What Type of Pre-emergent Spray for Lawns?

Types of Weeds

• Many pre-emergent herbicides are selective, meaning they are designed to work with specific types of weeds. Typically, those weeds fall into two categories: broadleaf and grassy. Broadleaf weeds include dandelions and creeping wood sorrel. Grassy weeds get their name because they resemble grass, such as crabgrass or dallisgrass. Both types can spread prolifically on your lawn if not treated before they emerge.

Broadleaf Basics

• These weeds are easy to recognize -- they look completely out of place in your lawn. The leaves often are large and flat, and many bloom into flowers. The seeds germinate throughout the warmer months, going dormant in the fall to lie in wait under the soil in your yard all winter. Several pre-emergent chemicals target broadleaf weeds, including isoxaben, benefin and oryzalin.

Good-bye, Grassy Weeds

• Grassy weeds need different chemical applications to prevent germination. Many of these chemicals also prevent grass seed from growing, so use these only on established lawns. Control these weeds with chemicals such as pendimethalin, dithiopyr and benefin mixed with trifluralin. Read the labels carefully to ensure these chemicals won't harm your lawn; not all work with every type of grass.

When to Treat

• Use pre-emergent before the weeds sprout; after you see green poking through, it's too late for pre-emergent interference. For weeds that typically arrive in early summer, such as crabgrass, apply the herbicide in late spring or when the nighttime temperatures hit 55 to 60 degrees Fahrenheit consistently. Broadleaf weeds might need additional treatments, every six weeks or so starting in late spring or early summer.