If you’re looking at a bottle of Preen and wondering, “does preen kill grass,” you’re asking the right question before you apply. Preen is a pre-emergent weed preventer designed to target weed seeds, not established grass, when applied as directed. Understanding this distinction is key to using it effectively and safely on your lawn or in your garden beds without causing unintended damage.
This article will explain exactly how Preen works, where and how to use it correctly, and what mistakes to avoid. You’ll get clear, step-by-step guidance to ensure your grass stays healthy while those unwanted weeds are stopped before they even start.
Does Preen Kill Grass
The straightforward answer is no, Preen should not kill your established, healthy grass when used properly. Its primary job is to create a chemical barrier in the top layer of soil that stops weed seeds from developing roots after they germinate. Since your grass is already growing with a mature root system, it is not affected by this barrier.
Think of it like a security system for your soil. It keeps new, uninvited guests (weed seeds) from moving in, but it doesn’t harm the current residents (your grass and plants). The active ingredient, typically trifluralin, works on seed germination, not on living plant tissue.
How Preen Works In The Soil
Preen is classified as a pre-emergent herbicide. The “pre-emergent” part is crucial. It must be applied before weed seeds begin to sprout. Once you water it in, it forms a thin shield in the soil about 1 to 2 inches deep.
When a tiny weed seed tries to put out its first root (the radicle), it contacts this barrier. The chemical disrupts cell division in the root tip, preventing the seedling from establishing itself. It never breaks the soil surface, so you never see the weed.
The Critical Difference: Seeds Vs. Plants
This mechanism is why timing and application are everything. Here’s the breakdown:
- Targets Seeds: Preen only affects seeds during the germination process. It is inert in the soil until a seed tries to grow.
- Ignores Roots: It does not move through soil to kill existing roots. The mature root systems of your grass, perennials, shrubs, and trees are safe.
- No Foliar Action: Spraying it on the leaves of existing weeds or grass does nothing. It must be applied to the soil and watered in.
When Preen Could Harm Your Grass
While Preen is safe for established grass, there are specific situations where damage can occur. These are almost always due to incorrect application or timing.
- Applying Over New Grass Seed: This is the most common and serious mistake. If you are seeding a new lawn or overseeding to fill in bare patches, Preen will prevent those grass seeds from germinating just as effectively as it stops weed seeds. You must wait until the new grass has been mowed at least 3-4 times before applying a pre-emergent.
- Using On Stressed Or Dying Lawns: If your grass is already severely weakened by drought, disease, or insect damage, applying any chemical can add stress. It’s best to address the underlying health problem first.
- Extreme Overapplication: Using vastly more than the label recommends can potentially lead to chemical buildup that might affect grass roots. Always follow the package rates.
- Using The Wrong Product: Ensure you are using a standard Preen lawn product. Some Preen formulations are for garden beds only and may have different instructions.
Correct Application On Lawns And Around Grass
To use Preen effectively and safely around your grass, a methodical approach is essential. Following these steps will give you the best results.
Step-By-Step Lawn Application Guide
- Check The Calendar: Timing is the most important factor. Apply Preen in early spring before soil temperatures consistently reach 55°F (when forsythia bushes start to bloom is a classic indicator). This stops summer annual weeds like crabgrass. For winter weeds like poa annua, apply in early fall when soil temps drop below 70°F.
- Mow And Clear: Mow your lawn at your regular height. Rake away any thick layers of thatch or debris so the granules can reach the soil.
- Use A Spreader: For large lawn areas, use a broadcast or drop spreader for even coverage. Calibrate your spreader to the setting recommended on the Preen bag. This ensures you don’t use to little or to much.
- Apply Evenly: Walk at a steady pace, slightly overlapping your passes to avoid streaks or missed spots. Uneven application leads to uneven weed control.
- Water It In: This step is non-negotiable. Within 1-2 days of application, water the lawn lightly. You need about 1/4 to 1/2 inch of irrigation to wash the granules off the grass blades and activate the barrier in the soil, without washing it away completely.
- Delay Further Disturbance: Avoid aerating, dethatching, or heavy railing for at least 4-6 weeks after application. These activities break the chemical barrier, making it ineffective.
Applying Preen In Garden Beds Adjacent To Grass
Using Preen in flower beds, around shrubs, or in vegetable gardens (using the appropriate vegetable formula) requires care to protect nearby grass.
