CHARLES R. HART, Texas Agricultural Extension Service, Ft. Stockton, TX

A variety of soil-applied herbicides are available for use on rangelands in Texas. Included in this category are Velpar L, Tordon 22K, and Spike 20P. These herbicides are taken up through roots of the plants and are applied to the soil, not directly to the plant. Rainfall is required to move the herbicide into the root zone of the target species. Soil-applied herbicides have advantages and disadvantages as listed below.

Advantages:

• Can be applied at any time of the year
• Generally broad spectrum control
• Can be held inactive until optimum growing conditions exist
• Extremely low drift potential
• Highly selective
• Requires no special equipment

Disadvantages:

• Can take up to three years for complete root kill
• Potential to be tied up by the soil
• Kills most any kind of plant
• Pelleted herbicides may be carried off by wildlife
• May not be useful as a broadcast application
• Can be washed away by rainfall

Application Techniques

Soil- applied liquid herbicides are applied undiluted to the soil in measured quantities with a spot application (Figure 1.). This can be done with an exact delivery gun, livestock drench gun, or a disposable auto-syringe. Care must be taken to thoroughly rinse the equipment after every use or it will freeze up. Velpar L in particular should not be left in equipment after use. Small backpack bladders can be purchased for ease of application or it can be applied directly from the container. It is effective on a broad range of woody range plants. Generally a dose of 2 to 6 ml per 3 feet of canopy height is used. Refer to herbicide labels for specific application rates.

Soil applied pelleted herbicides are applied directly to the soil surface by individual plant treatments (Fig. 2.) or broadcast methods. It can be applied using protective gloves or a whirly-bird spreader. It is generally applied at 3/8 to 3/4 oz. per 100 square feet as an individual plant application. Refer to herbicide labels for specific application rates. This type of application should not be made on areas that are frequently flooded, areas with steep terrain, or along interior ditch banks.

For individual plant treatments, soil applied herbicides should be applied to the soil just inside the drip line of the plant (Fig. 3.) at the rate specified on the label. It is important to remember that Velpar L and Spike 20P inhibit photosynthesis in the plant. Thus, susceptible species may defoliate and refoliate several times before they are killed.