Focus on Weed Seeds to Radically Reduce the Need to Weed.

Weeds are natural survivors, and a key part of their strategy is to produce many thousands of seeds per plant. Millions of these seeds lie dormant in garden soil, ready to germinate when conditions are right.  

Any soil disturbance – even the act of weeding – exposes buried weed seeds to the light they need to grow. Just a flash of sunlight is enough.  

A three-inch layer of mulch stops weeds by depriving them of light. But weed seeds can hide in the mulch itself, and wind and birds carry in new weed seeds daily. Sprinkling a pre-emergent such as Preen on top of mulch adds a one-two weed prevention punch. It forms an invisible control barrier that stops weeds from rooting in the top layer of garden soil and mulch for up to 3 months per application. No roots. No weeds. No weeding.  

Early spring is the optimal time to start a weed prevention routine to stop the first weeds of the season from growing. But any time is a good time to start. There are always new weeds ready to germinate as different weeds germinate at different times, spring, summer, fall and even winter.  

Over time, repeating a preventive mulch-and-Preen routine each year can radically reduce a gardener’s need to weed. When weeds don’t grow, there’s no need to pull them!  

Preventing vs. Killing Weeds  

A pre-emergent prevents weed seeds from getting established – so they never grow. Weeds that are already growing, on the other hand, must be killed by pulling, digging, spraying, or burning.  

How does Preen work?  

Preen is a pre-emergent. It stops weed seeds from sprouting or forming roots. Preen’s active ingredient, trifluralin, constitutes 1.47% of the product, with the rest being the delivery medium, small granules made of corncobs and recycled paper. Once activated by watering-in, Preen bonds with soil particles and mulch to create an invisible weed control barrier. The barrier stops seeds from sprouting by inhibiting cell replication in the root hairs. Preen remains effective for up to three months. It does not persist in the soil, and will not leach into groundwater. Preen’s active ingredient breaks down and biodegrades within six months to a year. The little corncob/paper granules also break down in the soil.  

When does spring weed season open?  

When spring weather warms the top layer of soil to 55° F, conditions are right for two things to happen: forsythia reaches full bloom and crabgrass seeds begin to germinate. Once you see yellow forsythia buds it’s time to apply a preemergent to stop the earliest weeds. This early prevention is optimal. But remember, different weeds germinate spring through fall so anytime is a good time to apply Preen.  

Should my customers re-apply?  

Preen is effective for up to three months per application. Re-apply Preen to stop late-summer and fall-germinating weed seeds.  

How Preen Prevents Weeds

Weed Prevention Q & A 

Why do Preen labels list certain plants only?  

Preen has been formally tested and cleared for use with a broad range of plants including more than 200 different types of flowers, perennials, roses, ornamental grasses, bulbs, ferns, groundcovers, shrubs and trees. By law, Preen labels can only list plants formally trialed with Preen. Has every garden plant been tested? No, the range is simply too vast. For ornamental plants and shrubs not listed on the label, you might apply Preen around a few sample plants to see what you think. In all cases, when planting, keep the Preen granules out of planting holes.  

After application, should my customers stay out of beds?  

Only briefly. After Preen granules are watered in, people and pets should stay off the area until the surface is dry to avoid tracking the wet granules on the lawn or elsewhere where they might stain porous surfaces. Once activated by water, the active ingredient in Preen bonds with soil particles and stays there until it biodegrades. 

Can my customers plant where Preen is already applied?  

It’s fine to add new plants, including bare root perennials and bulbs, to beds where Preen has already been applied as long as you take care to keep soil with Preen granules out of new planting holes. The best technique is to dig the planting hole, position the plant and then refill the hole with fresh soil and compost. 

Does unusually rainy weather affect Preen? 

Yes, excessively warm, wet conditions can shorten the effective period of a Preen application by speeding up microbial activity in the soil. This causes the carbon-based active ingredients in Preen and Organic Preen to break down and disperse faster than usual. The durations listed on Preen packaging are based on averages.  

In fall, will Preen affect newly planted bulbs? 

No, as long as the bulbs are planted four or more inches deep. Preen is applied on top of garden soil or mulch. Because Preen bonds to soil particles and doesn’t leach, flower bulbs planted at least four inches deep are not affected.  

Can my customers use Preen by a water garden?  

No, do not use Preen near ponds, streams, rivers or water gardens or near drains or sewers that may empty into an aquatic ecosystem. Avoid using where eroding soil might wash into bodies of water. When granules spill on walks or driveways, sweep them up instead of hosing them off. 

Is there an organic Preen weed preventer?  

Yes, Organic Preen Vegetable Garden Weed Preventer is made of 100% corn gluten, a by-product of corn processing. Corn gluten contains amino acids that stop seeds from forming roots, making it an effective organic pre-emergent weed preventer. It’s especially suited for use in vegetable gardens. Apply Organic Preen around transplants and seedlings once they have developed true leaves and are two to three inches tall. Reapply after four to six weeks.  

Is there a bagged Preen mulch available?  

Yes, Preen Mulch Plus is the only bagged mulch available that is guaranteed to prevent weeds for up to 6 months. Preen Mulch Plus features shredded natural woods plus two weed preventers. It comes in three non-fade colors: midnight black, russet red, and chestnut brown. It is packaged regionally, made variously of local sustainable cedar, fir, pine, cypress, or shredded hardwood mixes, and tinted with iron oxide or carbon black. No treated or recycled wood products are used. Note: apply Preen Mulch Plus 2-inches deep instead of the usual 3-inch depth recommended for other mulches. The 2-inch depth means that each bag of Preen Mulch Plus covers up to 12 square feet, or 30% more than average mulches while delivering stronger weed protection.