By Cale A. Bigelow, Ph.D.
Rapid establishment of a dense, persistent turf from seed relies on planting a viable optimal target seedling density. Target seeding rates, however, vary by individual species and seed size. For example, spreading grasses such as Kentucky bluegrass with smaller but numerous seeds per pound have a lower suggested seeding rate (e.g., 1-2 pounds/1,000 ft2). By contrast, bunch-type grasses such as perennial ryegrass possess large but less numerous seeds per pound and, in turn, have a higher suggested seeding rate (e.g., 6-9 pounds/1,000 ft2).
Seeding rate guidelines have been developed over decades of research and practical observations, and take into account factors such as seed size, growth habit, the presence of seed hulls, seed age, and additional considerations (such as if one is planting on bare soil or into an existing turf). Other factors may include target planting date, soil temperature at seeding, the availability of water to keep the seed moist during the germination process, and sometimes even differences in soil texture (e.g., seeding into a sand-based rootzone, etc.).
Ultimately, there are numerous sources for seeding rate guidelines (See table above from Columbia Seeds or visit Seed Research of Oregon at https://www.sroseed.com/technical-specs/seeding-specifications). While subtle differences may exist, in general, published guidelines are similar with the goal of rapid, healthy establishment.
Once a target seeding rate is determined, there is another important but often overlooked factor that must be considered if you want to achieve your target planting density. This is referred to as pure live seed (PLS). In other words, what percent of the desired species in the seed bag is actually viable and most likely to establish under optimal field conditions? In simple terms, PLS is the percentage purity of a given species/cultivar multiplied by the percentage germination. For example, if a given turfgrass species listed on a seed label contains 95% purity and has a lab-tested germination rate of 90%, the PLS would be equivalent to 85.5% PLS. 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 ft2. If PLS is not considered, you would be missing nearly one full pound of seed per 1,000 ft2 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 ft2, 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 6 pounds per 1,000 ft2. 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 ft2 planting rate – not 6 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 example seed label image below). 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 ft2 – you would be applying approximately 1,500 seeds per 1000 ft2 (1,500,000 seeds per pound divided by 1,000 ft2) or 10.4 seeds per inch2 (1,500 seeds divided by 144 square inches in a ft2). If the PLS of the bluegrass seed blend is 41.9%, that 10.4 seeds per inch2 reduces to 4.4 seeds per inch2. 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.
Cale A. Bigelow, Ph.D., is professor, Department of Horticulture and Landscape Architecture, Turf Science, Management and Ecology at Purdue. Bigelow, Cale A. He can be reached at cbigelow@purdue.edu or on Twitter @BIGTurfKnowHow.