Seed germination studies

Since most of the seeds we need have been collected, it's time to think about seed quality. How "good" are the seeds were are getting?

It's a relatively simple thing to check seeds to see if they have embryos, but that doesn't necessarily mean they will germinate. For the past several years I have been running germination tests in petri plates. I put a circle of filter paper (paper towel would do as well) in the bottom of the petri plate, moisten the paper, and add about 25 seeds. I usually do at least 2 plates of each species.

The plates are wrapped with Saran and put in the refrigerator for a month or so to induce them to germinate. (The seed biologists call this procedure stratification.) Then the plates are remoistened and brought to room temperature under fluorescent lights or in the greenhouse. They are rewetted as required and checked daily for germination. Care is taken to be sure the seeds are not overwetted, as this may result in mold growth. If they have not germinated in a month or two, I assume they are not going to germinate (but see below).

The photo shows a purple milkweed germination test. The seeds have broken out of their seed coats and have sent out long roots.



In 2008 I ran 25 different germination tests, with quite variable results. Some species germinated very well, some did moderately well, and some did very poorly. A few species did not germinate at all. The highest germination percentages were found with the milkweeds, almost all of which germinated very well. (See results below)

Butterfly milkweed: 84%
Short green milkweed: 84%
Poke milkweed: 86%
Purple milkweed: 100%

Eupatorium sessilifolium, a Special Concern species, germinated very poorly, around 2% at best (some batches did not germinate at all).

Two other Special Concern species did better: sweet Indian plantain, 28%; Giant yellow hyssop, 52%. With Napaea, another Special Concern species, the higherst percent germination was 6%.

I tested three grasses: Indian grass, 58%; little bluestem, 16%; and prairie dropseed, 22%. (These were tested without the cold treatment, since grasses don't require this.)

Not all plant species are induced to germinate by the cold, moist treatment described above. Some species require other kinds of treatments. There are reference books that provide details.

Although these tests are fairly quick and easy, and mimic what the seed testing laboratories do, not all seeds germinate in this short time period. The seed testing labs have an alternate procedure, the tetrazolium test, which checks to see if the seeds are viable. This measures the ability of the seed tissue to metabolize the tetrazolium compound and produce a colored product that can be seen by the eye. If the seed does not germinate but is tetrazolium positive, then the seed lab concludes that the seed is alive but dormant. (There are also other, more technical, methods of checking seed viability.) The advantage of buying seeds is that the seed companies sell based on pure live seed (PLS). If you collect your own seeds, you are assured a local genotype, but viability is not guaranteed. With seed of unknown viability, much higher planting seed densities are required.

The above is a very abbreviated discussion of a complicated process. For more on seed quality, see The Tallgrass Restoration Handbook (Packard and Mutel) or A Practical Guide to Prairie Reconstruction (Kurtz).