as crabgrass, foxtail, and barnyardgrass, while allowing the desired turfgrasses to grow.  Mesotrione is only labeled for preemergent use on newly seeded Kentucky bluegrass lawns. All of the other preemergent herbicides kill the seeds of the cool-season grasses and cannot be used at the time of seeding.

Fertilizer-herbicide combinations are sold at most retail stores. This allows homeowners to combine the two operations into one application. A disadvantage of the combination is that the proper time for weed control often does not coincide with the optimum time to fertilize. Combinations with preemergence herbicides are generally effective in controlling annual grass weeds as long as applications are made at the appropriate time and recommended amount.

In addition to annual grassy weeds, a spring application of a preemergence herbicide will control annual broadleaf weeds, such as prostrate knotweed and spurge. A second application at a reduced rate may be necessary for season-long control.

Paying attention to herbicide timing, application uniformity, product and rate, and ensuring (1/2 inch) irrigation within 3-5 days of application will help prevent annual grass (crabgrass) invasion. Below you will see two pictures of crabgrass in an early leaf-stages.

The last picture is a general guide of preemergence application dates via Quali-Pro's Prodiamine label:



March 24, 2012

I received these pictures from Grad Student Andrew Hoiberg on March 23. They were taken in a very protected area by animal science on ISU campus. I did another check of the areas on campus where I see crabgrass first each year on the morning of March 24 and still see no crabgrass germinating in these areas. I'll keep you posted on other developments.

These are the real thing. It is large hairy crabgrass. Notice the pointed leaf tip and the fine hairs on the leaf margin.

I would like more pictures from around the state as you see crabgrass emerge. I will get them up on the blog. This does appear to be one of the earliest springs in my 33 years here in central Iowa.


Crabgrass is Everywhere

August 23, 2016

Many people have been calling and emailing us the past couple of weeks with questions over crabgrass (Digitaria spp.). If you have a light yellow green grass with wider blades than the rest of the turf, that seems to be growing at a much quicker pace than other parts of your yard, you more than likely have crabgrass.

Crabgrass with seed heads.

These plants germinated in the spring, but you didn’t notice them until recently probably. The good news is this grass is a summer annual and the growth will soon slow with the cooler temperatures and the plant will die after a killing frost. If a seed head forms on crabgrass, it can produce thousands of seeds on each plant, so your problem is already started for the next year.

Crabgrass can take over quickly.
Crabgrass quickly moves into areas where turf has been lost.

If you have crabgrass you have a few options:

  1. Since it is so late in the year it is best to just let the cool temperatures remove the crabgrass for you.
  2. Chemical controls exist, but at this stage in the plants development you will need to apply a couple of applications and it still may not kill the plant.
  3. Preemergence herbicides don’t help once the crabgrass has emerged.

Tips for preventing crabgrass:

  1. Fertilize your existing turf this fall. A healthy turf stand will have less weeds due to a more uniform canopy with few openings for crabgrass to move into.
  2. Seed thin spots in the next couple of weeks to help thicken the turfgrass you want.
  3. Maintain turf at 3-4” height of cut. Taller turfgrass stands tend to have less weed pressure since the leaves shade out the germinating crabgrass.
  4. Put out a preemergence herbicide by May 1st in central Iowa, since crabgrass germinates from seed each spring. Crabgrass will germinate once soil temperatures reach 55 degrees Fahrenheit.Try to time this application with a ½ inch rainfall or irrigation within 3-5 days of applying the preemergence herbicide.

Crabgrass is excellent as self seeding.
This area may need some additional grass seed after the first frost.




May 14, 2018

I have been watching for the first crabgrass plants to emerge for each of the past 40 springs.  The reason for my interest in this is that I always have some kind of crabgrass treatment going out each spring and I don’t want to miss germination.  In Ames, the crabgrass usually germinates by about May 1 and I tell people to try to get their preemergence herbicides out mid-April.  It has amazed me over the years how consistent that date for crabgrass germination has been.  There have been warm springs when one would expect early germination and cold, late springs where late germination would be expected. However, each year the crabgrass emerges around May 1, 


This spring was unusually cold and we still had snow into late April.  The weather service tells us that it was the coldest April on record.  Yet, on May 2, 2018, the first crabgrass began to emerge at the research station. 


