The wonder of seeds never ceases to amaze. A white pine seed is quite small but carries the embryo and genetic code for an entire tree. If you've planted snapdragons, you know how tiny a seed can be. 

We talk in gardening circles about how to germinate seeds, but what we are really doing is setting the conditions for seeds to germinate. When seeds requirements are met, they’ll perform the act with no further instruction from us. 

In Nature, no intervention is needed, but germination rates can be poor or excellent, depending on environmental conditions. Plants overcome this issue by producing a great abundance of seeds. Thousands of seeds may be needed to get a few seedlings established. For gardeners, such a poor success rate would be untenable, so we strive for the perfect conditions, including careful watering and care once germination starts. 

Seeds are stout, hardy, resilient structures and may stay dormant but viable for years or even decades, depending on storage conditions and the species. However, once they begin the germination process, they are fragile and easily killed. 

Requirements for Germination

Seeds need the proper warmth (temperature), moisture, oxygen, and sometimes light to germinate. While these values are ranges and not specific levels, there is always a sweet spot. It's the reason we sometimes use heat mats and humidity domes. 

When a seed begins to germinate, its metabolic processes are activated, resulting in embryo growth and, eventually, a new seedling emerging. Most of the activity is concealed from our eyes until the new green shoots pop up and we celebrate, but a lot is going on before that event.

Temperature

Of the three requirements for seeds to germinate, temperature or warmth is the one which serves as the alarm clock. Temperature requirements come from the need for activation of enzymes which serve as a catalyst for chemical reactions. Too cold, and the enzyme processes are slow. Too hot, and the enzymes may be destroyed. Like Goldilocks, the enzymes need a temperature in the middle to do their thing efficiently. That's why a seed may germinate at a cool temperature but take longer. In warmer temperatures, that same seed may germinate several days or even weeks faster. 

Seeds don't all need the same temperature to begin the process. Some plants are adapted to cooler temperatures than others. Lettuce and spinach will germinate in cool temps, but corn or melons may stay dormant and succumb to rot before they ever sprout in cold soil. A beet seed will germinate in soil as cool as 40℉ but will sprout faster at 50-85℉. Lettuce seeds will germinate at 35℉ but are quickest around 75℉. 

Oxygen

We often think of plants as an oxygen supplier, which is true, but they also use oxygen for respiration. Without oxygen, a seed can't germinate. Seeds, like plants, use oxygen to carry out cellular respiration. Oxygen is required to burn carbohydrates and use the energy stored within to begin growing. Until the roots and leaves are established, all energy for growth must come from that stored in the seed.

The need for gas exchange is part of the reason a seed starting mix or soil bed should be loose and aerated. With no exchange of gases, as would happen in compacted or saturated soil, those processes are slowed or nonexistent. 

Moisture

Moisture is a requirement for seeds to germinate and is the kickoff for the whole process. We can see this plainly because our garden seeds don't germinate in their package, even when the temperature is right. When exposed to moisture, seed coats take up water in a process called imbibition. It's a fancy word for a liquid being absorbed by a solid and expanding in volume. If you've soaked a pea seed, you've seen the seed seem to expand and swell–that’s imbibition. 

The amount of water available for imbibition affects germination. If there is no available water, the seeds can't absorb it and start the process. 

It's always good practice to water in freshly planted seeds unless you’re starting seeds indoors in a premoistened medium. Tuck seeds into bone-dry soil, and you'll be waiting longer for them to sprout. But don’t overdo it and saturate the soil.

What about soil and light?

At first guess, we might assume seeds need soil for germination. After all, that's where we put them when we want them to sprout. But soil is not needed. In fact, if you've conducted a germination test on seeds by sprouting them in a damp paper towel, you've proved no soil is required. 

Most seeds are perfectly able to germinate in the dark. Even a half inch down into the soil, there isn't much light getting through. However, a few, like coleus and petunias, need light to sprout and do well when sown on the surface. For those plants, either press the seeds into the soil for good contact, or cover them with a light dusting of seed starting mix and mist them for moisture. Of course, all seedlings need light after germination. 

How Seed Germination Happens

A seed is a tiny enclosed packet of energy waiting for the right time. If you dissected the seed, you’d see several distinct parts. Inside the seed is a plumule, which will become the first shoot, and a radicle, which will become the first root. Also stuffed inside are the cotyledons, which we often call seed leaves. The cotyledons, sometimes called the endosperm, are not only seed leaves, but food storage for the embryonic plant. As they emerge from the soil, cotyledons become green, able to carry out photosynthesis until the first true leaves arrive.

For seeds to germinate, the protective seed coating must be altered so water and oxygen can get through. Depending on the species, this could be from moisture, abrasion, or even the digestive juices of a bird or animal. For example, sweet pea seeds (Lathyrus odoratus) are hard as rocks, and germination is greatly improved by scarifying the outer seed coating, soaking, or both. Morning glory seeds are another example, needing soaking or nicking to germinate reliably. At the other end, tomato seeds are quite willing to germinate without special treatment, given a few days of moisture and warm temperatures. 

Once the seed coating has been breached, moisture and oxygen start the biological process. Enzymes catalyze chemical reactions and carbohydrates stored in the cotyledons are burned (cellular respiration) to provide the needed energy to begin cellular division and growth. The embryonic cells begin to enlarge, the seed coating ruptures and the radicle emerges.

The radicle, which becomes the new root system, is often the first part of the new plant to emerge from the cell. Watching a time-lapse video of a bean seed germinating is an excellent way to see the exterior processes unfold. The radicle grows downward due to gravitropism (plant root tips can sense gravity and which way is up or down) long before the new shoot emerges above the soil. 

Next, the plumule emerges from the soil, uncurling and growing upward, breaching the soil surface and eventually unfurling the cotyledons. The new shoot elongates and begins forming the first true leaves. Voila, we have a seedling! 

Some seeds germinate quickly, while others take practically forever, but the processes are the same. An understanding of the germination process can help you troubleshoot problems when starting seeds. Pay close attention to moisture and temperature (oxygen usually isn’t a problem unless the soil is saturated) and you’ll have better success.