Seed Dormancy: The State in Which a Living Seed Does Not Germinate
Seed dormancy means that a seed is alive and still does not germinate, even though warmth and moisture are right. In hemp it mainly affects freshly harvested seed: in a study of two varieties it lasted five to six weeks after harvest and then cleared by itself. Anyone checking how well their seed germinates during that window gets a figure that is too low. A seed that stays put is not necessarily a dead one.
An example shows what this looks like. Someone harvests their own seed, lays a hundred of them on damp kitchen paper three weeks later and counts after a week. Some have sprouted, the rest look unchanged, and the obvious conclusion is that the harvest is poor. Exactly this situation has been measured in hemp: twenty-one days after harvest, 79 % of one variety’s seeds germinated, although a staining test, which marks living tissue red in the cut seed, found 97 % of the same seeds alive. The gap between the two figures was made up of resting seeds, not dead ones.
Telling a dormant seed from a dead one
Seed dormancy, a living seed that fails to germinate, cannot be recognised from a germination test alone. A germination test sets out a counted number of seeds under fixed moisture and temperature and, after seven days, counts the seedlings, the young plants that have emerged. A dormant seed and a dead one look the same there: both simply lie where they were put.
A second test tells them apart, the tetrazolium test. The seeds are cut open and placed in a solution that stains living tissue red. The result is called viability, the capacity to live, and it does not depend on whether a seed happens to be dormant. Dormancy therefore shows up as the distance between two figures: many seeds alive, few germinating.
How long it lasts in freshly harvested hemp
Seed dormancy, living seed that does not germinate yet, has been followed in hemp through the weeks after harvest in one study. Both varieties tested carried it for thirty-five to forty-two days, which is five to six weeks. Twenty-one days after harvest, seed from the upper part of the plants germinated at 79 % and 91 %, while the staining test for living tissue found 97 % and 96 % of the same seed alive. The authors confirmed that the gap was dormancy by staining specifically the seeds that had not germinated.
The dormancy then wore off without any intervention. This breakdown through simple storage is called after-ripening: from twenty-eight days after harvest, treated and untreated samples germinated equally well. The duration is documented for two varieties, not for hemp in general.
Ending it sooner
Seed dormancy, the state of living seeds that do not germinate yet, can be cut short by anyone unwilling to wait five weeks. The measured method is prechilling: the seeds are moistened and held at 10 Β°C, a little warmer than a household fridge, for five days before the actual germination test begins. On seed twenty-one days from harvest this lifted germination to the level of the staining test. The authors recommend it for freshly harvested seed and whenever the age of a lot, meaning a batch of seed from one harvest, is unknown. From the twenty-eighth day after harvest it gave no further advantage in the same study.
In the authors’ reading it works two ways at once: it lifts dormancy and lets the seed take up water in advance, so that it germinates faster and more evenly once moved into warmth. The two effects were not measured separately.
A second study adds another method: moistening with potassium nitrate, a salt solution, and a three-day prechill at 5β10 Β°C, roughly fridge temperature or a little above, raised germination in two of five lots well above the test methods recommended for hemp. The third method is time itself, after-ripening. Landraces from South Africa have been tested with further treatments; they are covered in the separate entry on dormancy.
Where the measurements disagree
Not every hemp lot that has been tested shows dormancy, that is, living seeds that fail to germinate. A second study examined five lots of one variety. There the share of living seeds lay close to the total of all seedlings, normally grown and misshapen together, and the authors concluded that no dormancy was present.
The same study, however, found that a potassium nitrate solution and a three-day prechill raised germination markedly in two of the five lots, and elsewhere reads this itself as a sign of residual dormancy. The age of the seed would settle the question, and that is exactly what this study does not report. How common dormancy is in hemp therefore remains open. A lot of unknown age is therefore prechilled before testing, as the authors of the first study recommend for that case.
What it means for a germination test
Anyone wanting to know how well a lot of hemp seed germinates has to allow for dormancy, the period in which living seeds do not germinate yet. The harvest date belongs with the germination figure: a result three weeks after harvest and one after two months do not measure the same thing.
With fresh seed or seed of unknown age, prechilling before the test is the route the authors recommend. The alternative is to wait until after-ripening is complete, which in the measured study was the case after six weeks at the latest. And a poor germination figure on fresh seed says little about quality on its own. It says far more next to a staining test for living tissue: if that figure is high and germination low, the seeds are resting rather than dead.