In the picture, you can see some knotweed that is beginning to expand.  Knotweed, which emerges in late March,  is often mistaken for crabgrass in early spring (see the blog from March 23, 2012 and May 29, 2012).It does look like a grass when it first emerges and does not appear to be a broadleaf species until it begins to expand.  The crabgrass in this picture is to the right and left of the pen.  It has a wide, pointed leaf and is often a light green color at emergence.



April 12, 2012

We have had three mornings with frost on April 10, 11, and 12. I took a walk on campus on the afternoon of the 12th to see what the impact of the frost had been on the crabgrass that I observed on April 2 (see earlier post). Crabgrass is very susceptible to cold damage, particularly in the seedling stage. I expected the crabgrass to be dead, but to my surprise, it appears to be doing fine. The crabgrass is in a somewhat protected area around the horticulture building, but it is an area that did have some frost.

I am still not seeing much germination of crabgrass in more exposed areas on campus and we have not seen any emergence at the horticulture research station as of April 12. It did get down to 20 F at the research station on the morning of the 11th. If any crabgrass was beginning to peak through, I would expect it to be dead. I'll continue to monitor the early germinating crabgrass over the next few weeks and let you know what is happening.

The trial in which we are applying Baricade and Dimension on April 1, 15 and May 1 to separate plots is continuing. As soon as I have some results from that work, I will put up a post.

Figures 1 and 2. Crabgrass by the horticulture building on the afternoon of April 12 after three mornings with frost.



May 2, 2013

It is May 2 and we have a snow storm going on in Ames.  Normally, when people ask me for a date by which they should have their preemergence herbicide applied for crabgrass control, my standard answer is to have it down by May 1.  The question that I have received most often in the last two days, is whether this cold weather will delay germination. 

The expected answer would be that the cold weather will delay germination,  however, I have been watching crabgrass germination dates for 34 years here in central Iowa.  What I have observed is that the germination date does not vary much from year to year.  When we have a warm spring and you would expect early germination, the crabgrass still germinates about the same time (around May 1).  When we have a cold spring, like this one, the crabgrass still seems to germinate about the same time.  It may be a few days early or late, but not a few weeks.

I am still expecting crabgrass to germinate soon.  If you do not have your preemerge down, I would recommend applying it as soon after the snow melts as possible.  I put mine on last week.

Horticulture Hall

Horticulture Hall

Horticulture Hall



May 29, 2012

On March 23, 2012, I put up a blog post about knotweed.  This annual broadleaf germinates very early each season and is often mistaken for crabgrass.

At that time, I posted a picture of an unknown weed that had just emerged at Veenker Memorial golf course in Ames, Iowa that had some of the characteristics of knotweed, but did not look like the other knowtweed germinating on the course at that time.  This patch did look a lot like crabgrass, but it was clearly too early for crabgrass to emerge.  The first picture below is the way that weed appeared on March 23.

I mentioned that I would return later and see what the weed was.  The second picture was taken in late May in the same location on the course.  It is clearly knotweed.  I'm not sure why the newly emerged seedlings looked so much different than the other knowtweed on the course.  You can see why people mistake some knotweed for crabgrass each year.

Photo 1.  Unknown weed on Veenker golf course as it appeared on March 23, 2012.  It had many of the characteristics of crabgrass, but was clearly a broadleaf.

Knotweed at Veenker Golf Course


Photo 2.  Picture taken in late May on the same location where the first photo was taken in March.  It is clearly knotweed.



April 2, 2012

I did another check for germinated crabgrass today, April 2. I'm starting to see more of it peaking through in protected areas on campus. This is weeks earlier than normal.

I am not seeing it in rural areas or more exposed areas on campus yet.

Nick Dunlap got the first applications of our preemergence timing trial out this morning. It includes Baracade and Dimension at label rates. We will apply again on previously untreated plots on April 16 and May 1. I'll let you know what we see. We are not seeing any germinated crabgrass at the research station yet.

I previously recommended that you consider going two weeks early this year, which would be about April 15 in central Iowa. If your areas are protected sites in an urban environment, I would recommend applying as soon as you can. We will do Reiman Gardens this Wednesday, which is earlier than we have ever made applications to that site.

Remember that crabgrass is very susceptible to frost. If we get a late freeze, which I still think is likely, the problem will take care of itself. In the absence of a freeze, however, we could see a lot of crabgrass this summer.

Germinating crabgrass, courtesy of Andrew Hoiberg.



March 23, 2012

The warm weather and the unusually large number of growing degree days (GDD) for late March have led to a lot of concern about early crabgrass germination.

