Weed Control in Landscapes: Herbicides

Summary

• Effective landscape weed control involves using different ways to suppress weeds, such as weed-free mulch and preventing weeds from going to seed near landscape beds.
• Preemergent and postemergent herbicides are also effective ways to control weeds in landscapes.
• The tables below list homeowner herbicide products that, when used according to the label, will assist in maintaining weed-free landscape beds.

Weed free landscapes require an integrated approach, including preventing weeds from going to seed, weed free mulch, and herbicides.
Ted Whitwell, ©2023, Clemson University

Introduction

Effective landscape weed control requires more than applying herbicides. It also includes using weed-free mulch and reducing weed pressure by removing weeds before they produce seed. When the label instructions are followed, preemergent and postemergent herbicides are effective in controlling weeds.

Preemergent herbicides should be applied 1 to 2 weeks before anticipated weed emergence so as the weed seeds begin to germinate, they absorb the herbicides and stop further development. Existing weeds should be removed either by hand or by postemergent herbicides before a preemergent herbicide is applied. Table 1 lists products for homeowner use to control weeds and not damage desirable landscape plants. The herbicide label will specify the weeds it will control and include a list of landscape plants where the herbicide is safe to use.

After making the herbicide selection, it is critical to read and follow the herbicide label to obtain the best results. Generally, preemergent herbicides are effective in controlling small seeded annual broadleaf and grassy weeds but are not effective in controlling large seeded annual weeds such as morning glory or established perennial weeds. It is usually better to apply the herbicides on the top of an organic mulched bed to prevent weed seed from becoming established in the mulch.

Preemergent herbicides are available in both granular and liquid formulations. Granular formulations should be applied when the landscape plant foliage is dry. Usually, granulated herbicides have less potential for injury to desirable landscape plants than liquid formulations and are easier to apply. Irrigation (about 0.25 inches) should be applied soon after applying preemergent herbicides to dissolve the granules over the surface of the mulch and to encourage the active ingredient to move into the mulch or soil surface. This uniform herbicide layer will prevent susceptible weed seed from emerging. Do not wait longer than 5 days to irrigate; otherwise, the products may not work well, and unnecessary exposure may occur. More preemergent herbicides are labeled for woody landscape plants, with fewer products labeled to use safely in herbaceous annuals and perennial landscape plantings.

Herbicides have active chemical ingredients (called common names) and are sold by different brand names, like generic drugs. Many product brand names are available for homeowners, such as Roundup, Snapshot, Preen, etc. Always check the brand name label for the active ingredient to choose the correct formulation. Active ingredients, such as pendimethalin, trifluralin, prodiamine, oryzalin, and dithiopyr affect sensitive weeds similarly by preventing root growth and development; therefore, they must be watered in soon after application to the landscape beds so that newly germinating weed seeds can absorb the herbicide resulting in their inability to grow a root system.

Isoxaben is very effective on winter annual weeds and is popular to improve broadleaf weed control. Isoxaben affects the root and shoot growth of sensitive plants. Many preemergent herbicides are not as effective on broadleaf annual weeds with large seeds, such as morning glory, or established perennials, such as nutsedge and Florida betony.

Table 1. Preemergent Herbicides

Please carefully read the labels for specific safety instructions, specific weed control, and tolerance of specific landscape plants. Remember, the label is the law.

Postemergent Herbicides

Most postemergent herbicides are absorbed through the foliage of plants. Spray formulations are generally the most effective; however, granular formulations may be easier, safer, and more effective in certain home landscape situations.

Weed size and age are important considerations when using postemergent herbicides. These herbicides are more effective when applied to weeds during their life cycle’s immature and vigorous growth stage. Weather conditions also play an important role in the activity and effectiveness of postemergent herbicides. Warm, humid conditions with good soil moisture are optimal for weedy plant growth and effective herbicide activity.

Selective herbicides control only certain plants and are generally safer to use in the landscape without damaging desirable plants. Non-selective herbicides damage most plants, and care should be taken to avoid accidentally applying them to or near desirable plants. Non-selective herbicides, such as diquat, glufosinate, and glyphosate, are used routinely to control existing weeds.

Applications should be carefully applied where the spray is directed away from landscape plants. Avoid spraying on windy days or when rainfall is imminent Pelargonic acid, plant oils, and acetic acid are organic contact nonselective herbicides that will control very small weeds that have just emerged. Weed regrowth may occur when using both selective and non-selective herbicides. To help identify potential herbicide damage or to learn about preventive measures to use when using herbicides in a landscape setting, please visit HGIC 2349 Herbicide Damage to Landscape Plants.

There are a few selective postemergent herbicides that can be used in landscapes. Halosulfuron is labeled for use in several common landscape plants and effectively controls annual and perennial nutsedges. Repeated treatments will be needed in severe infestations. Annual and perennial weedy grasses can be effectively controlled with fluazifop and sethoxydim in broadleaf landscape plants.

Herbicide sensitivity varies greatly among the diversity of landscape species ranging from herbaceous annuals and perennials to woody shrubs and trees. In general, herbaceous annuals and perennials are more sensitive to herbicide damage, while woody shrubs and trees are more tolerant to herbicides. The product label should be consulted before using herbicides in the landscape. There are different types of herbicide concentrations for purchase, including those that are ready to use and those that will be mixed with water and sprayed.

Table 2. Postemergent Herbicide Active Ingredient Names, Product Names Available To Homeowners, And Weeds Controlled.

Originally published 08/23