Weed Control

The Two Types of Weed Control

Every weed control product falls into one of two categories: pre-emergent or post-emergent. Knowing the difference is the foundation of a weed-free lawn.

Pre-emergent herbicide

Applied before weeds germinate. Creates a chemical barrier in the soil that prevents weed seeds from sprouting. Doesn't kill existing weeds — only prevents new ones.

Post-emergent herbicide

Applied after weeds have already grown. Kills weeds that are visible. Most effective when applied to young, actively growing weeds.

The Iowa Weed Control Schedule

Late March to Mid-April: Pre-emergent for Crabgrass

Crabgrass is the #1 weed enemy of Iowa lawns. It germinates when soil temperatures hit 55°F at a 4-inch depth — which usually happens in mid-to-late April in this area.

Apply pre-emergent 2-3 weeks before crabgrass germination. In Iowa, that means late March to mid-April. Common timing markers:

• When forsythia bushes start blooming
• When soil temperature consistently hits 50°F
• About 2 weeks after the last hard frost

Common pre-emergent products contain prodiamine, pendimethalin, or dithiopyr.

Late May to Early June: Second Pre-emergent (Optional)

Most pre-emergents last 3-4 months. If you applied early, a second light application in late May extends crabgrass protection through the worst summer weeks.

April to October: Spot Treat Broadleaf Weeds

Dandelions, clover, ground ivy, plantain, thistle, and other broadleaf weeds need post-emergent herbicide. Best timing:

• Spring (April-May): Most effective. Weeds are young and actively growing.
• Fall (September-October): Second-best window. Weeds are moving sugars to roots, pulling herbicide deep with them. Most effective for tough perennials like dandelions.
• Avoid summer: Heat stress means herbicide can damage your lawn along with the weeds.

Common Iowa Lawn Weeds and How to Kill Them

Crabgrass

Annual grassy weed. Germinates in spring, grows through summer, dies at first frost. Prevention with pre-emergent is the only effective control. Once mature crabgrass appears in summer, post-emergent products like quinclorac work but it's much harder to kill.

Dandelions

Perennial broadleaf weed. Kill with 2,4-D, dicamba, or MCPP-based products. Most lawn weed-and-feed products contain these. Best applied in spring or fall to actively growing dandelions.

White Clover

Indicates low soil nitrogen. Best controlled by fertilizing properly so grass crowds it out. For active treatment, products with triclopyr work better than standard 2,4-D.

Ground Ivy (Creeping Charlie)

One of the hardest weeds to kill. Spreads by runners. Requires a product with triclopyr applied in fall when the plant is moving nutrients to the roots. May need 2-3 applications over multiple seasons.

Plantain

Broad flat leaves, common in compacted soil. Standard 2,4-D works. Aerate to fix the underlying compaction issue.

Nutsedge

Appears yellow-green and grows faster than your grass. Standard weed killers don't work. Requires sedge-specific products with sulfentrazone or halosulfuron.

The Best Weed Control is a Thick Lawn

Chemical weed control is the bandaid. A thick, healthy lawn is the cure. Weeds need bare soil and sunlight to germinate. A dense lawn smothers them before they start.

Three things crowd out weeds better than any herbicide:

• Mow tall (3-3.5 inches) so existing grass shades the soil
• Overseed annually to keep the turf thick
• Fertilize on schedule so grass outcompetes weeds for nutrients

Important Cautions

• Don't apply weed killer to newly seeded lawns. Wait until after 3-4 mowings.
• Pre-emergent kills new grass seed too. If you're spring overseeding, skip pre-emergent that year.
• Read the label. Some weed killers harm trees, shrubs, and gardens if they drift.
• Spot treat when possible. Blanket weed-and-feed products waste herbicide on areas that don't need it.
• Don't spray in wind or extreme heat. Both cause drift and lawn damage.