Weeds are classified into two major categories: grassy weeds and broadleaf weeds. Like grasses, weeds are also classified as coolseason and warm-season. Preemergent herbicides differ in their ability to provide control, so the first step in your program is to correctly identify the weed. Make sure you read the label, because herbicides that are designed for one type of species won’t n e c e s s a r i l y work on others.
K n o w i n g what type of weeds you’re dealing with will help you correctly time your pre-emergent herbicide application. Weed species germinate at different soil temperatures. Because pre-emergent herbicides need to be applied prior to seed germination, it is necessary to know when the weed species in question will germinate.
“Different weeds exist in different parts of the country,” says Matt Bradley, marketing manager for herbicides, Bayer Environmental Science, Research Triangle Park, North Carolina. “Florida and other Southern states have problems with weeds in warm-season turf grasses, including Bermudagrass, zoysiagrass, centipedegrass, St. Augustine, and seashore paspalum.”
Dean Mosdell, technical manager with Syngenta, says cool-season annual weeds such as annual bluegrass, chickweed, Carolina geranium, henbit and purple deadnettle can be prevented by applying a preemergent herbicide to the lawn well before weed seed germination occurs.
As the name i m p l i e s , p r e - emergent weed control works by applying a specially formulate d h e r b i c i d e early in the growing season. This creates a barrier—or a weed prevention zone—in the top one to two inches of soil, preventing weeds from emerging through the surface. A good pre-emergent product applied at the right time will ensure that your clients’ lawns will retain their well-manicured beauty and avoid unsightly weeds from popping up later in the year.
“Pre-emergent weed control works by inhibiting the development of newly germinated seeds.
Based on that, you have to get it down early, even in split applications. As soon as the weed seed germinates, it comes in contact with the barrier and, basically, dies. The herbicide doesn’t stop germination; the barrier stops the development of the weed seed,” explains Mosdell.
You don’t want to run the risk of penetrating the shield, so it’s important not to aerate or otherwise puncture the soil after application. If you do, you may allow weeds to grow through the aerated points, which would defeat the entire purpose of applying a pre-emergent product. You don’t necessarily have to saturate an entire yard with a pre-emergent product for it to be effective. Instead, prior to applying the herbicide, scout out the property a few months before, note any problem spots, then treat only those weed-prone areas.
It is also important to remember that pre-emergent herbicides must be watered in after applying, to create the barrier that prevents germination. This should be done at least seven days prior to the initial germination date, to allow time for the herbicide barrier to be established in the soil.
You should also be aware of the tolerance level of a particular turfgrass before you apply an herbicide.
There are many examples where a herbicide may be labeled for use on one turfgrass species, but may severely injure or kill another. This is of particular importance if your business serves clients who live in areas where both cool- and warmseason grasses are planted.
Timing is everything
When it comes to pre-emergent weed control, timing is everything. Pre-emergent herbicides are only effective if applied before the annual grass weeds emerge. Apply too late and the pre-emergent herbicide will be totally ineffective.
In general, the two dates to remember are March 15 and September 15. Those are the two dates of the year around which pre-emergent fertilizers should be applied so that they activate before seasonal weeds make an appearance. There are two application dates because generally, there are two types of weeds: winter weeds and summer weeds. This is especially true in regions that don’t completely freeze over in the winter.
To prevent summer weeds, the application date is March 15th, or when average soil temperatures reach above 50 degrees. Major summer weeds like crabgrass, dallisgrass, goosegrass, and Johnsongrass will only emerge once the soil is consistently over this temperature. In warmer areas, the date will probably be earlier.
September 15 is the fall application date that will limit any late fall growth and, hopefully, eliminate weeding work the following year.
Sounds simple, right? Wrong. One thing about Mother Nature: she is quite unpredictable.
“Dates are fine in general, but climate conditions vary year by year, so it is important that applications of pre-emergent herbicide products be coordinated with soil temperatures and the type of weed rather than just the date,” says Ken Klopp II of LebanonTurf, Lebanon, Pennsylvania. “For instance, crabgrass is a weed that starts germinating when the soil temperatures r e a c h 5 5 degrees for three consecut i v e d a y s .
Goosegrass, on the other hand, needs soil temperatures around 60 degrees to start germinating,” he added.
Not only is temperature an important aspect of weed germination, but environmental factors that break down the herbicide also play a role. Weather conditions can wreak havoc and alter not only your schedule, but the herbicide’s effectiveness as well.
“Too much rainfall or not enough can either break the herbicide down more quickly than expected, or slow the activation down and allow time for the weed seeds to germinate. For this reason, repeat applications may be necessary to improve and prolong weed control,” Klopp said.
Other weather conditions will have an effect on your pre-emergent herbicide. A prolonged drought or unusual cold weather that freezes the ground or covers it with ice and snow will both play a role in determining your application schedule.
“This past year in the Philadelphia, Pennsylvania area, it was tough for our pre-emergent application division, because we had excessive heat and that seemed to create a failure in the pre-emergent products we used,” says Andy Sykes, owner of Garrett Churchill, Abington, Pennsylvania. “We usually only apply the pre-emergent herbicides once, but based on the weather this past summer, we’re going to split our applications this year, in order to ensure a successf u l e x t e n d e d w e e d c o n t r o l process.”
In unusual weather situations like this, w h e r e t h e u n e x p e c t e d occurs, you’ll need to quickly adjust your program. Successful landscape contractors don’t lock themselves into a set routine and are willing to add additional applications when conditions warrant.
“We are going to apply a first round of pre-emergent the first week of March and schedule our second round for April 15th, with an additional round the first week of June,” said Walter Cornett, owner of Walter Cornett Landscaping, Sandston, Virginia. “The third round is new this year, because we experienced a very hot and dry summer. Between areas of dissipating grass and the fact that weeds just seem to maintain water longer, these conditions have produced some plentiful crops of weeds in the late summer, so we’ve had to adjust our schedule accordingly.”
Types of herbicides
When it comes to choosing the proper pre-emergent herbicide, there are basically two types: chemical and organic. One of the most commonly used family of preemergent herbicides is the dini-troanilines, sometimes referred to as DNAs. As the seedling germinates, the herbicide is absorbed by young roots and emerging shoots, preventing cell division and effectively killing the plants. Because this family of herbicides is not very water soluble and readily binds to soil particles, they tend to remain near the soil surface and do not leach through the soil profile. The chemical barrier remains intact until soil microorganisms and other factors degrade the herbicide.
“The biggest issue with preemergent herbicides is that they don’t have residual that lasts long enough to withstand 12 months of a growing season,” Bradley said. “So, multiple applications need to be made. If you’re in the Carolinas, one to two applications a year is normal, but in areas of south Florida, you may need to get up to four applications a year.”
In the area of organic lawn care, there has been a great deal of discussion in the past two decades about corn gluten meal. Corn gluten is a biological alternative that has been shown to be an effective fertilizer and pre-emergent treatment. Though some lawn care professionals are not yet convinced, if your client is adamant about not using chemical herbicides, corn gluten is an effective alternative.
Remember, pre-emergent herbicides don’t destroy weeds or their seeds. They simply stop them from growing. Some seeds are known to last 50 years, so effective weed control has to be an ongoing program set on a regular schedule, or the weeds are going to continue to grow. This is the one area where weeds are an advantage to your business. A good weed control program gives you the opportunity to have a regular relationship with your clients and a continual source of revenue all year round.