Seed Dormancy

A single flower produces a single seed. When you see a flower with hundreds of seeds on its seed head, it means that the flower was a compound flower – with hundreds of miniature ‘flowers’ in the middle. When you pick a flower like this, gardeners will call the entire thing a ‘flower’, but botanists will call it a cluster of small florets. A single flower producing hundreds of seeds is very common for some plant families (e.g., Asteraceae), while other plants produce single flowers spaced out separately (e.g., Gaura).

A seed is such a fascinating thing. All life on our planet depends on plant seeds. 60% of the world’s food is seeds of wheat, rice, corn, and soy. All of these are actually annuals – i.e., they grow, live, set seed, and die all within 12 months (or thereabouts). In other words, 60% of the world’s food depends on the behaviour of seeds.

The seed is comparable to a human baby inside a mother’s womb. A seed is enclosed by a seed coat, which is something like a womb. The seed coat actually has the same DNA as the mother plant – not the baby plant. The properties of the seed coat are inherited from the mother plant. Inside the seed, the embryo is the baby plant – which is genetically different from the seed coat. Plants produce sexually, so the embryo is genetically a mix of the mother and father plants.

When a seed forms on the mother plant, the hormones within the seed start to change – and the seed begins to enter a state called dormancy. When a seed becomes dormant, it does not germinate under ‘normal’ circumstances. Poetically, you could say that the seed is sleeping.

What’s interesting is that the environment around the mother plant helps determine the depth of dormancy for the developing seeds. There’s evidence that a colder autumn can create a deeper dormancy for the seeds (Chen et al, 2025). Also poetically, it’s as though the mother plant is whispering to her babies that they need to bunker down for a longer winter – and they shouldn’t germinate until they’re absolutely sure that it’s spring.

Dormancy and Germination
As the seed is forming, it starts to develop a thick layer of cells between it and its mother (called an abscission layer). Almost like tying off an umbilical cord. For most seeds, this is when they are heaviest – when they are at their maximum moisture content. Sometimes dormancy-setting can be interrupted if seeds are harvested before they’re fully mature – e.g., green seeds of Wild Oats can be harvested while still green, which give a higher germination compared to mature seeds (Baskin and Baskin, 2014). But most seeds need to develop to maturity before they are primed for proper germination.

The state of dormancy is different for every species, but there are only a handful of different types. Inside the seed, there are biomolecular underpinnings for how dormancy is maintained. Seeds come out of dormancy in one of two ways: either (1) they get damaged over time, such as with many seeds after dry storage; or (2) they experience specific environmental signals that it’s time to germinate.

The process of germination is an entirely different internal state compared to dormancy. Similar to the difference between being asleep and being awake. When dormancy is broken, germination is most likely to happen.

‘Breaking’ dormancy is often a one-time deal: once broken, it doesn’t re-enter dormancy. This is true for most seeds that are difficult to germinate (i.e., deeper levels of dormancy). For most seeds in the northern hemisphere, however, their dormancy fluctuates with the seasons. With these seeds (which have what is called a non-deep physiological dormancy), their dormancy fluctuates over time. For example, Field Poppy (Papaver rhoeas) behaves this way – where it comes out of dormancy after the warmth of summer, germinating in the autumn; but if it doesn’t germinate in the autumn, the cold winter actually pushes it back into dormancy. This cycling behaviour of dormancy is common with many seeds. Often these are very long-lived seeds. (This is also why you should be careful using a heat mat with seeds like these, as you might just push seeds back into dormancy rather than germinating them.)

Almost always, seeds from plants that are native to regions that experience cold winters (i.e., so-called temperate biomes) will only break dormancy after they experience winter in some way. It’s their single greatest ‘cue’ for knowing when to germinate (i.e., spring comes after winter). Seeds from tropical regions (such as Nasturtium) don’t rely on cold winters as a cue to germinate; often they just need moisture and light. The vast majority of seeds that are tough enough for gardens Zones 6 are from plants in the northern hemisphere.

There are only a handful of different ‘types’ of dormancy. I am often referencing these in my daily articles, and hopefully this helps you understand the terms a bit better.

Dormancy in Seed Batches
To make things a bit more complicated (too late), if you were to take a handful of seeds from the same seed pod and try to germinate them, you may not get 100% germination. This is often because (1) some seeds are dead; and (2) not all of the seeds have the same dormancy.

