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Preemergence Herbicides for Landscape Use

Posted: March 4, 2013

Preemerge herbicides reduce hand weeding and the need for post-emergence treatments.
Preemergence herbicides do not control emerged weeds. The seedlings in this picture are would not be controlled by application of a preemergence herbicide.

Preemergence herbicides do not control emerged weeds. The seedlings in this picture are would not be controlled by application of a preemergence herbicide.

Annual weeds such as crabgrass, foxtails and ragweed commonly infest landscape plantings. While mulches provide some control, the addition of a preemergent herbicide can improve the level of weed control and customer satisfaction. In addition, preemerge herbicides reduce hand weeding and the need for post-emergence treatments. The saying “An ounce of prevention is worth a pound of cure” applies here.

As a practical matter, many landscape firms use granular products because of the ease of application. We’ll focus on granular products in this discussion.

Rarely does a single active ingredient provide control of all annual weeds, so, it’s not surprising that herbicide manufacturers often combine an ingredient that works best on broadleaved weeds with an ingredient that works best on grassy weeds. For instance, Snapshot, which has been a very useful product for decades, is a combination of trifluralin (Treflan) and isoxaben (Gallery). Trifluralin provides control of grasses while the isoxaben controls broadleaves. Neither ingredient used alone would provide satisfactory weed control.

A relatively new product, Freehand, combines pendimethalin (Prowl) with dimethenamid (Tower). In this case pendimethalin is the primary grass control ingredient and dimethenamid picks up the broadleaves. In addition, dimethenamid provides preemerge control of yellow nutsedge. This quality distinguishes Freehand from most of the other preemerge granular products.

Because of their plant safety and broad spectrum weed control, Snapshot and Freehand are excellent starting places if you’re new to the preemergence herbicide game.

Penn State’s publication Controlling Weeds in Nursery and Landscape Plantings lists eight additional granular, preemergence herbicides labeled for use in landscape plantings. Each of these contain an ingredient (oxadiazon or oxyfluorfen), that injures bulbs or herbaceous perennials that emerge through the herbicide treatment. For this reason, consult labels carefully and double check the components of plantings where you intend to use them.

Note that annual bedding plants are not found on the labels of the products discussed so far, with a few exceptions on the Freehand label. Products containing oxadiazon and oxyfluorfen are especially problematic for bedding plants.

Where annual bedding plants are used, consider granular formulations of pendimethalin (Pendulum 2G), napropamide (Devrinol 2G ) or trifluralin (Treflan 5G) and consult the Freehand label for annuals that are tolerant of it.

In addition to the products we’ve discussed, several herbicides available in sprayable formulations exist. The newest is Spect(i)cle (indaziflam) which is new chemistry for the ornamentals industry. For a complete list of herbicides for use in landscapes see Controlling Weeds in Nursery and Landscape Plantings.

Finally, note than none of the herbicides discussed here control perennial weeds such as Canada thistle, bindweed, quackgrass and many others. And, preemergence herbicides must be applied before weed germination and also be moved into the soil with irrigation or rainfall. In Pennsylvania, applications of preemergence herbicides in late March thru early April are generally effective.

As always, consult the full label before applying any herbicide. The Crop Data Management System is a handy site that lists labels. While researching this article, I noted that the Freehand label warns that unacceptable injury to all ornamental grasses may occur where the product is used. Who would have guessed that? Proceed with caution.