Sprouting plants from seeds indoors can sometimes be a tricky task. Before trying your hand at growing plants from seeds, you should understand the three stages of seed germination.
The three stages of seed germination are:
- Imbibition: the seed intakes water, swells, and extends its first root (radicle)
- Dormancy: the seed digests the nutrition inside of it and sprouts
- Growth: the seed grows exponentially into adulthood
That’s just a simple summary of the three phases—in reality, it’s more complicated than that and there are many processes going on. Understanding exactly what is happening, why, and being able to identify what stage your new seedlings are in is extremely useful knowledge for any indoor gardener. We’ll cover all of the stages in depth below!
A Seed Is Born: Pre-Germination
Before we can jump into the first stage, we need to have a quick overview of what happens before the seed even gets to that stage. Understanding where the seed comes from and what exactly a seedling contains will help us better understand and explain steps that occur later.
When a plant flowers, the male pollinates the female’s ovaries which then produces an “embryo seed” (or multiple seeds, depending on the species). The embryo, much like an animal embryo, is like a little “mini plant” made from multiple types of cells.
While it grows on the mother plant, it feeds on the endosperm, which is where essential nutrients like carbohydrates, proteins, and lipids (fat) are made and stored. Once it leaves the mother plant, the endosperm will be its primary energy source while it sprouts.
Germination: Activate!
The germination process itself begins with a process called metabolic activation. This includes several hormones, processes, and nutrients, including:
- Gibberellin, a growth hormone which is responsible for stimulating the plant to grow upwards, encouraging germination, and even later processes like flowering and ripening the fruit
- Amylase synthesis, the process by which the seedling can convert the stored starches in the endosperm into sugars to fuel growth before it can photosynthesize
Pre-germination and the process by which germination gets started is very complicated, so we’ll stop there. Those are two important processes to remember going forward. However, in summary, just remember that pre-germination, the seed is:
- Storing nutrients
- Secreting growth hormones
- Preparing to convert stored nutrients into energy
First Stage of Seed Germination: Imbibition
The first stage of seed germination is Imbibition, or water uptake. In this stage, the seedling begins to absorb water and swell in size.
This is the stage that occurs when you begin providing the dried seedling with water, either in soil or directly in water itself. The seed swells in size because the structural components—and the cells themselves—are being rehydrated.
During this first stage, there is also a stage called the “Lag Phase.” This is unlikely to occur in an indoor gardening setting, but some seedlings do better when they are exposed to it, so we will discuss it briefly. The lag phase occurs during a drought or other environmental conditions. The seedling is still active, but prepares more slowly to sprout due to decreased resources.
The last part of the rehydration phase is when the plant shoots out a radicle. Once the seed has swollen with enough water, it shoots out its first little baby root (the radicle). It’s that cute little green root that you can see when you are first sprouting your seeds.
Second Stage of Seed Germination: The Dormant Phase
This phase is also called the “Latent Phase.” At this point, the seed has swelled with water and extended its little baby radicle root. Once that occurs, the seed seems to enter a dormant state, hence the name.
In reality, the seed is just beginning to break down and digest all of those compounds that were stored inside its endosperm that we talked about earlier. These include things like:
- Carbohydrates
- Lipids (fats)
- Proteins
- Phosphates
All of these little compounds are essential to the seed’s growth, and they’re all provided from storage within the endosperm at this point.
One of the more important pieces of this process is the breakdown of proteins. The seedling breaks the proteins further into sucrose, which is then sent to the rest of the seed to be consumed for energy. In essence, the seedling takes the stored proteins, breaks them into sucrose, then uses that sucrose as energy to synthesize new proteins that it needs for its growth.
In the latent phase, the necessary food with water helps the seedling prepare. After a while, it releases combustive energy powerful enough so that it becomes ready to sprout.
Oh, Shoot! The Seed Sprouts
Depending on the right conditions, the cotyledon pokes through the dirt and displays the shoot’s leaves. Cotyledons are often called seed leaves because they are actually part of the seed embryo. They are basically that first little sprouting stalk the plant shoots above ground.
The cotyledon continues to feed from the stored nutrients inside the seed until the real leaves can grow and start photosynthesizing. Since it is small, the cotyledon will shed off as the truer leaves continue to develop.
Afterwards, another little piece called the hypocotyl grows to become the stem while the plumule becomes the first mature “adult” leaf the plant grows. Those are the scientific terms, but all you need to remember is:
- The baby sprout emerges
- Another piece grows to strengthen and become a stem
- The baby leaves shed as the first mature adult leaf begins to grow
Factors of Growth
At this stage, there are many outside factors that can influence when the seed begins to actually sprout and in what orientation. Chief among these is the light conditions, however the baby leaves (the plumule) won’t actually spread, and they only turn green when exposed to light! It’s a somewhat complicated process, but it all relies on a light sensitive pigment called phytochrome.
Don’t worry if your seedling sprouts at an odd angle. If a seedling is in soil when germinating, it will sprout at a bend to protect the delicate tip of the plumule. Seeds sprouted in water may sprout straight instead. Either way, it will eventually straighten up when it is exposed to enough light.
Exposing the seedling’s shoot (plumule) to lots of light has an effect on its viability. If the sprout is exposed to enough light, it will high what’s called a “positive phototropism.” Sprouts with a high level of positive phototropism have the best change of achieving photosynthesis. This can vary somewhat by plant, so refer to guides on sprouting your specific type for a more in-depth discussion of how much light a new sprout need.
Third Seed Germination Phase: Growth into Adulthood
The third and last phase of seed germination is both the longest and the broadest. The third phase is usually defined by the baby leaves (cotyledons) falling off and the adult, or “true leaves,” continue growing.
From there, the plant is off to the races! Technically this third phase covers its growth into adulthood, including:
- Growth and strengthening of the stem and leaves
- Maturity of the reproductive system
- Flowering, pollinating, and producing more seedlings
However, that’s obviously a huge range. Think of it more like adulthood in human terms—much of the change in your life happens during your childhood and teenage years, and once you’re an adult you just continue growing. At this point, the seed germination process hands off to further processes in adult plants that are outside the scope of this discussion.
The Environment Affects the Seed
Before planting your garden, it is essential to keep in mind that seeds respond to their surroundings. Depending on the plant variety, all seeds grow to maturity with the proper temperature, water intake, oxygen, and light.
Temperatures can affect the seed’s growth:
- A seed may not germinate below its minimal temperature
- A seed may not germinate above it maximum temperature
- Improper temperature might damage or cause seed to remain dormant
As important as the proper environmental conditions, proper aeration is needed to propagate a seed’s germination because it:
- Allows optimal air exchange between embryo and soil
- Allows carbon dioxide (CO2) to be dispelled away from seed
Improperly aerated soil can cause seed to drown from overwatering.
The Three Stages of Seed Germination: Summary
We covered a lot of information today, including scientific terms and other things that may have left your head spinning. Let’s summarize the important points:
- During the first phase (Imbibition), the seed intakes water, swells, and extends its first root (radicle)
- During the second phase (Dormant) the seed digests the nutrition inside of it and sprouts
- During the third phase (Growth), the seed continues to grow exponentially into adulthood
Additionally, making sure the environmental conditions are correct is essential to properly sprouting seeds. Thankfully, this is often easier for indoor gardeners because we have much more control over our growing space!
Good luck, and happy planting!