The most common example is a mix of non-deep and intermediate PD seeds. This is very common. If you decide not to cold stratify the seeds, you might get 25% germination, and which is fine. Maybe this is acceptable for your goals. But the rest of the seeds would have germinated if you cold stratified the seeds. Furthermore, you might get more germination if you cold stratify for 3 months as opposed to 2. This is because seeds are almost never all the same dormancy – you need to treat the seeds based on deepest level of dormancy.

Some gardeners will swear by a certain method for germinating seeds and are hesitant to try a different method (especially nursery growers). But, better results are possible with just a little bit more work sometimes.

Another kind of combination is actually MD + MPD. Some seeds from the same plant will have just morphological dormancy (MD) where the embryo is immature but no winter is needed. The rest of the seeds have morphophysiological dormancy (MPD) where the embryo is immature and the seeds also need winter. This is extremely common with Aquilegia species, where it’s almost always a mix of MPD and MD seeds. The best results will always be to cold stratify the seeds based on the more difficult of seeds in the mix. But of course you’ll always get a small percentage of seeds germinating without cold stratification (because the MD seeds just need a little time to grow).

Small Reminder
Lastly, I wanted to quickly point out that that cold stratification is not really possible when seeds are in their dry state. They must have a relatively high moisture content (i.e., they are imbibed) in order to undergo cold stratification. This is because seeds are pretty much inactive when they’re in a dried state – i.e., they’re not doing anything, they’re just in ‘standby mode’. Seeds must be moistened again before they can do anything, including cold stratification. Simply putting a dry seed packet into the fridge does not cold stratify seeds. (It is dry storage, so some seeds will lose dormancy over time through after-ripening, but this is not the same as cold stratification.)

Second, a casual reminder that all seeds eventually run out of energy, especially if they’re imbibed. If you’re cold stratifying seeds in an imbibed state, the lower temperature will allow them to last for longer – but once you start bringing imbibed seeds up to room temperature, they only have 2 or 3 months of energy reserves. They will not last forever. So, if you start off trying to germinate the seeds in the wrong state (e.g., you forgot to cold stratify) and then try and correct this by transferring to another temperature regime, it might not work because the seeds have run out of energy. It’s always wise to research the germination requirements before you start.

Published by Brian Wagner on 11/2/25 in the “Growing Flowers from Seed” Facebook Group.

One of the simplest types of dormancy is called physical dormancy (abbreviated as PY in scientific studies). Physical dormancy is where the seed coat is impermeable to water – i.e., you can soak it in water and it doesn’t rehydrate. It has a physical barrier preventing water from getting in. An example are Baptisia species – the seeds are impermeable to water, and you would need to physically damage the seeds (or poke a small hole in them) to let water come into the seed. This is the only barrier to germinate. Once the seed takes in water, it germinates.

In nature, seeds with a physical dormancy don’t germinate for several years after they drop to the ground. The elements wear the seed down over time – freezing, thawing, sun exposure, cracking, physical crushing, animals, etc. It’s a ‘delay’ for germination that lasts several years or more.

Published by Brian Wagner on 10/25/25 in the “Growing Flowers from Seed” Facebook Group.

The vast majority of seeds that have an immature embryo (as explained in the MD section) also have a physiological dormancy as well. They have both. Seed ecologists would say they have a morphophysiological dormancy (MPD). For example, Aquilegia oxysepala has an MPD and in order to germinate the seeds, you would first cold stratify the seeds for 10 weeks at 4C (40F), followed by incubation at 15C (60F) during the day and with light. With this type of MPD, the embryo only grows during the cold winter period – and it also breaks its physiological dormancy at the same time.

MPD are very complicated, and there are many different types. If you’ve ever tried growing any plant in the Ranunculaceae family, you’ll have experienced failure to some degree. Most if not all species in this family have an MPD. Also, MPD is very common in Apiaceae, Ericaceae and Lillidaceae family plants. I could write a book on MPD, it’s very complex – and they have some strange behaviours.

There are two fundamental classes of MPD: simple and complex. Simple MPD is where the embryo grows during autumn or spring temperatures, and it can only grows after its physiological dormancy has first been broken. (There are also three levels of simple MPD, but I will not cover this now.) Complex MPD is where the embryo only grows during winter temperatures, and the physiological dormancy is broken at the same time (usually). Again, it’s complicated.

Published by Brian Wagner on 10/25/25 in the “Growing Flowers from Seed” Facebook Group.

