What Can Be Done to Weed-Infested Lawns in the Spring?

Rake and Aerate

• Dethatching and aerating lawns in early spring helps grass grow strong enough to successfully compete with weeds. Thatch is a layer of organic matter that forms at the base of grass stems, preventing air, nutrients and water reaching grass roots. Rake small lawns vigorously with a lawn rake, removing dead grass, moss and other plant debris, or use a machine dethatcher on large areas. Aerating opens up compacted lawns, allowing grass roots to penetrate the soil and improve their water and nutrient uptake. Remove plugs of soil every 4 inches with a hand aerator, or use a machine aerator. Complete dethatching and aerating before applying weedkillers.

Get Them Young

• Pre-emergent herbicides create a barrier on the soil surface where weed seeds can't grow. Crab grass and other annual lawn weeds die down in winter, but leave behind seeds that sprout in spring. To prevent annual weed seed germination, spread a granular weedkiller containing 1.71 percent pendimethalin, or a similar pre-emergent herbicide, in midspring. Apply at a rate of 3 ounces per 100 square feet, or according to the manufacturer's instructions. Irrigate to activate the chemical barrier. Pre-emergent herbicides prevent grass seeds from germinating, so wait at least four months before reseeding your grass unless the manufacturer's instructions direct otherwise. Raking, digging or otherwise disturbing the soil surface disrupts the weed control barrier and reduces herbicide effectiveness.

Target the Long-Timers

• Applying selective weedkillers in late spring controls established broadleaf weeds in lawns. Selective herbicides for broadleaf weeds, such as mecoprop, dicamba and 2,4-D, don't harm lawn grasses. Spray individual weeds with a ready-to-use herbicide, such as a solution containing 0.22 percent mecoprop-p, when temperatures are 65 to 85 degrees Fahrenheit. Hold the bottle 2 feet from the weed and spray it in the center, aiming to lightly cover the plant, or use according to the manufacturer's instructions. Cover desirable plants and don't spray on windy days, when the herbicide might drift. Clear children and pets from the area before spraying, and heed all warnings on the label.

Water, Feed, Cut

• Watering too much or too little, mowing too short and soil with poor nutrient levels can stress and weaken grass, which allows weeds to take over. Winter rain and snow provide enough water for dormant grass, but lawns require additional water when rainfall reduces in spring. Irrigating deeply but infrequently promotes the best grass growth. Water your lawn until the soil is moist 6 to 8 inches deep, and wait until the top 2 inches are dry before watering again. Apply lawn fertilizer in mid spring, such as a slow-release 10-6-4 fertilizer at a rate of 1 1/2 pounds per 100 square feet. Mow your lawn when it grows 3 inches tall, removing the top inch of growth. Mowing your lawn closer than 2 inches encourages weeds.