Preen is a widely used granular pre-emergent herbicide applied to control weeds in garden beds and landscaped areas. The product works by creating a protective layer in the soil that prevents weed seeds from successfully germinating. Understanding which specific types of weeds this product targets and the correct application method determines its effectiveness.
Understanding Pre-Emergent Action
A pre-emergent herbicide functions by stopping weed seeds from sprouting rather than killing established plants. The standard Preen product contains the active ingredient trifluralin, which interferes with a plant’s cellular processes. Specifically, it inhibits the cell division process (mitosis) in the root tips of a developing seedling.
When spread on the soil and activated by water, the product forms a shallow chemical barrier in the top one to two inches of the soil surface. Any weed seed that begins to germinate and extends its first root into this treated zone absorbs the chemical, halting its growth immediately. This ensures the weed dies before it can break the soil surface and become visible. The barrier typically remains active for nine to twelve weeks before requiring reapplication.
Annual Weeds Controlled by Preen
Preen is designed primarily to prevent the germination of annual weeds, which complete their life cycle within a single growing season and rely on seeds for propagation. The product label states it can prevent the sprouting of over 200 different broadleaf and grassy annual weeds. This broad spectrum of control includes many common garden invaders.
One frequently targeted weed is Crabgrass, a grassy annual that germinates when soil temperatures consistently reach 55 degrees Fahrenheit. Preen also effectively controls other grassy annuals like Annual Bluegrass (Poa annua) and Foxtail. Preventing these grasses from sprouting eliminates the difficult task of hand-pulling them later.
The herbicide is also highly effective against numerous broadleaf annual weeds. By blocking seed germination, Preen interrupts the annual cycle of self-seeding, leading to a long-term reduction in the weed seed bank.
Common Broadleaf Weeds
Specific examples of broadleaf annuals stopped before they emerge include:
- Chickweed
- Common Lambsquarters
- Oxalis (woodsorrel)
- Pigweed
- Purslane
Weeds Preen Does Not Prevent
Preen is a preventative product and will not eliminate weeds that are already actively growing. Any weed that has broken the soil surface and developed leaves and a root system is considered established and cannot be controlled by a pre-emergent herbicide. Established weeds must be manually removed or treated with a post-emergent product before Preen is applied.
The product is also ineffective against perennial weeds, which live for multiple growing seasons. Perennial weeds often reproduce through structures other than seeds, such as rhizomes, tubers, or deep taproots. Common examples include Dandelions, Clover, and Nutsedge. Since these plants do not rely on seed germination, Preen’s chemical barrier has no effect on their established root systems or spreading underground parts.
Proper Timing and Application
The success of a pre-emergent herbicide application depends entirely on correct timing, as the product must be in place before the weed seeds begin to sprout. For most annual weeds, apply the granules in the early spring, typically before the soil temperature rises above 55 degrees Fahrenheit. A second application in the late summer or early fall is often necessary to prevent the germination of cool-season annual weeds, such as Annual Bluegrass.
Before applying the granules, all existing weeds must be removed to ensure the soil surface is clean. The granules should then be spread evenly over the soil or mulch surface at the rate specified on the product label. Immediately following application, the product must be activated by watering it in thoroughly.
This watering process dissolves the active ingredient and moves it into the top layer of soil, forming the necessary chemical barrier. If the soil surface is disturbed after activation, such as by deep raking or tilling, the barrier will be broken, resulting in inconsistent weed control. To maintain the preventative effect, reapplication is typically required every nine to twelve weeks.