Controlling Weeds in Nursery and Landscape Plantings

Bulletin 867

Weed Control Program

Too many people look at weed control as a defensive measure that involves killing weeds when they appear. However, it is much easier and cheaper to prevent weed growth than to kill existing weeds. Preventive measures also are safer and longer lasting.

Anyone growing or maintaining ornamental plants should have a weed control program. This means planning how to control weeds in a crop before it is planted. The program has three parts:

1. Eliminate weeds in and around the growing area and kill seeds or vegetative parts prior to planting. It is especially important to kill all perennial weeds or their parts because they are not controlled by mulches or preemergence herbicides, and cultivation only serves to propagate them. Some postemergence herbicides that can be used to control perennial weeds prior to planting must be used with extreme caution after planting.

2. Prevent weed growth in and around the growing area. Mulches and/or preemergence herbicides work very well for controlling weeds from seed.

3. Eliminate weeds as they appear. Since few preventive methods provide total control, hand weeding, cultivation, or careful spot treating with a postemergence herbicide usually is necessary.

Mulches help control weeds by blocking the light needed for germination of small weed seeds.

When developing a weed control program, consider the following guidelines:

1. In most situations, one application of a preemergence herbicide at the recommended rate will not provide season-long control. Repeat applications will be needed. Growers who get season-long control from an application probably are applying herbicide at too high a rate and may be stunting the growth of their crop.

2. No one preemergence herbicide controls all weeds. Some control broadleaved weeds better than grasses, while others control grasses better than broadleaved weeds (see Table 3, page 16). Some postemergence herbicides control most weeds, but their use is restricted in established plantings.

3. If one type of weed is controlled and another type in the same area is not, the uncontrolled one will eventually cover the area. For example, if an herbicide controls broadleaved weeds but not grasses, grasses will fill the entire field. To get preemergence control of a broad spectrum of weeds, combinations of herbicides should be used. When more than one application is made in a season, herbicides should be alternated from one application to the next.

4. Growers who spend the time and money to make an area weed-free should take measures to prevent weed regrowth by mulching or applying a preemergence herbicide to the area.