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Seed Calculators, Seed Count, & Seeds per Square Foot for Dummies

Bean counting has come a long way, but why would anyone use a calculator made for seeds?  Let’s break down some common terms and bring you up to speed.

Seed Count

Not all seeds are created equal, some plants make seeds that are quite large, like corn, while others are as small as dust.  Because of this fact, some species will have many more seeds in a pound than other species will.  For example, there are more dust-sized Hairy Mountain Mint seeds in a pound than corn-kernel sized Eastern Gamagrass seeds.  To be precise, 4.5 million Hairy Mountain Mint seeds weigh in at 1 pound, while a pound of Eastern Gamagrass is merely 7,500 seeds.  That’s 600 times more Hairy Mountain Mint seeds in a pound than Eastern Gamagrass!  So, in essence, seed count is the number of seeds in one pound.  Seed counts are often expressed with units of seeds/pound or PLS Seeds/lb.  

Seeds per Square Foot

If you were to measure out a square where each side was 1 foot long, that would be a square foot.  The question then becomes, “How many seeds are needed in that square foot to reach the desired stand of plants?”  The number of seeds in a square foot is often referred to as seeds/ft2, but it may also be written as PLS seeds/ft2, or PLS/ft2.  A generally accepted number of seeds needed in a square foot for a full stand is 40-50. 

An Example Using Eastern Gama Grass & Hairy Mountain Mint

Let’s look at an example of seeds per square foot, and let’s say that the goal is to plant 40 seeds per square foot on a field that is 1 acre in size.  The desired mix is equal parts Eastern Gamagrass and Hairy Mountain Mint.  So, a little basic math reveals that 20 seeds of each species would be needed in one square foot. 

Now, if the goal was to ruin a perfectly good day, you could measure out the planting area, foot by foot, and, by hand, drop 20 seeds of each species onto each and every square foot.  However, if you’d rather keep your good days good, some quick math can reveal how many pounds of each type of seed to put on an acre.  It is an important first step to know that there are 43,560 square feet in one acre.  That means that on each of these 43,560 square feet there should be 20 seeds of Eastern Gamagrass.  43,560 square feet with 20 seeds in each would total up to just shy of 1 million Eastern Gamagrass seeds (871,200 seeds to be exact) to evenly spread across the 1 acre area.  Given we are also planting 20 seeds/ft2 of Hairy Mountain Mint, an equal number of its seeds would be needed.  

Buckle up, here’s where the math gets complicated.  We know that there are 7,500 seeds of EGG in a pound so 871,200 Eastern Gamagrass seeds divided by 7,500 seeds/lb would be 116 pounds of Eastern Gamagrass seed needed for 1 acre.  To put it another way, if there are 871,200 seeds of Eastern Gamagrass, they would weigh 116 pounds.  That much Eastern Gamagrass seed would fill up half a bathtub! 

Let’s do the same calculation for the Hairy Mountain Mint, which has 4.5 million seeds in a pound.  871,200 seeds divided by 4,500,000 seeds/lb is 0.19 pounds.  So, for the same number of seeds of this species, the weight required is much, much less, and this quantity of Hairy Mountain Mint seed would fit in a coffee cup. 

In the end, to keep from going crazy while tediously placing 20 seeds of each species in each square foot, we can simply mix 116 pounds of Eastern Gamagrass and 0.19 pounds of Hairy Mountain Mint together and scatter it across the 1 acre planting.  To make this process even easier, what if the math didn’t have to be done by hand?

Seed Calculator

Many states have designed computer files to do the multistep math required to convert seeds per square foot to pounds of seed.  These computer files help design plantings and have come to be known as seed calculators.  To avoid doing calculations by hand, seed calculators have detailed information about various species built in, such as seed count.  So, for example, a user might input the number of acres in the planting then input the plant species and each plant’s desired percentage of the mix (as defined by seeds/ft2).  The computer file would then calculate information like the pounds of seed of each species to plant.  This means that the above example of Eastern Gamagrass and Hairy Mountain Mint could be calculated digitally using a seed calculator, and in all probability, it would be much faster than doing the math by hand.

A Few Curious Notes   

Here are a few oddities and a common assumption that deserve closer attention.

Seed Counts Differ Between States

Different seed calculators and different states use different seed counts (seeds/pound) for a given plant species.  This doesn’t mean that the number of seeds in a pound magically changes when a state line is crossed.  Rather, this is because there is not one correct seed count for a given species.  There is natural variation in seed size from year to year caused by growing conditions such as drought.  There can also be differences in processing of seeds such as debearding, hulling, and grading and this can have a significant impact on the number of seeds in a pound.  So, in the end, the goal is to have a number that is at least in the right ballpark.     

Percent of Stand

Another important note is that percent of the mix, as measured by seeds per square foot, should never be assumed to be the percent of the stand that results in the field.  Fox an example of this, we can use the above example where the mix was planned with 50% of the seeds in each square foot being Eastern Gamagrass and 50% of the seeds being Hairy Mountain Mint.  It is natural to assume that since half of the seeds are Eastern Gamagrass and half are Hairy Mountain Mint that these two plants, when the planting matures, will have equal cover in the field as well.  However, this is not true in the least.  If indeed the 116 PLS pounds of Eastern Gamagrass and 0.19 pounds of Hairy Mountain Mint were planted on 1 acre (that is an equal number of seeds of each), the resulting stand would be something like a 1000% stand of Eastern Gamagrass plants and 5% stand of Hairy Mountain Mint.  “But,” you say, “there were an equal number of seeds, so how can this be?!”  Other factors must be considered such as the final footprint that the plant occupies, which for Eastern Gamagrass is the size of a chair and for Hairy Mountain Mint is more akin to a coffee cup, and the survivability of the seed. 

Learn More

A related concept is PLS or Pure Live Seeds.  To learn more about this, check out the PLS for Dummies article.

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