Tennessee Turfgrass - April / May 2012

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Make the Most of Your

GRANULAR APPLICATIONS By John Street, Ph.D., and Pam Sherratt, Dept. of Horticulture and Crop Science, OARDC & OSU Extension, The Ohio State University


turfgrass managers continue to rely on the use of preemergence herbicides as a preventative control for annual grassy weeds. Preemergence herbicides provide a chemical barrier or chemical blanket at the soil surface that intercepts the weed and prevents its growth and development. Efficacy of the herbicide product used is determined by many factors: time of application, soil temperature, product rate, product formulation and uniformity of coverage. Making sure that granular products are uniformly applied is critical to success. Application uniformity is determined by particle size, particle-size uniformity, application equipment and application speed.

Particle size Particle size is defined by the size guide number (SGN) value (see Figure 1). The SGN is determined by calculating the average particle size of the product granules in millimeters and multiplying by 100. Materials and products used on turfgrass generally have an SGN range of 75 to 350. Greens products range from 75 to 100; close-cut turf, from 125 to 150; and higher-cut turf like lawns, from 150 to 240. SGN is important in preemergence herbicide efficacy because it reflects the number of particles applied per square inch. Estimated particles per square inch for SGN values of 100, 150, 200 and 240 are 14 to 16, 8 to 10, 6 to 8 and 4 to 6, respectively. A target of 6 to 8 particles

Figure 1. Particle Size – Size Guide Number (SGN).

Size Guide Number

More Examples


A product with an average particle size of 1.5mm will have an SGN of 150

1.5mm 2.3mm

UI 33


Uniformity Index (UI) A UI is a value representing the relative particle-size variation of the granules within a product. A UI of 50 means that the ratio of the smallest particles is one-half the size of the largest particles (Figure 2). A UI of 100 means that all of the particles within the product are equal in size. UI values range from 35 to 60, and a value of 50 is considered acceptable for good uniformity of spread. UI is important because, with rotary spreader equipment, larger granules travel a greater distance than do finer

Figure 2. Uniform Particle Distribution – Sizing: Uniformity Index (UI).

The “average particle diameter” of the product expressed in millimeters multiplied by 100

For Example:

per square inch is considered the minimum desirable number for optimum preemergence herbicide effectiveness.

UI 50

TENNESSEE TURFGRASS APRIL/MAY 2012 Email TTA at: tnturfgrassassn@aol.com

UI 100

UI 50 means the smallest particle is 1/2 the size of the largest particle.