Does Preen Kill Grass

If you’ve ever used a weed killer in your lawn, you’ve probably wondered, does Preen kill grass? It’s a common concern for homeowners trying to balance a beautiful lawn with effective weed control. The short answer is no, Preen is not designed to kill established grass, but using it incorrectly can definitely harm your lawn. Let’s look at exactly what Preen is, how it works, and the right way to use it so you can keep your turf healthy and weed-free.

Preen is a popular brand of weed preventer, also known as a pre-emergent herbicide. Its primary job is to stop weed seeds from sprouting. It creates a barrier in the top layer of soil that disrupts the growth of seedling roots. Since your grass is already grown from established plants (not seeds you’re trying to sprout), it should not be affected. However, the keyword here is “should.” Misapplication is where the problems start.

How Preen Actually Works in Your Soil

Preen products contain active ingredients like trifluralin or dithiopyr. These chemicals work by forming a thin layer in the soil. When a tiny weed seed tries to germinate and send out its first root, the chemical stops that root from developing properly. The seedling dies before it ever breaks the soil surface. This is why it’s called a “pre-emergent”—it works before weeds emerge.

Your grass plants have a mature root system deep below this barrier. They aren’t trying to germinate from seed, so the pre-emergent doesn’t target them. But there’s a catch. If you apply Preen over newly seeded grass areas, you will kill those grass seeds just like weed seeds. The product can’t tell the difference.

The Critical Difference: Weed Preventer vs. Weed Killer

This is the most important concept to grasp. Mixing these two up can lead to lawn disaster.

Weed Preventer (Preen): Stops seeds from growing. Applied before weeds are visible. Safe for established plants.
Weed Killer (Post-emergent herbicide): Kills existing, visible weeds. Applied after weeds have sprouted. Often kills any plant it touches, including grass, unless it’s a selective formula.

Using a weed killer where you meant to use a preventer will absolutely damage your lawn. Always read the label carefully to know which product you have in your hands.

When Preen Can Become Harmful to Your Lawn

Even though Preen is labeled as safe for established lawns, certain mistakes can cause yellowing, thinning, or even dead patches. Here are the main ways Preen can hurt your grass:

1. Applying on New Grass Seed or Sod

This is the number one cause of Preen-related lawn damage. If you’ve recently seeded your lawn or laid new sod, you must wait. New grass needs time to become firmly established. The roots need to grow strong and deep enough to be safe from the pre-emergent barrier.

Wait at least until you’ve mowed new grass 3-4 times.
For sod, wait a full growing season before applying any pre-emergent.
Always check the specific product label for its recommended waiting period.

2. Using the Wrong Preen Product

The Preen brand makes several formulations. Some are designed for garden beds, not lawns. Using a garden bed product on your lawn can introduce ingredients or concentrations that are not ideal for turfgrass. Always choose a product clearly labeled for use on lawns, such as “Preen Lawn Weed Control.”

3. Over-application or Uneven Spread

More is not better. Applying too much Preen can overwhelm the soil and potentially stress grass roots. An uneven spread can result in concentrated spots where the chemical barrier is too thick, preventing even your grass from taking in water and nutrients effectively. Always use a calibrated spreader for even application.

4. Applying at the Wrong Time

Timing is everything with pre-emergents. The goal is to have the barrier in place just before weed seeds germinate. For summer weeds like crabgrass, apply in early spring. For winter weeds, apply in early fall. Applying during peak grass growing season can sometimes stress the lawn, especially during heat or drought.

Step-by-Step: How to Safely Apply Preen to Your Lawn

Follow these steps to get the weed prevention benefits without risking your grass.

Identify Your Grass and Target Weeds: Know what type of turf you have (cool-season or warm-season) and the primary weeds you battle (e.g., crabgrass, poa annua). This informs your timing.
Choose the Correct Product: Purchase a Preen formula made specifically for lawns. Double-check the label.
Prepare Your Lawn: Mow your grass to its recommended height. Remove any debris or thick thatch. If soil is compacted, consider aeration before application, as this helps the product move into the soil.
Check the Weather: Apply when no rain is expected for 24-48 hours. You also want a calm day with little wind to prevent drift.
Calibrate Your Spreader: This is crucial. Set your broadcast or drop spreader to the rate listed on the Preen bag. Fill the hopper on a hard surface, not on the lawn.
Apply Evenly: Walk at a steady pace, slightly overlapping your passes to avoid missed strips. Apply half the product walking north-south, and the other half walking east-west for superb coverage.
Water It In: Lightly water the lawn after application. This washes the granules off the grass blades and into the soil where the barrier forms. Use about 1/4 to 1/2 inch of water.
Mark Your Calendar: Note the application date. Most pre-emergents last 3-5 months, so you’ll know when to consider a second application.