First of all, let me tell you about my experience with this over the past 33 springs in central Iowa. I watch crabgrass germination dates very closely because of my research with preemergence herbicides. I have often been worried about early germination because of warm conditions in March and early April, only to find that cragbrass germinates at about the normal time, around May 1 in the Ames/Des Moines area. This is also true in cooler years when I anticipate late germination may occur later than usual. The germination is still around the normal time.

This year is one of the warmest in my experience and I’ll be watching the situation closely. At the research station we plan to apply applications of Dimension (dithiopyr) and Baracade (prodiamine) separately in test plots on April 1, April 15 and May 1 to determine what kind of crabgrass control we can expect in a warm year like this. I’ll keep you informed of the results as things develop. We will also discuss this work at field day on July 19.

We have had several calls and e-mails in the last few days about emerging crabgrass. I spent March 21 looking for any emerged crabgrass around town and around Veenker golf course. I did not find any. What I am seeing is a common problem at this time of year. Prostrate knotweed (Polygonum aviculare), which is a broadleaf species common in compacted areas around sidewalks and on in the center of sports fields, looks a lot like crabgrass as it emerges. This is one of the earliest germinating weeds in the landscape and is often observed in March and even in late February. While it is a broadleaf, it can look a lot like a grass when it first emerges. It is often mistaken for crabgrass by lawn care customers early in the season. It is a problem for lawn care specialists because the customers will often tell them that they have applied their preemergence herbicide too late. It is not until the leaves begin to mature that it becomes clear that this is a broadleaf and not a grass.

While I found knotweed everywhere I looked on the 21st, I did not see any crabgrass, even on bare soil on southern exposers where the crabgrass usually emerges first.

I did notice some interesting things about the knotweed. While this species is considered to be an annual in this location, it does appear that some of the runners may not have died during the unusually warm conditions this winter and that leaves were emerging from the runners. That is very unusual. I'm not quit sure of this yet and I will continue to watch this development over the next couple of weeks.


The picture below is of knowtweeed germinating from seed in March.

A close up of the plants shows that it is clearly not crabgrass, but it
does look like crabgrass from a distance.


I also saw quite a bit of annual bluegrass (Poa annua) which usually germinates in fall, but which can germinate in spring. This species can also be mistaken for emerging crabgrass.



The last picture is of an annual broadleaf that looks so much like crabgrass that it fooled me when I first saw it. Its texture and color and size exactly matched germinating crabgrass. The main difference is that the leaves are rounded on the end, whereas crabgrass will have more of a pointed leaf tip. I can see how people are easily fooled by these seedlings at this stage, however. I think that these emerging plants are knowtweed, but they are a little different and I'm going to continue to watch them over the next few weeks. If it turns out to be another species, I will let you know in a future blog.

So, what am I recommending as far as the best time to apply preemergence herbicides this spring?

I am currently telling people to go about two weeks earlier than they normally would. If you are used to getting them down by May 1, try to get them down by April 15. The timing will be later in Northern Iowa and earlier in Southern Iowa.

Dithiopyr (Dimension) has some post activity on newly germinated crabgrass and if you have to go later, that may be your best product.

If people call me on May 1 and ask if they should still treat, I'm going to tell them yes. I'm still betting that crabgrass germination will occur close to its normal time.





August 6, 2012

On July 3, I put up a post about Windmill grass (Chloris verticilatta) and on August 2 a post on Bermudagrass (Cynodon dactylon).  You will also find several posts on crabgrass (Digitaria spp) over the last two years.

Right now is early August, the seedheads from each of these species are visible in lawns.  I thought it would be a good time to get some pictures of the three species to help with identification.

Here is the seedhead of Windmill grass.  This warm-season grass has been moving into central Iowa and north for the past few years and there is quite a bit of it in Ames this yeas.  It branches off in all directions and rolls like a tumble weed when it breaks lose from the stem.  It is a little larger than most crabgrass seed heads.

Windmill grass forms a tight-knit mass of slolons in the lawn and at this time of year it is covered by seedheads.  (See picture 2)

Here is bermudagrass.  It also has a branching seedhead, but notice how each branch arises from the same point.   It is quite rare in central Iowa, although it looks like it may be expanding in this region.

This is crabgrass.   It is one of the most common weeds in Iowa.  Unlike the other two which are perennials, this one is an annual and has to come back from seed each year.  It is similar to bermudagrass, but notice how it branches off from varying areas along the stem.