Controlling Weeds in Nursery and Landscape Plantings

Bulletin 867

Herbicide Formulations

The formulas for calculating the amount of herbicide needed for an area depend on the formulation of the herbicide. The amount of active ingredient in dry formulations of herbicides is presented as a percentage of active ingredient (ai). The amount of active ingredient in liquid formulations is presented as pounds of active ingredient per gallon (lb ai/gal).

All of the dry formulations of herbicides are stable and maintain their activity for many years if protected from moisture. Granular products are more expensive per pound of active ingredient than are sprayable formulations, so they are generally used only in high-value areas, such as landscapes and container nurseries. Wettable powders have one problem. They are dusty. During weighing and mixing, they often form a dust cloud around the equipment. The dust is messy and may be a health hazard. To minimize this problem, chemical companies developed the dry flowable and water dispersible granular formulations. They are pelletized versions of the wettable powders and produce much less dust during handling. There is essentially no difference between DF and WDG formulations.

Water soluble formulations of herbicides mix thoroughly with water and stay in solution even without agitation. Products that are labeled as liquids or aqueous solutions will readily mix with water, but they form suspensions in the spray solution and can settle out if not agitated regularly. Emulsifiable concentrates are oil-based products that maintain the herbicide in suspension. When mixed with water, the oil and herbicide are suspended in the water and can separate without agitation. Unless clearly stated otherwise on their label, all liquid formulations should be protected from freezing. Roundup is an example of a product that can freeze and thaw repeatedly with no loss of activity. Liquids also have a shorter shelf life than dry products. Except for the water soluble products, they can separate into phases in the containers that may or may not remix. Liquid or aqueous solutions may separate into a liquid and solid phase, with the solid phase accumulating on the bottom of the container. If stored too long, it may be impossible to properly mix them again even with vigorous agitation. Emulsifiable concentrates are less likely to separate, but may do so if stored too long. Buy only as much herbicide as you should need in one year. All of the products can be stored for at least that long.

Because many herbicides have more than one formulation, recommendations are often given in terms of pounds of active ingredient per acre (lb ai/A). For example, simazine is available in two formulations: 4L and 90DF. A simple recommendation to a grower might be, "Apply Princep at 2 lb ai/A to your field." The grower then has the option of which formulation to use. When a formulation is chosen, the grower must then calculate the amount of the product purchased to use.

Example:

Princep 90DF (90% ai): 2 lb / .90 = 2.2 lb

Princep 4L (4 lb ai/gal): 2 lb / 4 lb/gal = 0.5 gal, or 2 qt

In this example, the grower can get 2 lb active ingredient of simazine from 2.2 lb Princep 90DF or 2 qt Princep 4L. To find the formulations of herbicides available for nursery use, see the sections titled "Preemergence Herbicides" (page 16) and "Postemergence Herbicides" (page 24).

a. Formulation for granular materials and wettable powders:

lb ai/A x sq ft to be treated x 100 = lb required to treat area
          ___________________   ___
           43,560 sq ft/acre  %  ai 

b. Formulation for liquids:

lb ai/A x sq ft to be treated x     1      = gal required to treat area
          ___________________   ________
          43,560 sq ft/acre     lb ai/gal 

Sample calculations:

a. How much Ronstar 2G is required to treat a field measuring
800 ft by 210 ft at 3 lb ai/A?

800 ft x 210 ft = 168,000 sq ft  
 
2G - Granular containing 2% ai
 
3 lb ai/A x 168,000 sq ft x 100 = 578 lb Ronstar 2G
            _____________   ___
             43,560 sq ft    2

b. How much Princep 90DF or Princep 4L is required to treat an 
800 ft by 210 ft field at 2 lb ai/A? 

800 ft x 210 ft = 168,000 sq ft 

90DF - Dry flowable containing 90% ai

2 lb ai/A x 168,000 sq ft x 100 = 8.5 lb Princep 90DF
            _____________   ___
             43,560 sq ft    90

4L - Liquid containing 4 lb ai/gal 

2 x 168,000 sq ft x 1  = 1.9 gal Princep 4L
    _____________  ___
     43,560 sq ft   4

Table 1 shows the amount of formulated herbicide required to supply the equivalent of 1 lb ai/A on 1 acre or 1000 sq ft.

Examples:

1. To apply Princep 4L at the rate of 3 lb ai/A plus Surflan 75W at 2 lb ai/A to 1 acre of land, note that 1 qt Princep 4L is required to supply 1 lb active ingredient. To get 3 lb active ingredient, you need 3 (1 x 3) qt Princep 4L. To get 1 lb active ingredient requires 1.3 lb Surflan 75WP. To get 2 lb active ingredient requires 2.6 (1.3 x 2) lb Surflan 75WP.

2. To apply Ronstar 2G at the rate of 3 lb ai/A plus Surflan 4AS at 2 lb ai/A to a bed that contains 5000 sq ft, note that 18.4 oz (522 g) Ronstar 2G are required to treat 1000 sq ft at a rate of 1 lb ai/A. Multiply this by 3 to get 3 lb ai/A and again by 5 to cover 5000 sq ft: 18.4 oz x 3 = 55.2 x 5 = 276 oz /16 oz/lb = 17.3 lb or 522 g x 3 = 1566 x 5 = 7830 g / 454 g/lb = 17.3 lb.

The table also shows that 1.5 T. (22 ml) Surflan 4AS are required to treat 1000 sq ft at a rate of 1 lb ai/A. Multiply this by 2 to get 2 lb ai/A and again by 5 to cover 5000 sq ft: 1.5 T. x 2 = 3 x 5 = 15 T. / 2 T./oz = 7.5 oz or 22 ml x 2 = 44 x 5 = 220 ml / 29.6 ml/oz = 7.5 oz. This shows that 17.3 lb Ronstar 2 G is needed to cover 5000 sq ft at a rate of 3 lb ai/A and 7.5 oz Surflan 4AS is needed to supply a rate of 2 lb ai/A to the same area.

When measuring herbicides, the following conversions (approximate) may be of use: