Many gardeners rely on Preen to prevent weeds, but its effect on desired flowers is a common concern. So, does preen kill flowers? The short answer is no, when used correctly, but the full explanation is crucial for protecting your garden.
Preen is a pre-emergent herbicide. It works by creating a barrier in the soil that stops weed seeds from sprouting. It does not kill existing plants. This distinction is the key to using it safely around your flowers.
Understanding how Preen functions will give you the confidence to use it effectively. You can have a beautiful, weed-free flower bed without harming your prized blooms.
Does Preen Kill Flowers
Preen is designed to prevent weeds, not kill established plants. Its active ingredient, typically trifluralin, targets germinating seeds. It inhibits root development in tiny, sprouting weeds.
Because your flowers are already growing with established root systems, Preen does not affect them. The chemical barrier works at the soil surface level, right where weed seeds try to germinate.
However, there are critical exceptions and proper techniques you must follow. Misapplication is the primary reason gardeners sometimes see damage to their flowers.
How Preen Works In The Soil
Preen must be watered into the soil after application. This action activates the herbicide, creating a thin shield in the top 1-2 inches of soil. This shield remains effective for several weeks, blocking weed growth.
It does not move deeply into the soil profile. This shallow zone of activity is why it’s safe around the deeper roots of your perennial flowers, shrubs, and trees.
The barrier breaks down over time due to sunlight, water, and microbial activity. That’s why reapplication is necessary, typically every 9-12 weeks during the growing season.
The Critical Difference: Pre-Emergent Vs. Post-Emergent
This is the most important concept to grasp. Preen is a pre-emergent. It only works on seeds that have not yet sprouted. It has no effect on weeds or plants that are already up and growing.
A post-emergent herbicide, like glyphosate, kills existing vegetation. Confusing these two types is a common source of garden mishaps.
When Preen Can Harm Flowers
While Preen is generally safe, certain situations pose a risk to your flowers. Awareness of these scenarios will help you avoid accidental damage.
- Applying Over Flower Seeds: If you sow flower seeds directly in the garden, Preen will prevent them from germinating just like weed seeds. You must wait until your flower seedlings are established.
- Damaging Tender Roots During Transplanting: If you disturb the treated soil barrier by digging or planting after applying Preen, you can expose tender new roots to the herbicide. Always apply after planting and cultivating.
- Using the Wrong Formula: Some Preen products are formulated for specific areas like vegetable gardens. Always check the label to ensure it’s safe for use around ornamental flowers.
- Over-Application: Using more than the recommended amount can create an overly concentrated barrier that may affect shallow-rooted flowers.
How To Apply Preen Safely Around Flowers
Following the label instructions precisely is the single best way to protect your flowers. Here is a step-by-step guide for safe application.
Step-By-Step Application Guide
- Prepare the Bed: Remove all existing weeds by hand-pulling or using a post-emergent herbicide carefully. Cultivate the soil lightly if you need to, as you will not want to disturb it after applying Preen.
- Plant Your Flowers: Get all your transplanting and digging done first. Plant your annuals, perennials, or shrubs as you normally would.
- Apply Preen Evenly: Use a spreader or shake the container carefully to apply a thin, uniform layer over the soil surface. Keep it away from plant stems and foliage. The label will specify the correct amount per square foot.
- Water Gently: Lightly water the area. This washes the granules off plant leaves and activates the barrier in the soil. Use a gentle spray to avoid washing the product away.
- Avoid Disturbance: Once applied and watered in, try not to disturb the soil. Do not dig, rake, or cultivate the treated area.
Best Practices For Different Flower Types
Not all flowers interact with Preen in the exact same way. Your approach might vary slightly depending on what you’re growing.
Annual Flowers
Annuals like petunias, marigolds, and impatiens are often planted as transplants. Preen is excellent for these beds. Apply it after you have planted all your flowers for the season.
If you direct-sow annual seeds, you must wait until those seedlings have developed their true leaves and are several inches tall before applying Preen around them.