- Create A Clean Edge: Define the border between your bed and lawn clearly.
- Apply By Hand Carefully: Sprinkle the granules directly onto the soil in the bed, keeping them away from the lawn edge. Use a light touch to avoid granules bouncing onto the grass.
- Use A Shield: When applying near the edge, use a piece of cardboard as a shield to block granules from drifting onto the lawn.
- Water Gently: Use a watering can or a hose with a gentle shower setting to water the bed without causing runoff into the lawn area.
Choosing The Right Preen Product
Preen comes in several formulations. Choosing the correct one for your needs is part of using it safely.
Preen Lawn Weed Control
This is the product specifically designed for use on established lawns. It’s safe for most common turf grasses like Kentucky bluegrass, fescue, ryegrass, and Bermuda grass when used as directed. Always check the label for any specific grass type warnings.
Preen Garden Weed Preventer
This is for use in ornamental garden beds, around trees and shrubs, and on ground cover. Do not use this on your lawn. There is also a separate “Preen Vegetable Garden” formula.
Preen With Fertilizer
Some products combine weed prevention with a fertilizer. This can be convenient for a spring feeding and weed control in one step. Ensure the fertilizer analysis (the N-P-K numbers) matches what your lawn needs for that season.
Addressing Common Concerns And Mistakes
What To Do If You Accidentally Apply Preen Before Seeding
If you’ve already applied Preen but need to seed, you have two options. First, you can wait. The product’s effectiveness diminishes over 8-12 weeks, so you can seed after that period. Second, you can actively break the barrier by tilling the soil to a depth of 2-3 inches, which dilutes and disrupts the chemical layer. This is labor-intensive but allows for sooner seeding.
Can You Apply Preen After Weeds Have Sprouted
No, Preen is completely ineffective on weeds you can already see. Those plants have successfully germinated and are beyond the reach of a pre-emergent. For existing weeds, you would need to use a post-emergent herbicide or remove them by hand before applying Preen to prevent the next generation.
Safety For Pets And Children
Once Preen has been watered in and the granules have dissolved, the treated area is generally considered safe for pets and children to walk on. The label will specify a drying time, usually 24-48 hours. As with any garden chemical, it’s wise to store it out of reach and follow all label precautions during application.
Long-Term Lawn Health And Weed Prevention
Preen is a tool, not a complete solution. Integrating it into a broader lawn care strategy yields the best, healthiest results.
Cultural Practices That Reduce Weeds
- Mow High: Keeping your grass taller (around 3 inches) shades the soil, making it harder for weed seeds to germinate.
- Water Deeply And Infrequently: This encourages deep grass roots and discourages shallow-rooted weeds.
- Fertilize Appropriately: A well-fed lawn is dense and competitive, leaving little room for weeds to establish. Soil testing can guide your fertilization plan.
- Aerate Annually: Core aeration reduces soil compaction, improves water and nutrient uptake for grass, and can help break up the thatch layer.
The Role Of Pre-Emergents In A Healthy Lawn
Think of Preen as your first line of defense. It handles the preventative work silently. This allows you to focus your energy on nurturing your grass rather than constantly fighting visible weeds. A single spring application can provide months of control, saving you significant time and effort during the growing season.
Frequently Asked Questions
Will Preen Kill Existing Grass?
No, Preen will not kill existing, healthy grass. It works only on germinating seeds, not on established plants with mature root systems.
How Soon After Applying Preen Can I Plant Grass Seed?
You should wait at least 8-12 weeks after applying Preen before planting new grass seed. For the safest results, check the specific product label for the recommended waiting period, as it can vary slightly.
Is Preen Safe For All Types Of Grass?
Preen Lawn Weed Control is safe for most common cool-season and warm-season grasses when applied correctly. Always verify on the product label for your specific grass type, such as Kentucky bluegrass, tall fescue, or Bermuda grass, to be certain.
Can I Use Preen And Fertilizer At The Same Time?
Yes, you can apply them together. For simplicity, you can use a product that combines both weed preventer and fertilizer, or you can apply them separately one after the other. Just be sure to water the application in properly.
What Happens If It Rains After Applying Preen?
Rain is actually beneficial after an application, as it helps water the product into the soil. A light to moderate rain is perfect. Only be concerned with very heavy, torrential downpours that could cause runoff before the product has been absorbed.