Seed Dormancy

Some seeds have this odd thing about them. They don’t sprout at the first moment that conditions are ideal. Why would they do this? It increases the rate of their survival. These seeds are in a state of dormancy. This isn’t the same as hibernation. Hibernation is something that an organism chooses to go into. Seeds don’t choose to enter dormancy when winter is coming or the soil dries out. They have no choice. It’s something they start out in. Dormancy keeps seeds from sprouting during decent conditions because it’s forcing them to wait until a time they’d have a better chance of survival.

Do all plant species produce seeds that go through dormancy? No, because not all of them need to. A seed dropped during October isn't going to sprout. The conditions aren't ideal for germination (the point when a seed sprouts and the plant first starts growing). It’ll wait until the spring and then it will sprout up. Also, seeds from plants living in areas with a fairly constant year round climate (such as an equatorial rainforest) can often be non-dormant. The conditions on any given day are so similar to any other day that there’s no reason to go through dormancy.

However, a seed dropped in August may need dormancy. September might be a damp, warm month, and a non-dormant seed might thing it was okay to sprout. And for a few weeks or a month it might grow just fine. At least until it started to drop below freezing and started snowing. At that point the seedling would probably die. However, with dormancy the seed is prevented from sprouting during the fall. Some mechanism in the seed prevents it from sprouting until it receives a signal canceling the dormancy. The dormant seed then rides out the winter safely, and it’s not until spring that it finally sprouts and grows. Instead of having a month or two to grow in the fall, this new seedling will have six or seven months to grow. This extra growth might make all the difference in the plant’s survival.

Seed dormancy can take a variety of forms and require a variety of cues in order to be broken. These forms include physical, physiological, morphological, morphophysiological, and combination dormancy. The cues can vary wildly, anything from a period of cold to one of heat, a time of dryness to one of dampness. Let’s take a brief look at each of the different forms.

Physical dormancy is probably the simplest form. In this form water is blocked from entering the seed by the seed coat. It’s only after a period of time and environmental factors that the seed coat is made permeable (allows the passage of water). At this point the seed can germinate once favorable conditions arrive.

Physiological dormancy is caused by internal chemical suppression of germination. The environmental factors might be favorable for growth, but the cue has not come to release the stranglehold on germination. This cue (at least in our area) often takes the form of a certain time length of cold temperatures. In much hotter climates this could be reversed and require a period of high temperatures. Other seeds might require a period of darkness, one that might only occur in the short days of winter, or with snow cover.

Morphological dormancy is a product of having seeds that can’t germinate because they’re not ready yet. The plant dropped seeds that were not yet developed enough to germinate. The seeds need time on the ground just to reach the point where germination could start.

Morphophysiological dormancy is a combination of physiological and morphological dormancy. The seedling is both underdeveloped and chemically prevented from germinating. This means that one of the environmental cues must occur and then a period of time must pass before the seed will sprout.

The final form is a mix of physical and physiological dormancy. The seed has both a water impermeable seed coat and it requires environmental cues to allow germination.

Think about the work it’s taken to grow what we want, where we want, and when we want. The seeds we buy to plant in our gardens aren’t always non-dormant varieties. Many of them had their innate dormancy bred out of them by humans selecting for seeds that germinated easier. Other seeds may need to be tricked into thinking that all their environmental cues had been met. They may be dried out and put in a fridge for a few months, put in a dark room, or kept warm for a few weeks. Think of the effort it takes to trick these seeds into growing for our needs. And that’s not to say we know how to trick all plant seeds. There are still species we don’t understand which cues they need to break dormancy.