If seeds have an embryo inside that isn’t fully developed (or even formed), seed ecologists would say that they are developmentally immature embryos (either underdeveloped or undifferentiated). In other words, despite the fact that the seed was fully formed and dropped from the mother plant, the embryo inside needs more time to grow. It doesn’t do this by accident. It’s just a different kind of delay mechanism.

Columbines are a great example – all of them have immature embryos when the seeds are mature, and before the seeds can germinate the embryos must mature (i.e., grow bigger) inside the seed. Some species, such as Aquilegia buergeriana, do not require cold stratification at all – and they simply need a little time at the right temperature for the embryo to grow and then germinate. (This species is from very warm regions.)

Published by Brian Wagner on 10/25/25 in the “Growing Flowers from Seed” Facebook Group.

It’s a common observation: after years of buying packaged seeds without issue, a gardener harvests her own seeds one year – and the germination is low. What has just happened?

Anyone new to harvesting seeds will stumble on this topic through some painful lessons: many seeds remain dormant for 6-12 months after harvesting. In academic circles this natural release of dormancy over time is called after-ripening.

The vast majority of seeds packaged and sold in retail stores exhibit this behaviour. In fact, most seed companies in the mass market seed industry refuse to sell seeds that need cold stratification or other kinds of treatment. They only sell seeds that after-ripen with time. For example, the vast majority of vegetable seeds are, in fact, dormant when they’re freshly harvested; after 3-6 months they’re no longer dormant. No special treatment, just the passage of time. What’s happening?

Normally, a seed matures on the mother plant and then gets released into the world – dropped, blown by the wind, pecked by a bird, licked by a dog. If it’s a plant in the temperate world (i.e., temperate meaning that winters are cold and summers are warm, as opposed to tropical where temperatures are always rather warm) then the seed will always almost be dormant when it’s released (or ‘dispersed’ as is commonly said in seed ecology studies).

Dormancy is programmed into the seed at a genetic level, but often it’s influenced by the environment as well. For example, certain alpine species are known to have a very deep level of dormancy if their seeds are harvested in the wild (at high elevations) – but if grown domestically in warmer environments they might have weaker dormancies. Genetics and environment both play a role.

Dormancy is intended to prevent the seed from germinating at the wrong time. It would be very inconvenient if all the seeds germinated as soon as they were ripe, wouldn’t it? Imagine all our flower seeds germinating in autumn rather than spring. The current definition of dormancy is the failure to germinate within 4 weeks when provided with optimal conditions for that species.

There are other ways seeds ‘know’ when to germinate, and it isn’t always dormancy related. Even if a seed isn’t dormant, it almost always has a temperature range over which it germinates – and temperatures that are too cold (or too hot) tell the seed to stay sleeping.

If seeds are harvested as soon as they’re ripe, they will almost certainly be dormant. However, many ‘easy flower’ seeds are dormant only for a short period of time – for example, Bellis perennis (Field Daisy) seeds are dormant but only for a few weeks, and then they’re good to go. Others are dormant for a few months, and others for a year.

What seeds after-ripen?

There are many (many) seeds that have a very shallow kind of dormancy – called a non-deep physiological dormancy (PD) – which typically lasts a short period of time. Seeds that have a non-deep PD can often break dormancy in two ways: either (1) they experience winter for 2-4 weeks; or (2) they naturally lose their dormancy over 3-6 months, especially if kept dry and at room temperature.

A great example is Yarrow (Achillea millefolium), one of the loveliest perennial flowers for full sun. At maturity, freshly harvested seeds are physiologically dormant – and they will not germinate when they’re fresh. To break the dormancy, you can cold stratify the seeds at 4C (40F) for 4 weeks when the seeds are moist. But, there’s another way to germinate the seeds: keep them dry and at room temperature for 6 months, and they should germinate just fine. If you harvest the seeds in August, then by spring these seeds won’t need cold stratification at all. They will germinate without it.

Not all seeds do this! Only seeds that have a non-deep PD experience after-ripening with prolonged dry storage. Seeds that have a deeper level (or different type) of dormancy don’t come out of dormancy with dry storage. For example, Rudbeckia fulgida (Black-Eyed Susans) for the most part don’t come out of dormancy with dry storage: they will need cold stratification no matter how old the seeds are. These seeds have an intermediate PD, not the non-deep PD.

Almost all annuals, weeds and vegetable seeds have a non-deep PD. In other words, almost everything commonly sold in seed packets at the garden centre will be seeds that have a non-deep PD.

