TURFGRASS MATH
95 % purity x 90 % germination = 85.5 % PLS So , why does PLS even matter ? Pure live seed is important because , as we just showed and calculated , it is extremely unlikely that any seed package will contain 100 % pure live seed . In addition to the fact that germination is less than 100 %, there are numerous other factors reducing seed purity . These may include small bits of inert debris left over from the seed cleaning process , the presence of other species , as well as other components the producer may have added prior to packaging . Also remember that as seed ages and / or as seed is stored in less-than-optimal conditions ( e . g ., summer heat ), the germination percentage can decrease dramatically and affect PLS .
Therefore , the concept of PLS is important because the lower the percentage PLS , the fewer the number of target viable seeds on the ground that may be capable of rapidly establishing a healthy turf . Remember , seeding rate guidelines are based on 100 % PLS . Using our example above — with a fairly high-quality grass ( 95 % purity and 90 % germination ) — this 85.5 % PLS can dramatically affect the number of viable seeds being planted . You may wonder how much 85.5 % PLS actually affects the seeding rate . Let ’ s use an example of a bunch-type grass with a target seeding rate of 6 pounds per 1,000 ft ². If PLS is not considered , you would be missing nearly one full pound of seed per 1,000 ft ² at planting . 85.5 % PLS x 6 pounds = 5.13 pounds of pure , viable seeds . That is roughly 15 % fewer seedlings available to germinate and fill in on the ground .
So , how do we use PLS to meet our target seeding rate ? In the above example with a target seeding rate of 6 pounds per 1,000 ft ², we would take our target seeding rate and divide that by the decimal fraction of percentage PLS . In other words , 6 pounds / 0.855 = 7.02 pounds of seed to achieve the target of 100 % PLS with six pounds per 1,000 ft ². Thus , with this new target in mind , you can use this value when calibrating your seeder or drop spreader and work to achieve a 7.02 pounds per 1,000 ft ² planting rate — not six pounds .
More recently , there is one other thing that can dramatically affect the PLS value . This has to do with the amount of inert ingredients in many contemporary seed packages . For example , let ’ s consider the high-quality Kentucky bluegrass athletic field blend ( shown in the seed label image above ). In this seed package , the seed contains 50 % by weight of a proprietary seed coating . Many of these coatings have some evidence of helping improve seed water uptake during germination and improving seedling vigor . The coating , however , substantially reduces the PLS value . To calculate the PLS of this blend , you simply total the percentage purity of all the cultivars , which equals 49.32 % and multiply by the percentage germination ( e . g ., 85 %) and the PLS is 41.9 %.
By not taking into account this 41.9 %, you may end up planting much fewer seeds than anticipated . Presuming you are using the above Kentucky bluegrass blend , let ’ s use a general average Kentucky bluegrass seed count of 1,500,000 seeds per pound . If one would seed Kentucky bluegrass at the lower end of the suggested planting rate — 1.0 pound per 1000 ft ² — you would be applying approximately 1,500 seeds per 1,000 ft ² ( 1,500,000 seeds per pound divided by 1,000 ft 2 ) or 10.4 seeds per inch ² ( 1,500 seeds divided by 144 square inches in a ft ²). If the PLS of the bluegrass seed blend is 41.9 %, that 10.4 seeds per inch ² reduces to 4.4 seeds per inch ². For spreading grasses , such as Kentucky bluegrass planted when the soils are warm and moisture is available , this is probably not the end of the world ( it will eventually fill in ). For bunch-type grasses , however , this could really affect early density characteristics , and the stand may be more prone to weeds .
Pure live seed really is a thing ! It requires a bit of calculation , but is an especially important factor to consider when using the lower ends of suggested seeding guidelines and / or during non-optimal seeding times of the year . SFM
Cale A . Bigelow , Ph . D ., is professor , Department of Horticulture and Landscape Architecture , Turf Science , Management and Ecology at Purdue . He can be reached at cbigelow @ purdue . edu or on X ( formerly Twitter ) @ BIGTurfKnowHow . sportsfieldmanagementonline . com May 2024 | SportsField Management
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