No matter how hard a gardener tries to develop or maintain a healthy, weed-free lawn, these undesirable plants are bound to pop up every now and then. Lawn weeds are a gardener's bane because they not only reduce the aesthetics of a lawn, but they compete with desirable grass plants for nutrients and moisture in the soil. Fortunately, readily available formulations called weed killers or herbicides kill these invasive, nuisance weeds.
There are two main types of weed killers--pre-emergent and post-emergent. Post-emergent weed killers are classified into selective and non-selective that are systemic or contact.
Pre-emergent weed killers target annual grass weeds that germinate from seed, grow during the season, develop more seeds and die after 12 months, such as goosegrass, crabgrass, annual bluegrass and foxtail. These weed killers inhibit cell division in young roots of annual grass weeds, causing young seedlings to die soon after germination.
Post-emergent weed killers kill actively growing weeds that are clearly visible in the soil. Selective post-emergent weed killers control annual, perennial and biennial broadleaved weeds without damaging nearby vegetation as compared to selective herbicides that kill any vegetation they come into contact with. Post-emergent weed killers are either systemic, that move throughout the plant after being absorbed by the leaves, or contact that only kill that part of the weed it is applied over.
Pre-emergent weed killers are effective before the emergence of annual weeds. Apply it at least a week before the initial date of germination so it establishes a barrier in the soil. According to the website All About Lawns, gardening experts around the United States suggest applying pre-emergent weed killer around mid-March for summer annual weeds and mid-September for winter annual weeds.
Apply post-emergent weed killer to a relatively young and actively growing weed in spring or early fall, when soil temperature is between 75 to 80 degrees F.
Wear a face mask, protective clothing and gloves prior to spraying any weed killer.
Spray the pre-emergent weed killer evenly over the soil or spread granules and water lightly afterward to assist soil penetration.
Spray the post-emergent weed killer directly over the undesirable grass weed, working toward maximum coverage and contact.
If pre-emergent weed killer is applied in spring, wait until fall before reseeding the lawn. For conditions where immediate reseeding is necessary, rake the soil to break the chemical barrier and add an even layer of peat moss over it. Also, heavy spring rains break the soil barrier, so apply the weed killer when there is no chance of rain for at least 24 hours.
Do not apply post-emergent weed killer during spring green-up when the weeds end dormant period, and refrain from mowing the lawn for up to three days afterward.