Admittedly, understanding the composition of a seed mix can be confusing. So we’re trying to clear some of the fog surrounding native seed mixes and how PLS & composition effects mix pricing.
Pure Live Seed (PLS)
PLS is just a means of indicating live seed. It is a measure used by the seed industry to describe the percentage of a quantity of seed that will germinate.
The success of any planting depends on seed quality, planting techniques, and the correct type and amount of seed needed. Bargain bulk seed with a low PLS percentage will end up costing more to properly seed a project than a higher-priced seed with a higher PLS percentage.
ALL of our native grass and wildflower seed mixes are sold in Pure Live Seed (PLS) to ensure you pay for the seed that will germinate, unlike buying in bulk and paying for ‘inert matter’ and other non-germinating seeds.
This means you might be paying for a 7.65-pound bag of native seed mix, but you’re getting 10 pounds. The extra weight in the bag is not viable seed, and therefore customers are not charged for it. The additional material could be chaff, duff, inert matter, or other organic materials that could not be cleaned out of the seed mix any further. Native seeds are often very tiny, sometimes as small as grains of sand or powder!! At some point, it becomes too costly and time-consuming to keep cleaning seed in an attempt to get 100% pure seed in a lot.
Once our seed is cleaned, it is then sent to a 3rd party laboratory to test for germination rates & purity. The combination of germination rate and purity make up the PLS. Let’s use Butterfly Milkweed as an example: 10 bulk pounds of seed is tested and found to be 90% pure seed and 85% germination rate. Meaning there would be 7.65 Pure Live Seed (PLS) pounds in this example. Another way to say this would be that in this 10-pound lot of seed, 7.65 pounds is the live, viable seed and will germinate. Because we can’t separate the live, viable seed from non-viable seed, this lot of seed will still contain 10-bulk-pounds, and the additional weight is inert matter, chaff, duff, etc.
Once the lab confirms the purity and germination rates, we enter the PLS % into our inventory and sales system to begin building a seed mix.
Building A Mix
The USDA regulates native seeding rates in “seeds per square foot” (PLSft2), which is precisely what it sounds like, how many seeds are in a single square foot. There are 43,560 square feet in 1 acre. Using Butterfly Milkweed as an example, if you wanted 1 Butterfly Milkweed plant growing in every single square foot of your 1-acre planting site, you would need 43,560 PLS seeds in your 1-acre mix.
Because seed is a commodity and sold in pounds (or fractions of a pound), adding and removing seeds from a mix will change the price, but not always in the way you think. Remember when I said some seeds are the size of powder, well some are the size of a dime too. It’s because of this that seed mix pricing can be confusing!
Here are two examples. Golden Alexander has 10,750 seeds per ounce, while Cardinal Flower has 705,797 seeds per ounce (like powder!). If you wanted 1 Golden Alexander plant growing in every single square foot of your 1-acre planting, you would need just over 4 ounces of seed. Whereas if you wanted 1 Cardinal Flower growing in every single square foot, you would need a small fraction of an ounce. In the infographic, you can see this example play out and how a more expensive seed can cost you less than if you were to use a less expensive seed in the mix. It doesn’t always work like this, however.
Custom creating a native seed mix is a calculated and time-consuming task. Understanding native plant communities, composition, soil types, climate conditions, desired customer goals, and other determinants; all get factored into creating our mixes. Simply adding, removing, or substituting a species can substantially alter the entire mix, both in price and long-term success.