If you were to harvest your own Yarrow seeds and try to germinate them, they won’t work. You’ll have to use seeds from last season. Or, cold stratify the seeds. Once you start to get into harvesting your own seeds, you’ll be confronted with the topic of seed dormancy more often. It will be very frustrating at times. Especially because there isn’t a good resource yet that tells you what dormancy the seeds have when they’re fresh.

What is after-ripening?

For seeds that have a physiological dormancy, their little embryos don’t yet have enough push power to break through their seed coat. The outer shell of the seed (often called the seed coat) is like a little straight jacket – keeping the embryo from breaking free. It isn’t a very strong straight jacket, but it’s strong enough to keep the weak seed from germinating. Normally, when a seed experiences cold stratification (or winter), the internal chemistry of the seed shifts – and it moves away from dormancy and into ‘germination mode’ – and which strengthens the embryo. It gets stronger, better able to break through the seed coat.

When seeds don’t experience winter – and instead they’re just plunked on the shelf and kept dry – their internal chemistry also changes: a signaling molecule called Abscisic Acid (ABA) breaks down through oxidation (free radical damage). Because seeds have a low metabolism when they’re kept dry (i.e., like they’re in standby mode) they can’t repair themselves. So when they’re dried out and kept dry for long periods of time, they’re experiencing free radical damage but they can’t repair themselves. In doing so, the seed shifts away from dormancy (maintained by ABA) and into a state where germination is more likely to happen (maintained by Gibberelic Acids (GA)).

Sometimes this after-ripening process is reversible. For example, many seeds such as Papaver somniferum will cycle in and out of dormancy depending on what temperatures they experience – and even though they will after-ripen into a non-dormant state, they can get pushed back into dormancy if they experience winter again. This kind of cycling in and out of dormancy is common with non-deep PD seeds. (Also this is why you don’t put poppy seeds into ice cubes wink.)

After-ripening for vegetable seeds is typically irreversible – the seeds stay non-dormant. Most annual flower seeds are like this.

How to after-ripen seeds

Seeds after-ripen faster at warmer temperatures. If dry out seeds and then keep them in the fridge, they will not after-ripen very fast. It slows it down. They might stay dormant by the time you’re ready to use them. Room temperature storage (around 21C or 70F) is the ideal temperature for after-ripening. Anything warmer runs the risk of pushing some seeds into secondary dormancy (if they cycle), and anything colder will slow down the process too much. Place them on a shelf at room temperature, somewhere dry.

Also, whenever possible you should always keep seeds in paper bags (to keep them dry) or just exposed to the air. You can add moisture absorbing packs if you want, but it isn’t necessary. Seeds will reach an equilibrium with the humidity in the air after a period of time.

If you’re trying to ripen seeds (i.e., make them after-ripen in a hurry), you should keep them as dry as you can and keep them at room temperature. Possibly 25C (80F) might help them after-ripen faster, but you might push them into a secondary dormancy depending on the seeds (i.e., some non-deep PD seeds cycle in and out of dormancy with temperature). If you’re not sure, best to stay at room temperature. If you dry them out and then stick them in the fridge (after you harvest them), they might still be dormant by the time you’re ready to sow them. It helps preserve their longevity, but it also prevents after-ripening.

Also, another great tip for seed storage is to keep them in a room where ambient light is present. In other words, where they experience the natural rhythm of light during the day and darkness at night. There is interesting research that seeds that are kept in 24/7 darkness can get pushed into a strange kind of dormancy (called skotodormancy) where they need really bright light to germinate, whereas seeds kept in ambient light will germinate in darkness (or dim light) as per their natural custom.)

Short-lived seeds

A quick note about short-lived seeds. Some seeds don’t tolerate drying out for very long at all. So-called recalcitrant seeds rarely last more than a year, and they must be kept at room temperature and in a moist state the entire time. There are very few seeds that are truly recalcitrant, but there are certainly a lot of seeds that are okay drying out – but which won’t last more than a year. For example, Primula bulleyana seeds have a strong germination if used within 12 months, but the seeds start to die shortly after. Same with Agastache foeniculum. And both have a non-deep PD.

Seeds that have a non-deep PD are often 2 or 3 years in longevity before they start to die. You could achieve 100% germination of most non-deep PD seeds in the first year after harvest, but the second year could be 75% – and the third year 25%. This is a very common death curve for non-deep PD seeds.

Published by Brian Wagner on 11/2/25 in the “Growing Flowers from Seed” Facebook Group.