Germination of summer weeds can vary from year to year, but on the average, the best time to apply a pre-emergence herbicide to the lawn is the latter half of February through the first week of March. It is usually during this time when the soil temperature normally stabilizes above 55 degrees. Some summer weeds, such as crabgrass, begin germinating at 55 degrees.
Looking at the University of Georgia weather monitoring site in Dearing the last 31 days, the soil temperature at 4 inches bottom out at 49.1 on Jan. 25. It went above 55 degrees on Jan. 30 but then dropped back below on Feb. 1 and is still below 55 as of this writing.
After the next warm spell, whenever that may be (it is supposed to get pretty cold this weekend) it will get above 55 and may stay there. Of course there are no guarantees as the soil temperature many times will fluctuate above and below 55 degrees for a two or three week period this time of the year.
Another signal to put out pre-emergence herbicide is when you see forsythia and crabapple bloom and we started seeing that early this month (in some isolated cases, we saw them blooming in January). But remember, if any crabgrass germinated, below freezing temperatures should have killed it. So in essence, it is not too late to put your weed killer out.
Many people miss these early dates and often ask me even in late March and April if it is too late. Yes it is too late for some weeds, but you at least will be stopping many other weeds from that point on. Some summer weeds don’t germinate until the soil temperature gets 65 or above.
Keep in mind that pre-emergence herbicides, for the most part, only prevent annual weeds, not perennial weeds. Perennial weeds come back vegetatively and from seed while annual weeds come back strictly from seed. Perennial weeds will require multiple applications of post emergence weed killers or will require hand digging.
What are the main summer weeds we want to prevent? Well the main one is the crabgrass I mentioned earlier. It is our number one grassy summer weed. A couple more are goosegrass and carpet weed. If you have St. Augustine and don’t prevent crabgrass from germinating, you are out of luck once it comes up since there is not a good post emergence herbicide that will kill crabgrass that won’t kill St. Augustine.
Pre-emergence herbicides are a useful tool for several reasons:
• The susceptible weeds never show up in the lawn since you applied the chemical before the seed germinates.
• You can use them in granular form, which tends to be easier to apply evenly than sprays.
• Most ornamental shrubbery, trees, and flowers will not be injured, except with herbicides that contain atrazine. In fact some are even labeled for use on some ornamentals. Post emergence herbicides can often damage ornamentals unless you follow label precautions.
• Established lawns are not noticeably injured, while most post emergence herbicides cause slight injury to temporary yellowing of turf grasses.
Do not apply a pre-emergence herbicide if the lawn will be seeded, sprigged, or sodded this spring or early summer. These herbicides can persist in the soil for two to four months (depending on the rain), and can interfere with turfgrass establishment.
Pre-emergence herbicides for lawns are sold under a number of trade names. The most common ones you will find in the Augusta area are benefin (Balan, Crabgrass Preventer), pendimethalin (Halts, Pre-M Plus) and dithiopyr (Dimension, StaGreen CrabEx or Crabgrass Preventer, and Vigoro Crabgrass Preventer). All of these are virtually the same, so you have a lot to choose from.
Whichever pre-emergence you use, don’t get stuck on one and continue to use it year after year. It has been shown that some weeds can build up a resistance to a herbicide when it is used for many consecutive years so after two or three years, switch to another active ingredient, then switch again after the next two or three years.
For this time of year I don’t recommend weed-n-feeds because it is too early to be fertilizing. In all cases, you would be wasting the fertilizer, while in other cases, particularly on centipede, you could be setting yourself up for winter kill or having yellow-green grass after green-up. You can come back if you wish, in about three months with a weed-n-feed.
Always be sure to select herbicides labeled for specific turfgrasses, since they vary in their tolerance. And always read the label.
Remember that pre-emergence herbicides will not control all the weeds in a lawn, but they are useful in preventing most of them.
Reach Sid Mullis, the director of the University of Georgia Extension Service office for Richmond County, at (706) 821-2349 or firstname.lastname@example.org.