TLDR : it depends on the seed. The variety of conditions needed for plant germination will surprise you. But don’t get discouraged, most plants need the same basic conditions in order to get a jump start into life.
Most plants from temperate climates like the one we have in most of France will start growing as soon as “spring-like” conditions occur (longer days, sunlight, warmth and the right amount of water). Seeds are usually programmed to wait out the harsh winter months, in dormancy, and then boot-up once they detect that conditions are becoming ideal to start growing. Evolution has optimised the process so that each plant starts its growing season at the optimal time for each particular species.
This is crucial as germinating too early can be fatal – late frosts (or hail amongst other things) will quickly do tons of damage to seedlings – and germinating too late can mean reduced growth rates or crucial delays in flowering period for example. Germinating too late may also make some plants miss their pollinators and bear no fruit or miss the male flowers they need to bear their fruits. We have already highlighted the importance of water and warmth for germination. So how do we get our seeds to start growing, and why do we need to do this?
Some seeds will germinate as soon as good conditions are detected in their environment. Usually, this means sufficient water to hydrate a dry seed, which sets off a series of chemical reactions in the embryo, leading to germination. At this point, the embryo also needs sufficient warmth, or growth will stop, and the seed will probably rot and die. To avoid this, some seeds come with a dormancy period, giving them the ability to start the germination process when conditions are just right.
What is seed dormancy?
Seed dormancy is linked to processes that stop a seed from growing, even if the germination conditions are perfect. This means that some seeds, placed in optimal humidity and temperature conditions will still not grow.
There are a bunch of ways that plants induce dormancy, and we’ll just mention a few so you get the big picture.
One of the simplest dormancies is linked to the seed coat or external, usually hard and impermeable outer layer. Seeds with a hard seed coat sometimes cannot absorb water, and so remain dormant for long periods of time, until something (or someone!) breaks through the seed coat. In natural conditions, this can be mechanical, i.e. the seed dries out and cracks or gets burnt by fire, or chemical as in broken down by acid – typically by passing through an animal’s digestive system. Once the layer is broken down, the embryo can absorb water and start growing. Some seeds also contain high levels of growth inhibitors (plant hormones that tell the embryo to not start growing yet). These can be broken down or washed away over time, finally releasing the embryo after some longs months of slumber.
In another type of dormancy, the embryo itself needs some kind of modification for growth and germination to be possible. These generally are linked to environmental conditions that need to be fulfilled before germination is possible. Some seeds for example need very high temperatures (say over 30°C) before germination is possible. These are usually plants that need to germinate in the middle of the summer to be successful. Others require a long period of cold for dormancy to be removed (for example, over 3 months at less than 4°C), such as many species that want to make sure they don’t germinate during the winter. Some species need a long period of drought, or specific periods of daytime / night-time to be able to germinate.
In all of these cases, germination cannot happen until some conditions are met. If you place a dormant seed in optimal conditions straight after it falls off the plant, it will not grow for a long period of time – think months, years, or even decades. However, this means that we can artificially induce the conditions that will lift the dormancy, often in much less time than required by nature. Here are a few examples.
Scarification
When a seed is covered by a thick, impermeable outer coat that stops water entering the seed, and germination from happening.
The solution here is to remove the seed coat physically, exposing the embryo to the environment, 
thereby allowing germination to happen. There are many ways to break down the outer layer, depending on the species. In some cases, simply rubbing the seed on some abrasive surface (i.e. sandpaper, glasspaper), or making some small holes with a needle or sharp knife can be enough. Take extra care to not rub your skin off or cut yourselves while doing this! Some seeds need strong acids to soften the outer coating of the seed – such as concentrated hydrochloric acid or sulfuric acid. Depending on the species, the strength of the acid and the time required to break down the seed coat can vary. Once again these methods require extra care, especially while handling strong acids that can cause severe burns and injuries to human skin.
Scarification can be very delicate as it is very easy to break into the seed and kill the embryo, and also it can be hard to know how much seed coat to remove to effectively remove it. Some trial and error to be expected!
Stratification
For species that require a long period of cold temperature for germination, there are several stratification methods. The easiest is the most natural one: just leave the seeds outside in the ground over winter – of course this requires a climate with a cold winter! Some seeds even prefer the natural oscillations of daily temperature and you will have more success with this technique.
On the other hand, it is also possible to artificially stratify seeds in your refrigerator. First, you need to clean the seed of any plant material still attached to them (fruit pulp for example). If you want to grow nuts, do not remove the shells. It is often best to soak the seeds for a few hours beforehand, as this can speed up the process, that requires moisture as well as cold temperatures. Then simply place the seeds in moist sand or vermiculite and put small groups of seeds in different plastic bags in the fridge (between 1 and 4°C). Don’t forget to label the bags! The stratification period can vary from 1 to 3 months. Some species also require a period of warm stratification before exposing them to cold in the fridge. In these cases simply do the same thing but first expose the seeds to a temperature 15-20°C, for a period of also several weeks to 3 months (depending on the species).
For seeds with chemical inhibition of germination, soaking the seed in water for 12-24 hours is sometimes enough to remove dormancy.
Why should we artificially remove dormancy?
Well, letting nature do its’ thing is a good option, so if you have the required time and patience and a favorable climate, let your seeds go through winter in the ground outside. For many species from temperate areas, this will do the trick just fine. However, some species need two successive winters to lift dormancy, especially if the first one isn’t quite cold enough. Also, anything can happen to your seeds underground. A number of animals and critters can dig up the soil and eat your seeds. Some ground-living insects and animals will really appreciate your efforts to feed them nice, juicy seeds over the winter! If the ground is waterlogged for too long, they can simply rot away, and many seeds are vulnerable to various fungi as well. So, stratifying your seeds can increase survival and germination rates by quite a lot.
You may also want to speed up germination. Stratification can require some weeks or months, but it also allows you to start the germination process earlier, to have seedlings ready to go into the ground as soon as conditions allow it. You may also want to grow some plants in your home or in a greenhouse when it’s too cold outside. So these methods of removing natural inhibition to germination can be very useful.
Please be aware that the needs for each individual plant species can vary immensely, so read up on the ideal process for your seeds before you start working on removing their dormancy.
If you are really into growing from seed, you can record in a gardening book when you planted each seed, how long it took to germinate, whether you started your seeds too early or too late, or whether you grew too few or too many. You should also note your plants survival rate. This way your harvest will be that much more effective the following year.
Growing plants from seeds is a constant experiment which is a lot of fun. So we recommend trying out new stuff and letting your imagination roam free.
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