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Ferguson Caras LLC
989 Herrick Brook Road
Pawlet, VT 05761

Gardening Q&A Backgrounder

Pre-emergent & Garden Weeds
Preen Southern Weed Preventer

New gardeners are often confused about what pre-emergents do and why experienced gardeners reach for that yellow bottle of Preen.

In the South, even many experienced gardeners may be surprised to learn that the Preen sold now is an improved version formulated especially for Southern gardens. Preen Southern Weed Preventer was introduced in 2012, with a dithopyr formula designed to last longer in the South’s longer-growing season. Preen Southern prevents weeds for up to four months per application, a full month longer than original Preen, which is only sold in northern states now. Unlike original Preen, Southern Preen is not approved for use on vegetable gardens.

Following is a short Q & A about Southern Preen, preventing weeds in Southern vegetable gardens and notes on pre-emergents.

About Preen Southern

Q: How does Preen Southern work?
A: Preen Southern Weed Preventer is a pre-emergent that is applied to block weeds from growing in the top layer of soil or in mulch. Preen Southern’s active ingredient, dithiopyr, constitutes .27% 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 growing into weeds by inhibiting cell replication in the root hairs. Preen Southern remains effective for up to four 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.

Q: Name some problem weeds Preen Southern controls?
A: Summer weeds (spring/summer germinators): spurge, crabgrass, common lespedeza, carpetweed, mustard, oxalis, goosegrass, foxtail, pigweed, prostrate knotweed, and more. Winter weeds (fall/winter germinators): hairy bittercress, annual bluegrass, Carolina geranium, catchweed bedstraw (cleavers, stickywilly), henbit, chickweed, marestail (horseweed), corn speedwell and more.

Q: Can I miss the right time to apply pre-emergent?
A: It’s never too early or too late to apply a pre-emergent such as Preen. Different weed seeds germinate at different times: spring, summer, fall and even winter.

Q: Should I reapply Preen Southern?
A: For extended weed control, reapply after 4 months. Spring and summer applications stop summer weeds from growing. Fall applications stop winter weeds that sprout in fall, winter or early spring.

Q: What other southern Preen products are available?
A: There’s dual-action Preen Southern Weed Preventer Plus Plant Food which comes in a bottle with a shake-top applicator. Its weed preventer, chosen specifically for long southern growing seasons, gives up to four months of on-going weed protection. Its balanced 6-8-6 fertilizer boosts flowering and helps plants build strong roots for better heat and drought tolerance. There’s also Preen Mulch Plus – the only bagged mulch available that’s guaranteed to prevent weeds for up to 6 months. It comes with added Preen weed preventers already mixed in. Packaged regionally and sold at Lowe’s and independent garden centers, the southern product features, variously, shredded premium natural wood mixes of local sustainable cedar, fir, pine, cypress or shredded hardwood. Three non-fade colors are offered, tinted with carbon black or iron oxide: chestnut brown, midnight black and russet red. It’s effective against weeds when applied 2 inches deep, instead of the 3 inches recommended for regular mulch. This means that one bag can cover more than 12 square feet, about 30% more coverage per bag than other mulches.

About Vegetable Gardens & Weed Control

Q: Can I use Preen Southern in the vegetable garden?
A: Preen Southern is not approved for use in vegetable gardens. It is for use in flower and landscape beds. Instead use Organic Preen Vegetable Garden Weed Preventer.

Q: What’s in Organic Preen for vegetable gardens?
A: Corn gluten, a by-product of corn processing, contains amino acids that stop seeds from forming roots, making it an effective organic pre-emergent weed preventer. Organic Preen Vegetable Garden Weed Preventer is made of 100% corn gluten in granulized form so it’s not dusty and is easy to apply from the bottle’s shaker-top applicator. Use it around transplants and seedlings once they have developed true leaves and are two to three inches tall. Reapply after four to six weeks.

Pre-Emergent & Garden Weeds

Q: I just finished weeding – why are there more weeds?
A: Weeds are programmed by nature to survive. Many propagate by seed, producing tens of thousands of seeds per plant, per season. Common lambsquarters, for example, can produce up to 70,000 seeds per plant, per season. The seeds collect in surface soil where they can stay viable for decades. A yard overrun with weeds is essentially a monstrously efficient weed seed-producing machine. Ironically, the very act of pulling up weeds and their roots drags buried seeds to the surface where they quickly germinate. Just a flash of sunlight can spark growth. Even the best-intentioned efforts to clean up a weed-ridden bed are only temporary unless steps are taken to stop weed seeds from sprouting.

Q: How can I get ahead of weeds?
A: First, remove any existing weeds from flower and landscape beds, including the roots. To this clean slate, add a layer of protective mulch to starve buried seeds and newly sprouted weeds of sunlight. Next, apply a pre-emergent weed preventer such as Southern Preen to prevent weed seeds in the top layer of soil, and in the mulch itself, from forming roots. No roots means no weeds. Repeat the routine every four months and you will drastically reduce the need to weed over time.

Q: When’s the right time to apply pre-emergent in spring?
A: When spring weather warms the top layer of soil to 55° F, conditions are right for two things to happen: daffodils bloom and crabgrass seeds begin to germinate. Once you see daffodils in bloom, it’s time to apply pre-emergent to stop crabgrass and other early-germinating weed seeds. Other signals that it’s time to apply: when yellow forsythia blooms; when dogwood blooms or Knockout roses leaf out. Remember, these cues are only reminders of the earliest opportunities to prevent weed seeds from sprouting. Different weeds germinate at different times all season, so it’s never too late to apply a pre-emergent.


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