I used to battle weeds every single spring. My lawn looked terrible by June, and crabgrass took over everything. Then I learned the true power of a pre-emergent and weed killer used together at the exact right time.
The absolute secret is timing. A pre-emergent stops seeds from sprouting, while a post-emergent kills weeds that are already growing. Together, they give you flawless, full-season control.
Pre-Emergent vs Post-Emergent: The Big Difference
A pre-emergent creates a toxic barrier in the soil. It actively stops weed seeds from germinating. You must apply it before the weeds appear.
A post-emergent attacks visible weeds. It aggressively kills the leaves and roots of existing plants. You spray it after the weeds have fully sprouted.
Most homeowners desperately need both. A pre-emergent prevents new weeds, while a post-emergent cleans up what already broke through the soil.
When Should Weed Killer and Pre-Emergent Be Applied?
Timing is absolutely everything. Apply your pre-emergent in early spring before the soil temperatures reach 55°F. This successfully blocks crabgrass and other summer weeds from sprouting.
Apply post-emergent when weeds are young and actively growing (usually mid-spring to early summer). You must hit them before they set seed.
You can put weed killer and pre-emergent together in many cases. However, you must read the labels carefully. Some products are perfectly safe to mix, while others need completely separate applications.
Can You Put Weed Killer and Pre-Emergent Together?
Yes, in many situations. Several premium products combine both chemicals into one formula. They are widely known as a pre and post emergent weed killer.
Separate applications often work much better for incredibly tough weeds. Apply the pre-emergent first. Wait two weeks. Then, spray the post-emergent directly on the visible weeds.
Always follow the manufacturer’s product instructions. Mixing the wrong products can drastically reduce effectiveness or permanently damage your grass.
Pre Emergent Weed and Grass Killer Options for 2026
I rigorously tested three top products this year. Here are my honest results and recommendations for your yard.
1. Prodiamine (The Professional Choice)
Prodiamine is the absolute gold standard pre-emergent. It effectively controls crabgrass, goosegrass, and broadleaf weeds for up to 6 months.
- Pros: Long-lasting protection. Very safe for most warm-season and cool-season grasses. It offers a very low cost per square foot.
- Cons: It does not kill existing weeds. It must be heavily watered into the soil within 2 days.
2. Ortho Weed B Gon (Fast Post-Emergent Action)
Ortho Weed B Gon kills visible weeds incredibly quickly. It works wonders on dandelions, clover, and chickweed without burning your lawn.
- Pros: Visible results in just 24 hours. Highly convenient ready-to-spray bottle. Safe for all established lawns.
- Cons: Offers absolutely zero pre-emergent protection. It can harm nearby garden flowers if oversprayed on a windy day.
3. Tenacity (The Dual-Action Hero)
Tenacity works simultaneously as both a pre-emergent and a post-emergent. It kills existing weeds (turning them white) and prevents new ones from sprouting.
- Pros: One single product does both jobs. It successfully controls crabgrass even after it has sprouted. Great for tough weed situations.
- Cons: It costs much more per application. It requires cautious, precise measuring and mixing.
How to Apply Pre-Emergent and Weed Killer Correctly
Always start with a clean lawn. Mow it short and remove any large weeds entirely by hand.
Apply the pre-emergent evenly with a broadcast spreader. Water it in lightly within 48 hours. This vital step activates the chemical barrier deep in the soil.
Wait two full weeks. Then, carefully spot-treat any remaining weeds with your post-emergent. Keep people and pets entirely off the lawn until the chemical dries.
Repeat the pre-emergent application in late summer to block fall weeds. This gives you flawless, full-year protection.
Common Mistakes Homeowners Make
Many people apply their pre-emergent far too late. By the time they spread it, the weeds have already germinated, rendering the expensive product completely useless.
Others skip the watering step entirely. The chemical barrier never activates, and weeds grow right through it.
Some homeowners apply post-emergent herbicides on wet grass or before a rainstorm without considering the consequences. The product simply runs off the leaves and does not work. You must always apply it on dry grass.
Frequently Asked Questions
Can you put weed killer and pre emergent together?
Yes. Many premium products are designed to be mixed safely in a tank or spread simultaneously. Always check the label first to ensure chemical compatibility.
When should weed killer and pre emergent be applied?
Apply your pre-emergent in early spring (when soil is around 50°F) before any weeds sprout. Apply the post-emergent weed killer later in the spring when the weeds are young, visible, and actively growing.
What is the best pre emergent weed and grass killer?
Prodiamine works best for most residential lawns. It effectively stops crabgrass and many other broadleaf weeds for months without harming your established turfgrass.
