How To: Calculate pure-live seed

Turfgrass seed is frequently under-applied. Obviously, anyone who simply ignores proper seeding rates has only himself or herself to blame if the stand comes up too thin. However, even conscientious operators may find themselves wondering why a stand didn't establish as thickly as they hoped. How can this happen? It's all in the numbers.

When seeding-rate recommendations are developed, they generally are based on a target of 1,000 to 2,000 viable seeds per square foot. When you hear that a turfgrass should be seeded at 8 pounds per 1,000 square feet, the often-unspoken assumption is that this is for seed that is 100 percent pure and viable (a standard never completely attained in reality).

This target is used for most turfgrass seeding recommendations, a fact that many don't realize. Perhaps this is because suggested rates, usually given in pounds of seed, can vary so widely. But because seeds of different species (and even varieties of the same species) may vary so much in terms of size and weight, achieving the same number of seeds per unit of area means very different weight-based rates must be used to achieve the same goal of 1,000 to 2,000 seeds per square foot.

Seed size is the main factor that affects seeding rates. However, two other factors — seed purity and seed germination — should be accounted for. When you measure out 10 pounds of seed, that 10 pounds includes seeds (hopefully not very many) of other plant species as well as seed (and other “inert” material) that won't germinate. Thus, your 10 pounds of seed is actually something less than 10 pounds of the desired turfgrass seed.

Better-quality seed should have higher levels of live seed as well as lower amounts of contaminants. But don't take it for granted that your seeding rate doesn't need an adjustment. It's a relatively simple matter to check the seed tag and make the necessary calculations to adjust for impurities. You can do this by calculating what's known as pure-live seed (PLS). You might be surprised at the difference this can make, not only in achieving the optimum seeding rate, but also in finding the best buy.

Calculate pure-live seed

The sample seed tag featured on page 33 shows how the needed figures are presented on seed tags. Seed tag information is regulated, so all tags will contain essentially the same information as that shown.

First, find the percent germination. Then find the percent purity. Multiply them and divide by 100 to find the PLS. For example:

(95% germ. × 80% purity)/100 = 76% PLS

Use PLS to determine the best seed buy

PLS often varies among seed lots, even of the same variety. Thus, the amount you're paying per pound of seed must be adjusted according to PLS. To find out what you're really paying, first determine the PLS of seed you're considering, as described above. Then divide the cost per pound by the PLS to get the cost per pound of PLS.

For example:

$2.00 per pound/.6175% PLS = $3.24 per pound of PLS

Using this calculation, you can compare seed lots. As the table above shows, Lot C is the least expensive per pound of PLS, even though it is the highest gross cost per pound.

Use PLS to adjust seeding rate

If necessary, you should adjust the seeding rate according the calculated PLS. Let's say you have seed with a PLS of 76 percent. If the recommended rate for this seed is 10 pounds per 1,000 square feet, you should actually use 13.2 pounds to achieve the target rate. You calculate it this way:

10 pounds/.76% PLS = 13.2 pounds

This is a significant difference and if you hadn't made this adjustment, you'd be roughly 25 percent under the target rate.

Many times, purity and germination are high enough that a significant adjustment will not be necessary. However, even seed with 90 percent purity and 90 percent germination will yield 20 percent less viable seed than you think if you don't consider PLS. So don't overlook PLS when you buy or apply seed.


Generic Seed Co. Mow Low creeping bentgrass
Lot# 25-OR-624
Pure seed 97.0%
Inert matter 2.4%
Other crop seed 0.4%
Weed seed 0.2%
Germination 92.0%
Test date 6/96
Free of noxious weeds
Oregon grown

Turfgrass type. All labels must list the turfgrass type (species). Most labels list a named variety as well, but not all states require this. Obviously, companies marketing proprietary varieties list them by name. However, generic seed also is available. (Be wary of such seed — for all you know, it could be pasture grass.)

Seed lot. This number corresponds to the specific lot from which the seed came. Though not usually important to the end-user, it allows regulators or seed companies to trace the history of the seed should a problem arise.

SEED PURITY. This is one of the figures you need for calculating PLS. It is the percent, by weight, of seed of the listed type. A seed mix must list this percent for each component. Quality seed usually is not too far below 100% purity.

Inert matter. This figure represents the portion of the lot that will not grow. It can include various debris, chaff, seed parts or other material. While not usually a problem in itself, more inert matter means lower purity and, perhaps, lower value.

Other crop. Seed of other plants considered to be crop plants are listed here. Even though considered “crop,” these can include perennial grasses that may cause serious infestations in your turfgrass stand. Therefore, seed should contain as little “other crop” as possible.

Weed seed. This indicates the percent, by weight, of weed seed present in the seed lot. It includes all weeds, not just noxious species. Of course, this figure should be low, and high-quality seed will contain few or no weed seeds.

GERMINATION PERCENT. This is another of the figures you use to calculate PLS. It is the percentage of the grass seed that will actually grow, as determined by germination tests. The label must show this for each component of a mix.

Test date. This is the most recent date on which inspectors tested germination. In mixes and blends for which components were tested separately, the label will list the oldest test date.

Shipper identification. The label always identifies the shipper. However, this sometimes is in the form of a code, perhaps because a resaler wishes to have their own name on the label rather than that of the producer.

Origin. Many states, but not all, require the label to indicate the seed's origin.

Noxious weeds. The label will indicate the presence of noxious weed seeds, if any. Because noxious-weed lists vary by state, labels list any weeds so defined by the state to which the seed is being shipped.

Certified seed. Another tag (or seal) you may see on seed is a certification label. Seed that state inspectors have found meets certain standards of genetic purity receives certification. This has no bearing on other aspects of seed quality, but certified seed ensures that the stated variety is what you're actually getting. Obviously, this is a crucial matter, so you should always buy certified seed, which will have a separate certification tag. State agencies conduct such programs, so certification criteria vary somewhat among states. However, all ensure that certified seed has met some minimum standard.

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