For the best success with seeds, we need to understand them and how they work. Seeds come in as many different packages as the plants that produce them. They may be enormous and clunky or tiny like specks of dust. Some have extremely hard protective coatings, and others are soft and oily. The common link among all plant seeds is that they are dormant embryos that will, in the right conditions, germinate and sprout one or two tiny leaves and the beginning of a root. If favorable conditions persist, these seedlings will begin to take moisture and nutrients from the soil and energy from the sun, and grow into mature plants.
Even the most novice gardener can have success with seeds by understanding how to provide those favorable conditions for seeds to grow in. Seeds vary from plant to plant, and may have different requirements for germination. But, generally speaking all seeds need three things in order to sprout: Water, Oxygen, and Warmth.
Water: Moisture softens the hard outer shell of seeds. Some seeds don’t need much water to germinate, but others absorb a large amount of water relative to their dry weight. Seeds absorb water by a process called imbibition. Once water has been imbibed, enzymes inside the seed are activated, which enable the plant embryo to use its stored food supply. Seeds contain just enough of that stored food to produce that root, and unfurl that first leaf or leaf pair. Without water, seeds cannot germinate. With too much water, they may drown, or you may encourage the growth of mould or fungus, which can both harm seedlings. Moderation is key.
Oxygen: This gas is required for the respiration of the plant. Yes, seeds and seedlings (and all mature plants) actually breathe, and exchange gases as part of their metabolism. This gas exchange is an essential part of photosynthesis, and allows plants to draw energy from sunlight. Oxygen is present in soil in the tiny spaces between soil particles. If too much water is present, or if the seed is planted too deeply, it will be oxygen starved and unable to germinate. Typically, in wet conditions, the seed will just rot. It is for this reason that seed starting mixes are designed to retain moisture, but drain off the excess. This is also why it’s vital that all containers, including seedling trays, have drainage holes on the bottom.
Warmth: The temperature of the soil directly impacts the metabolic rate of the seeds and seedlings. Most seed types have a specific temperature range that is ideal for their germination. In some cases, this is just above comfortable room temperature (16-24°C/60-75°F), and in others it is warmer still (24-32°C/76-90°F). We can control the temperature of the soil by using seedling heat mats, or by placing our seed trays someplace reliably warm. This used to be the on top of the refrigerator before fridges became energy efficient. If you have no way to heat the soil in your trays, or if you need to direct sow your seeds into cool soil, the seeds will just take longer to sprout.
Germination in some kinds of seeds also involves light. They may require being started in a dark cupboard, or they may need to be sown on the surface of the soil and exposed to bright light. If either of these are the case, your seed packet instructions will say so.
The dormancy of some seed types will not be broken, even in favorable conditions, unless they undergo vernalization. This is a process used to simulate the seasonal changes and winter conditions the seeds might experience if they fell off their parent in the fall, and sat in place until spring. Vernalization typically consists of planting seeds in moist soil in a tray or container, and then putting the whole tray inside a sealed plastic bag in the refrigerator for 2 weeks or longer. The tray is then brought out into a warm place, and the seeds germinate as usual.
Timing: Another distinguishing factor among seeds is the rate at which they germinate—and the “days to maturity,” or the time it takes between germination and harvest time. Vegetable gardeners have a limited growing season: The soil and air have to be warm enough, the sunlight has to be strong enough, and the days have to be long enough. Traditionally we have marked the extreme beginning and end of the growing season with the last average frost date in spring, and the first average frost date in the fall.
Depending on the seed type, if you can provide the moisture, oxygen, and warmth it needs, you can predict (within reason) the time it will take to sprout and then mature. Working backward from your first frost date, you can calculate how early a given seed needs to be planted. Below are two very different examples.
Lettuce has an optimal soil temperature for germination of 10-22°C (50-72°F), and lettuce seeds take 7 to 10 days to germinate. This year you have chosen Cardinale as your main lettuce variety, and it matures in 60 days. If the first average frost date in your growing area is November 2nd, you need to allow a minimum of 70 days combined germination and growing time to produce a mature head of Cardinale lettuce. You need to plant this variety on or before August 24th for this to work. There are, of course, ways to accelerate growth and extend the growing season, but for the sake of argument, the principle of timing is important.
Habanero pepper seeds have an optimal soil temperature for germination of 25-29°C (78-85°F), so you will need to use a heating mat if you want the seeds to germinate in 10-21 days. Then the plants grow slowly, taking a full 120 days before they are mature enough to produce their spicy fruits. In this extreme example, it makes no sense to sow Habanero pepper seeds less than 141 days before you want to harvest, and peppers produce the most fruit when the weather is hot. Working backwards from a harvest date of August 20 (when hot days are on the decline), the latest possible date to sow the seeds is April 2nd.
So think about sowing seeds as marking a place in space as well as a point in time. With fast maturing crops like salad greens, it may make more sense to sow short rows every couple of weeks so that they mature over a longer period—rather than sowing one long row and ending up with a whole lot of mature salad greens all at once.
