Controlling Weeds in Nursery and Landscape Plantings

Bulletin 867

Steps To Take Before Applying An Herbicide

1. Identify the predominant weeds to be controlled and read the label to be sure the herbicide selected will be effective against them. Remember that no one preemergence herbicide will control all weeds. The continued use of one herbicide may result in the eradication of the original problem weeds, while other weeds that are resistant to the herbicide begin to thrive. Be aware of a changing weed problem. Appendix 3 provides a listing of all the weeds (classified as annuals, biennials, or perennials) named on the labels of the preemergence herbicides.
   • Annuals complete their life cycle in one year. They grow from seed, flower, produce seed, and then die. Summer annuals germinate in the spring or summer, while winter annuals germinate in the fall. Some weeds, such as chickweed, common groundsel, and bittercress, may germinate and grow at almost any time of year. These weeds may grow all year in container overwintering structures. Annuals are easily controlled by preemergence herbicides if the proper material is applied at the right time. To control summer annuals, preemergence herbicides must be applied in very early spring or in the summer after the soil has been cleared by cultivation or a postemergence herbicide application. To control winter annuals, preemergence herbicides must be applied in late August or September. Either contact or translocated postemergence herbicides can be used to kill annual weeds because they cannot resprout from their roots after their tops have been killed.

   • Biennials, such as wild carrot, require two growing seasons to complete their life cycle. In the first season after the seeds germinate, the plants develop an extensive root system and a dense cluster of leaves called rosettes. The next year they flower, produce seed, and die. Biennials can be controlled by preemergence herbicides at the seed germination stage, but postemergence translocated herbicides are required after biennials are established.

   • Perennials will grow and produce seed year after year. Herbaceous perennials, such as quackgrass, mugwort, and Canada thistle, die back to the ground each year. Woody perennials may drop their leaves, but they do not die back to the ground. The longer perennial weeds are allowed to grow uncontrolled, the larger their root systems become, the more they spread, and the harder it is to control them. Cultivating through patches of perennial weeds propagates and spreads them, as each root piece may grow into a new plant. As with biennials, perennial weeds can be controlled from seed by preemergence herbicides, but postemergence translocated herbicides are required after perennials are established.

2. Carefully read the label to make sure the herbicide is not toxic to any of the desirable plants around which it will be used.

3. Determine the proper application rate according to:

   -recommended range on the label

   -soil texture (e.g., Princep/simazine is more effective in light sandy soils than in heavy clay soils)

   -length of time since the last application (e.g., no more than 1.5 pounds active ingredient per acre (ai/A) of Factor should be applied in one year)

   -herbicide combinations (Reduce the rate if two herbicides are used in combination. Most herbicide labels list a range of recommended rates rather than a specific rate. If using combinations, choose the lower end of the range listed.)

4. Calibrate the sprayer or spreader. (See page 8.)

5. Determine as accurately as possible the area of the field to be sprayed.

6. Determine how much herbicide will be needed to cover the area at the rate selected in Step 3. Have at least two people make the necessary calculations independently of each other. Do not apply the herbicide if there is any doubt about the accuracy of the calculation.