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!!