Our early spring has brought us weeds and lots of them. The problem could have been abated with the application of a pre-emergence herbicide. I want to discuss what pre-emergence herbicide is, how it works, and why I recommend it so often.
Weeds can be divided into categories based on life cycles, including perennials, biennials and annuals. These weeds can also be broken down even further by the cool or warm season during which they are suited to grow.
Perennial weeds are usually more difficult to control than annual weeds. Besides reproducing by seed, perennial weeds reproduce by vegetative structures such as stolons , rhizomes, tubers, taproots and bulbs.
Biennial weeds live for two years. During the first year, they germinate from seed and produce vegetative growth. In the second year, biennials form a seed stalk, produce seed and die. Biennial weeds aren’t as common in lawns as annual and perennial weeds. Good examples include wild carrot (Queen Anne’s lace) and bull thistle.
Annual weeds complete their life cycle in less than a year and reproduce by seed. Annuals may be further divided into winter (cool season) and summer (warm season) weeds. Winter annuals germinate in late summer and early fall, live during winter, and die in late spring or early summer. Good examples include annual bluegrass, common chickweed, henbit and swinecress.
Summer annuals germinate in spring months, live during the summer, mature in fall, and are usually killed with frost. They include crabgrass, goosegrass, lespedeza and knotweed. Germination of annual weed seeds is mostly dependent on soil and air temperatures. A soil temperature of 55 degrees is about where we see the switch between cool- and warm-season weeds.
Our soil temperature is approaching 60 degrees. We‘re in an earlier-than-normal transition and get to have both warm- and cool-season annual weeds at the same time. Hooray.
Since annual weeds are only produced by seed, the best way to control them is to get the seed early. This is where pre-emergence herbicide comes into play. Pre-emergence herbicides are applied to lawns prior to weed seed germination – September to early October for cool-season weeds and March and again in June for warm-season weeds. This can vary depending on weather.
Pre-emergence doesn‘t kill seeds but causes abnormal cell development or prevents cell division when the seed does germinate. It stops the plant from growing by inhibiting cell division in the shoot and root tips while permitting other cell duplication processes to continue. Allowing this product to disturb the natural germination process is why getting the herbicide out in a timely manner is important.
Common turfgrass pre-emergences have the active ingredient Pendimethalin (Lesco Crabgrass Preventer), Dithiopyr (Preen Southern Weed Preventer) and Prodiamine (Barricade). The easiest way to broadcast these products is with a rotary spreader, so get a granulated formula. Choose one with the 0-0-7 fertilizer to add a little potassium to your lawn, which is important for root growth and disease resistance.
Different pre-emergences have varying lengths of effectiveness. Some will last three months, while others may last a year.
Some things to be conscious of concerning pre-emergence herbicides are: They don’t affect a weed that is already present, just the seed.
Apply only according to the written label on the packaging. Do not apply to an area that is to be a lawn or to a newly sodded lawn. They will adversely affect the root growth and can kill the sod. Do not apply where seeding may take place.
This includes overseeding with rye grass seed, new lawn bermuda and centipede seeds, wildflower seed or vegetable garden areas.
Make sure this herbicide gets watered in within a reasonable time frame with rain or irrigation. That usually means about ¼ inch within one to seven days.
Most pre-emergence herbicides are recommended for established turfgrass only. There are a few products that can be used in ornamental beds as well to keep down annual weeds. Look for the active ingredients Trifluralin (Preen) or Orzyalin (Surflan) for weed control in ornamental beds (again, read the label first).
Preemergence herbicides form the backbone of weed control programs. They do not control all weed seeds that may be present in a landscape, but they are effective for many of the most common weeds. Get it out soon or it will be a long, weedy summer.
Reach Campbell Vaughn, the agriculture and natural resources cooperative extension agent for Richmond County, by e-mailing firstname.lastname@example.org.