Preen is a popular brand of granular weed control used by home gardeners to maintain weed-free ornamental and vegetable beds. The most common formulations are pre-emergent herbicides, which create a chemical barrier in the soil. Understanding the nature of this barrier and the plants it affects is important for safe gardening. This guidance provides direction on which plants and situations require caution to protect desirable plants from harm.
Understanding Pre-Emergent Herbicide Action
Pre-emergent herbicides, such as the original Preen Garden Weed Preventer, do not kill existing weeds. They work by targeting the growth process of seeds attempting to germinate in the soil. The active ingredient is Trifluralin, a dinitroaniline herbicide, which forms an invisible layer in the top layer of soil after application.
The primary mechanism of action is the inhibition of cell division, disrupting the formation and function of microtubules in the plant’s roots and shoots. When a seed begins to sprout, the herbicide prevents the new root and shoot cells from dividing correctly, stopping the plant from establishing itself. Since this process targets cell division in new growth, it is safe to use around established plants that have mature root systems.
The Restricted List: Plants Grown from Seed
Any plant intended to be grown directly from seed in a treated area will be harmed or prevented from growing entirely. Pre-emergent products cannot distinguish between a desirable seed and a weed seed; they prevent both from sprouting. This restriction applies universally to all plants grown from seed in the area of application.
Gardeners must avoid applying Preen to areas where they plan to direct-sow annual flowers, as the seeds will fail to germinate or the seedlings will be stunted. Common annuals often started from seed should not have this herbicide applied near them until they are established transplants. Wait until flowering plants have germinated and reached a height of two to three inches before application.
The same restriction applies to vegetables that are commonly direct-sown into garden beds. If you intend to sow seeds for vegetables, you must avoid applying the herbicide to that soil entirely.
Direct-Sown Vegetables
If the seed goes directly into the ground, a pre-emergent herbicide should not be present. Examples of common direct-sown vegetables include:
- Carrots
- Beans
- Peas
- Radishes
- Corn
While some labels list certain vegetables, such as broccoli, cauliflower, and cabbage, as safe to plant, these exceptions are specific to transplantation or involve special application methods.
Application Timing and Plant Vulnerability
The vulnerability of plants to pre-emergent herbicides extends beyond seeds, encompassing newly transplanted material and specific plant types. New transplants, even those considered safe once established, can be damaged if the herbicide is applied too soon. The young root systems of new plantings are susceptible to the chemical’s action, which inhibits the formation of new cells needed for root growth.
When adding new plants to a previously treated area, the planting hole should be filled with fresh, non-treated soil to prevent root injury. It is recommended to wait until transplants are at least two to three inches tall or have developed several sets of true leaves before applying the product. For certain sensitive plants like Phlox, even established plants may show distress or damage when treated with Trifluralin-based products.
Gardeners must also be mindful of the “re-seeding interval” if they plan to sow grass seed or cover crops later. The herbicide creates a barrier that can last up to 12 weeks. Disturbing the soil barrier can bring up new weed seeds, requiring re-application. Applying a pre-emergent restricts the ability to sow any seeds in that area for several months after application.