There’s a wealth of chemical plant killers to choose from at your local garden center or hardware store. Pre-emergent herbicides promise that weeds won’t even get started. Unfortunately, neither will other plants starting from seed in the same soil. Use pre-emergent herbicides in beds where the plants have begun growing, or resolve yourself to a long wait before planting annual flower seeds.
How Pre-Emergents Work
Pre-emergent herbicides bond to a layer of soil in a process called sorption. The success of this process depends on many factors including moisture, temperature, soil pH and organic content. The higher the rate of sorption, the less likely the herbicide is to leach through the soil or run off its surface and end up in the water supply -- streams, rivers and lakes. This layer forms in the few inches below the soil’s surface, thanks to cultivation and watering-in of dry granules. It then disrupts the cell division that forms roots of plants as they germinate.
The second quality of pre-emergent herbicides that determines their effectiveness is how long that disruptive layer remains toxic to tiny roots -- its persistence in the soil at a level where it stands between newly born leaves and the sun. To express persistence, manufacturers borrow a nuclear term, half-life, to explain the length of time an herbicide retains toxicity under ideal conditions. Oryzalin, an herbicide recommended for use with ornamentals, has a half-life of approximately 20 days according to the Weed Science Society of America. This means that the weed killer will lose half of its toxicity at the end of three weeks and that half of its remaining capability -- and so on for months -- and could continue killing plants from seeds added during the growing season.
Many pre-emergent herbicides and herbicide combinations work well in ornamental beds. Each is labeled for the type of plant it kills best and conditions under which its sorption rate is highest. Persistence varies widely. Oryzalin and a combination of oryzalin and benefin, recommended for flower beds and ornamentals, have half-lives from 20 to 23 days, depending on the product. Other herbicides, such as isoxaben, which has a half-life of 85 days, should be used only where desired plants are mature, because the soil remains toxic for seeds for most of the year if applied as suggested in late summer to early fall for winter weeds and late winter to early spring for summer weeds.
What About Seeds?
Pre-emergent herbicides kill all seeds, so planting flower seeds before their half-lives expire guarantees failure, and waiting until the following year might not guarantee success. The University of California suggests an integrated approach to weed control using pre-planting cultivation and weeding and post-planting mulching to keep weeds at bay. If herbicides must be employed, use them after perennials are well up and nursery pack annuals have begun growing in their new homes.
- University of California Integrated Pest Management: Weed Management in Landscapes
- University of North Carolina Extension: Using Pre-Emergence Herbicides for Weed Control in the Home Landscape
- University of Florida IFAS Extension: Pre-Emergence Herbicides for Use in Ornamentals
- Star Nursery: Preventing Weeds With Pre-Emergents
- Oregon State University Extension: Herbicide Residues in Field Soils
- U.S. Environmental Protection Agency: Environmental Fate Review: Benefin and Oryzalin
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