The onset of spring means it’s time for one of the most important turf maintenance practices of the year: the application of pre-emergent weed control. Pre-emergent herbicides keep grassy weeds—like crabgrass, goosegrass, foxtail, and a handful of broadleaf weeds—from popping up in your lawn.
In the world of turf management, there is a lot of theory and myth; however, here are some good “rules of thumb” to follow.
The magic number
When soil temperatures reach 55 degrees Fahrenheit and hover there for three to seven days, crabgrass seeds begin germinating. This typically occurs in mid March in my home state of Missouri, but every region will vary.
When soil temperatures escalate to 62-65 degrees Fahrenheit, crabgrass will start emerging from the soil. At this stage, it is still very hard to see, and can often be easily misidentified as knotweed. When soil temperatures climb to 73 degrees Fahrenheit, significant emergence occurs.
Monitoring your soil temperatures can help you get the upper hand on potential weeds.
Monitoring your soil temperature
There are multiple approaches to monitoring soil temperatures. For example, you can insert a thermometer at a depth of one inch into your soil profile, record the temperature each day for 3-7 days, and then calculate the daily average.
This approach can lead to varied results depending on your microclimates—such as shade or sun “hot spots”—so it’s a good practice to take all of your temperature measurements in the same area of turf in full sun.
Many people also use referential measurements, such as the use of a flowering ornamental during its blooming period, to assess soil temperature. Forsythia, redbud, dogwood and daffodil—which tend to bloom between March and April—may be good references, depending on your area.
When is the best time to apply pre-emergent?
Timing is critical when applying pre-emergent. Ideally, a pre-emergent application would take place before your soil reaches the 62 degrees Fahrenheit temperature threshold mentioned above.
If you have missed the ideal window for pre-emergent applications, better late than never! Putting down a late application of pre-emergent can still help prevent 75-90 percent of future weed growth. Late applications should be watered in immediately. One-quarter inch of irrigation (or rain) does the trick. When doing a late application, Dithiopyr, found in pre-emergents like Dimension, is labeled for controlling crabgrass in the one-to-three leaf stage.
I’m often asked whether or not you can put down a late application of pre-emergent (to combat future weed growth) at the same time as an application of post-emergent (to kill visible weeds); the answer is that it is possible, depending on what type of chemicals you are using.
If using a granular form, it is not a good idea to have both applications overlap, because the combination can result in nitrogen overkill on your lawn. If, however, you are using a liquid application—say, for example, you are mixing a pre-emergent, post-emergent and fertilizer into one tank—then it is possible, because in this type of application, you can control how much of the liquid fertilizer (and nitrogen) you are adding into the tank mix.
Controlling grassy weeds of any type can be a small hassle, but not to fear—Ewing is here! If you are new to the business or if you’re just interested in some good old-fashioned turf talk, hit up your local Ewing.
Your local Ewing turf expert can fill you in on the different types of herbicides, and rates of active ingredient that work well in your area. Doing your homework can save you and your customer some hassle and hard-earned cash down the road.