Tennessee Smart Yards Native Plants

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Useful Info on a Practical Question — How Many Seeds per Square Foot?

In the past, when I am converting an area of lawn to native plants, I always go through the process of trying to figure out just how many seeds I need to order.  I am a bit variable in the number of seeds I use per square foot, and I got to thinking I need to get a better handle on this question. If you are doing a small garden and just want to plant seeds in a row, it is relatively easy. However, if you are going to broadcast seeds by hand over a larger area, for example 30’ by 40’, it gets a bit tricky.

This decision process is definitely not a science but more of a reasonable guess. So many factors influence the rate of germination of seeds. These include how well you have prepared the soil, whether or not you rake the seed into soil, how much weed seed is still viable at the soil surface, time of year you put down the seed, slope of the land, viability of your seeds, and even the size of the seeds. (Small seeds generally have less carbohydrate reserves and perish more easily than large seeds.)

In general, I found that the recommended rate varies by source, but it is usually between 20 and 60 live seeds per square foot, depending on their size. One source specified a minimum of 40 viable seeds per square foot, which seems like a good starting point since it falls in the middle of the range. Then you adjust this number according to your situation:

(1) If you are planting on a slope of 3:1 or more, you need to increase the seeds per square foot by 50%. (A 3:1 ratio means that for every 3’ of horizontal run, there is a vertical change of 1’ of ascent or descent, which is about a 4” change per foot.) On a slope, seeds are more vulnerable.

(2) If you are hand-broadcasting the seed, you need to increase the number of seeds by 30% to account for fact that seed is not uniformly buried in the soil.

(3) If you are putting seed down in a dormant period, e.g. in fall so that the seeds can go through the required cold stratification during the winter, you need to increase the number of seeds by 50% to take into account the extended time for loss of seed due to birds, rodents, being blown away by wind or washed away by rain.

(4) Most seed formulas count number of seeds needed based on the assumption that all seeds are viable. In reality, not all seeds will germinate. Growers calculate the percentage of “pure live seed” or PLS. PLS takes into account the percent purity of each seed lot and the percent of successful germination in that seed lot. Packages of grass seed usually have the PLS designated on the package, usually about 80%, which means 80% of the seed should germinate. However, PLS is rarely listed on flower seed. I talked to a rep at a large native plant nursery, and he said you can probably assume that flower seeds on average also have a PLS of 80%.

This calculation process is not as complicated as it might sound. It is much easier in an example. Assume a worst case example and that every situation listed above applies……you are planting a native seed mix on a slope in the fall by hand broadcast.

Minimum seeds per square foot = 40
Plus 50% for the slope = + 20
subtotal = 60
Plus 50% for dormant planting time = + 30
subtotal = 90
Plus 30% for hand broadcasting = + 30
subtotal = 120
Assume 80% live seed so divide by 80%
Grand total seed needed = 150 seeds per square foot

Just to repeat….this is not a science, and I know there are other ways to calculate and adjust the numbers. In fact, I find that sometimes an influential factor is how many seeds I have left over and I just want to add more to use them up. Still it is helpful to have a starting point that you can tweak to your own situation.  Plus I went back and looked at my seeding rate on my plantings, which so far have worked pretty well, to see how they compared, especially since all of the above conditions apply to me.  I was pleasantly surprised to find that I have been in the ballpark on my plantings, so it seems like the formula works for me.  Sorry for such a long blog but hope it is helpful.  Any related advice is appreciated!!

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One response to “Useful Info on a Practical Question — How Many Seeds per Square Foot?

  1. joystewart June 26, 2016 at 3:49 am

    I thought I would add a comment based on actual experience now that my seeds have come up this spring, particularly as it relates to increasing seed volume on slopes due to potential run-off of seed. I can see that you could need to use more seeds due to loss from runoff but in my case, the lowest point in my “run-off area” was also part of the planting. So I ended up with a huge germination rate at the bottom of the slope. My seedlings are so thick I can’t even see the ground. I bet instead of having a plant every square foot, I actually have a plant about every square inch. I am going to get a lesson in what happens when seed is planted much too thick. I suppose that does happen in nature where there is bare ground and then just the sturdiest plants survive. At least I hope that is the outcome and that I don’t end up with a lot of unhealthy, overcrowded plants. At least the weed seedlings are going to have one heck of a time surviving.

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