Lynn Sosnoskie, University of Georgia, Bugwood.org
Sometimes, turfgrass seed that is low in purity and germination is sold at a reduced price. The amount of seed required to compensate for poor purity and germination can be determined by calculating percent pure live seed (PLS). Pure live seed indicates the amount of seed in the container that is capable of developing into seedlings.
To calculate PLS, the percentage of pure seed listed on the seed label of a cultivar is multiplied by the percent germination (also listed on the seed label), and the product is divided by 100. For example, 92% pure seed of the cultivar x 80% germination / 100 = 74% PLS. To determine how much seed to plant, divide 100 by the percentage PLS (74% in this case). Thus, in this example, 100/74 = 1.4. Thus, 1.4 pounds of seed with a purity of 92% and a germination of 80% would be needed for each pound specified in the desired seed mixture.
One way of determining if seed with low purity and germination is really a good buy is to divide the PLS into 100 then multiply by the cost of the seed. A comparison of two seedlots is provided as an example.
(sold at 'reduced' price of $1.95 per lb):
PLS = (85% purity) x (60% germination) / 100 = 51%
100 / 51 = 2 lb of seed needed per lb of seed specified
Cost = $1.95 x 2 = $3.90 per lb of PLS
(sold at regular price of $2.50 per lb):
PLS = (99% purity) x (90% germination) / 100 = 89%
100 / 89 = 1.1 lb of seed needed per lb seed specified
Cost = $3.00 x 1.1 = $3.30 per lb of PLS
A comparison of actual cost per pound of pure, viable seed reveals that seed that appeared to be a bargain (Seedlot A) was actually more expensive.