Perennial Flowers
Established perennial beds are ideal for Preen. Apply it in early spring before weeds germinate and again in mid-summer. Since you are not replanting the entire bed each year, soil disturbance is minimal.
When adding new perennial plants to an existing treated bed, be careful. Dig your hole, plant, and then reapply Preen only on the disturbed soil around the new plant, avoiding its stem.
Bulbs
Preen is safe to use over the top of flowering bulbs like tulips and daffodils. Apply it after you have planted the bulbs in the fall. It will help prevent winter and spring weeds without affecting the bulbs beneath the soil.
Common Mistakes To Avoid
Even with good intentions, its easy to make an error that can put your flowers at risk. Here are the most frequent mistakes gardeners make.
- Applying Before Planting: This is the top mistake. If you put Preen down on empty soil and then plant flowers, you will be digging through the protective barrier and exposing your new plants’ roots.
- Using It On Wet Foliage: Applying granules to wet leaves can cause the herbicide to stick and potentially cause foliar damage. Ensure plants are dry before application.
- Ignoring the Label’s Plant List: Most Preen labels include a list of plants it should not be used around. Some very sensitive flowers may be on this list. Always check.
- Forgetting About Re-Seeding: If you have flowers that readily self-seed, like columbine or poppies, Preen will prevent those desired seeds from sprouting too.
Alternatives To Preen For Weed Prevention
If you are uncomfortable using a chemical pre-emergent, or have a garden full of self-sowing flowers, several effective alternatives exist.
Organic Mulches
A 2-3 inch layer of organic mulch is one of the best ways to suppress weeds. It blocks light from reaching weed seeds and retains soil moisture.
- Shredded Bark or Wood Chips
- Straw or Pine Straw
- Compost or Leaf Mold
Landscape Fabric
Woven landscape fabric allows water and air through while blocking weeds. It is best used under a layer of mulch for aesthetics and to protect the fabric from sunlight.
Corn Gluten Meal
This is a natural, organic pre-emergent. Like Preen, it inhibits seed germination. It also adds nitrogen to the soil as it breaks down. It requires precise timing and application to be effective.
Manual Weeding And Cultivation
Regular, shallow hoeing or hand-pulling of weeds when they are small is a highly effective, though more labor-intensive, method. It disturbs the soil very little if done correctly.
FAQ: Your Preen and Flower Questions Answered
Can I Use Preen After Planting Flowers?
Yes, this is the correct time to use it. Always apply Preen after you have finished all planting and transplanting for the season. This ensures you do not disturb the soil barrier afterward.
How Long After Applying Preen Can I Plant Flowers?
You should wait. The standard recommendation is to wait 12 weeks after application before planting new flowers or seeds. For the safest results, follow the specific waiting period on your product’s label.
Does Preen Kill All Weeds?
No. Preen is primarily effective against annual weeds that grow from seeds each year, like crabgrass and chickweed. It does not control perennial weeds like dandelions or bindweed, which regrow from existing roots.
Is Preen Safe For Roses?
Yes, Preen is generally considered safe for use around established rose bushes. Apply it to the soil surface around the base of the roses, keeping it away from the main canes, and water it in thoroughly.
What Happens If I Get Preen On My Flowers?
If granules land on the leaves, gently brush them off or rinse the foliage with water. A small amount is unlikely to cause harm, but it’s best to remove it to prevent any potential leaf burn or stress.
Final Verdict On Preen and Flower Safety
When used according to the label directions, Preen is a safe and effective tool for preventing weeds in flower gardens. The core principle is timing: apply it only after all planting is complete and the flowers are established.
The key takeaway is that Preen targets seeds, not plants. Your mature flowers with developed root systems are not its target. By understanding its pre-emergent action, you can leverage its weed-stopping power without fear.
Always start with a small test area if you are uncertain, and never deviate from the instructions provided by the manufacturer. With careful use, you can enjoy vibrant, healthy flowers in a garden that requires much less weeding.