What to Do If You Think Preen Has Damaged Your Lawn

If you see yellowing or thinning grass after an application, don’t panic. Take these steps to help your lawn recover.

Water Deeply: Give the area a deep, thorough watering. This can help dilute the chemical concentration in the soil and move it beyond the root zone.
Avoid Fertilizer: Don’t add fertilizer to try and “push” growth. Stressed grass can be burned by fertilizer. Wait until you see signs of recovery.
Reseed Carefully: You cannot reseed immediately. The pre-emergent barrier will still be active and kill new grass seeds. You must wait until the product’s effectiveness wears off, typically until the next seeding season.
Focus on Health: Keep the lawn mowed at a higher setting to reduce stress. Ensure it gets adequate water. The grass may grow out of the damage once the chemical dissipates.

Natural Alternatives to Preen for Lawn Weed Prevention

If you’re hesitant about using any chemical product, there are effective cultural practices that prevent weeds naturally.

Maintain a Thick, Healthy Lawn

The best defense is a dense turf. Weeds are opportunist that move into thin, weak areas.

Mow High: Keep your grass taller. This shades the soil, making it harder for weed seeds to germinate.
Water Deeply and Infrequently: Encourage deep root growth by watering less often but for longer periods.
Fertilize Appropriately: Feed your lawn based on a soil test to provide the nutrients it needs without excess growth.

Use Corn Gluten Meal

This is a natural pre-emergent herbicide. Corn gluten meal is a byproduct of corn processing that inhibits root formation in sprouting seeds. It’s safe for kids, pets, and established grass, and it also adds nitrogen to your soil. Apply it in early spring and early fall, just like synthetic pre-emergents.

Common Myths About Preen and Lawn Care

Let’s clear up some frequent misconceptions.

Myth: “Preen will kill all the weeds already in my lawn.”
Fact: No, it only prevents new weeds from sprouting. You must remove existing weeds first with a post-emergent herbicide or by hand-pulling.

Myth: “I can apply Preen and seed my lawn the next week.”
Fact: Absolutely not. You must wait the full period stated on the label, often 3-4 months, or your grass seed will fail.

Myth: “One application of Preen lasts all year.”
Fact: Most products last 3-5 months. For season-long control, you usually need a spring and a fall application.

FAQ: Your Questions Answered

Can I use Preen on all grass types?

Most Preen Lawn products are safe for common cool-season and warm-season grasses like Kentucky bluegrass, fescue, ryegrass, and Bermuda. Always verify on the product label for any specific restrictions, especially for sensitive grasses like St. Augustine or Centipede.

How long after using Preen can I plant grass seed?

You need to wait until the pre-emergent effect has worn off. This is typically one full growing season. For a spring application, you’d usually wait until fall to reseed. Check the specific product for its labeled waiting period, as it can vary.

Is Preen safe for pets and children?

Once the application is watered in and the granules have dissolved, the product is considered safe for people and pets to walk on. Always follow the label instructions, which usually advise keeping them off the lawn until the product has been watered in and the grass is dry.

What’s the difference between Preen and a fertilizer with weed preventer?

Weed-and-feed products combine fertilizer and pre-emergent (or sometimes post-emergent) herbicide. Preen Lawn Weed Control is primarily a preventer. Using a separate fertilizer and pre-emergent gives you more control over timing, as they often have different optimal application schedules.

Can I apply Preen after aerating my lawn?

Yes, aerating before an application is actually beneficial. It helps relieve soil compaction and allows the pre-emergent to move into the soil more effectively to form it’s barrier. Just be sure to apply the Preen after you’ve aerated.

Final Thoughts on a Healthy Lawn Strategy

Understanding that Preen is a preventive tool, not a curative one, is key. It will not fix an already weedy lawn, but it is a powerful part of a proactive lawn care program. The goal is to combine timely pre-emergent applications with excellent lawn maintenance practices. By mowing correctly, watering wisely, and feeding your soil, you create an environment where grass thrives and weeds struggle to get a foothold. Remember, the healthiest lawns are built from the soil up, not from the sprayer down. With careful use, Preen can be a helpful ally in your quest for a thick, green, and resilient turf.