Florence Grovida Gardening

Herbicide Types

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For controlling lawn weeds, there are generally two herbicide application types: preemergence and postemergence. Pre-emergence herbicides prevent weeds from becoming established. Postemergence herbicides are applied after weeds are established. Once established, certain weeds may be difficult to control. For example, trying to control an established grass weed in an established lawngrass can be very difficult. Herbicides with such a margin of selectivity are limited. Selectivity with a preemergence grass herbicide is achieved by application timing (applied to established lawngrasses, preventing grass seedlings from establishing). Postemergence herbicides are generally used to control emerged, broadleaf weeds in established lawns.

Preemergence (PRE)

Preemergence herbicides are applied to the soil and act as seeds germinate. These herbicides can act on summer and winter annuals, and some perennial weeds starting from seed. Preemergence herbicides need to be applied prior to germination. The time of germination for each weed species is life-cycle dependent. Preemergence herbicides generally provide some residual activity (meaning the weed control may last for several weeks after initial application). Preemergence herbicides may provide excellent grass weed control and have limited activity on broadleaf weeds. Except where specifically noted, do not apply preemergence herbicides to newly established or reseeded lawns for at least three months.

To ensure best performance with preemergence herbicides:

1. Remove trash, leaves and thatch to allow the herbicide to directly contact the soil.
2. Apply the preemergence herbicide uniformly over the treatment area (as directed on the product label).
3. After application, apply one-half inch or more of irrigation. This activates the herbicide by moving it into the soil. Preemergence herbicides will not be as effective if not immediately activated by irrigation (unless otherwise noted on the product label).

Postemergence (POST)

Postemergence herbicides act on weeds after they have germinated and are applied to the above ground plant tissue. Weeds are generally easiest to control when young (early in their life cycle). Postemergence herbicides may be contact or systemic. Contact herbicides only kill the top growth contacted by the herbicide. Systemic herbicides applied to the top growth, move within the plant to control below ground reproductive structures not contacted in the initial application. Systemic herbicides are preferred when treating established perennial weeds.