Should I Kill the Weeds Before I Put Down Topsoil?

Weed Growth Patterns

• Knowledge of weed types and growth patterns is a key to effective weed control before topsoil is added. Weeds such as crabgrass, goosegrass and foxtails, are annual grassy weeds that set seeds in summer and fall. Their growth cycle is interrupted by pulling them before they flower and set seed. Purslane, chickweed and thistles spread by underground rhizomes, rooting tendrils or seeds. Solarization is an effective method of weed control for a large flat area.

Solarization

• Weeds need sunlight to grow but will die if the heat is excessive. A pre-topsoil solarization treatment requires six to seven weeks. The seed bed is cultivated, raked smooth and sprinkled with water. A thick layer of clear plastic is applied to the entire area and secured at the edges with soil. Weed seeds germinate but are killed by the heat before they grow. Topsoil is then applied to a weed-free surface.

Weed Cultivation

• Weeds are also eliminated by irrigating the area, allowing weed seeds to germinate, and cultivating them with a scuffle hoe. Repeat the pattern of irrigation, germination and cultivation two or three times for best results. Cultivation is most effective to a depth of one inch. Eliminate weeds before they have grown more than four leaves. The soil should be allowed to dry thoroughly before adding topsoil.

Pre-emergent Weed Killer

• Corn gluten meal pre-emergent weed killer eliminates broadleaf weeds but not grassy weeds. It allows the weed plant to germinate but seedlings will not grow. Corn gluten meal weed killer is applied in spring to wet ground and must be kept moist. It is effective on weeds such as henbit, chickweed, dandelion, purslane and knotweed. Purchased topsoil or sifted backyard compost may be applied when weed seeds no longer show signs of germinating.