Annual weeds complete their life cycle in less than one year and reproduce by seed. Annuals may be further divided into winter (cool season) and summer (warm season) weeds.
Winter annuals germinate in the late summer and early fall months, live during the winter and die in the late spring or early summer with the onset of high air temperatures. Good examples include annual bluegrass (Poa annua), common chickweed, henbit and vetch. These are the weeds that you see invading your lawns now.
Summer annuals germinate in the spring months, live during the summer and mature in the fall months and are usually killed with frost. These warm annuals include crabgrass, chamberbitter, lespedeza and knotweed. The germination of these 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 season weeds and warm season weeds. Our warm-seasonal temperature means we are in a transition period and get to have both warm and cool season annual weeds at the same time. Hooray for us.
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 in to play. Pre-emergence herbicides are applied to lawns prior to weed seed germination. Our standard recommendations are Sept. 1-15 for cool season weeds and March and again in June for warm season weeds. This can vary depending on weather (like our current early spring).
Contrary to popular belief, pre-emergence does not kill seeds but causes abnormal cell development or prevents cell division when the seed germinates. They stop 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.
Some of the common turfgrass pre-emergents have the active ingredient pendimethalin (Lesco Crabgrass Preventer) and prodiamine (Barricade). The easiest way to broadcast these products is with a rotary spreader, so get a granulated formula. I also like to get one with the 0-0-7 fertilizer to add a little potassium to the lawn. Our lawns tend to lack potassium, which is important for root growth and disease resistance. Different pre-emergents have varying lengths of effectiveness. Some will last for three months while others may last a year.
Some things to be conscious of concerning pre-emergence herbicides are:
• Apply only according to the written label on the packaging.
• They don’t affect a weed that is already present, just the seed.
• Do not apply to an area that is to be a lawn or 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 over seeding with rye grass seed, new lawn bermuda and centipede seeds, wildflower seed or vegetable garden areas.
• Make sure that this herbicide gets watered in with about a 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.
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.