There are two schools of thought on how to best fill containers for starting seeds.
One idea is to fill the flat with dry soil mix and then moisten it by watering it from the top with a sprinkling can; or you can set the flat into a pan of water and let it wick up into the mix. Both ways are fine although watering from the top is often quicker if you need to get it moist in a hurry for some reason. The overhead water also packs down the soil mix slightly making it somewhat denser.
The second idea is to moisten the soil mix before placing it into the pot. This is done by adding water to the mix either directly into it’s original bag or by moistening only part of it in a bucket or tub. It’s easy to over-do the watering and end up with a water-logged soil mix which then has to be allowed to drain before using.
If you are using starting mix that you have moistened ahead of time, scoop up some mix and place it into the flat or pot and level it off. Then lightly bump the tray on the table top once or twice to settle in the mix. Resist the urge to pack the soil into the container with your fingers, that will reduce the needed air space in the soil. Remember, it’s not like making sand castles!
Different seeds have different germination requirements. Some need a cold period, some need to be soaked, some need to be treated with growth hormones, some even need to pass through the digestive tract of an animal! We won’t worry about any of those types of seeds in this discussion, for now we’ll stick to the most common requirements.
The most important thing you need to know is whether or not the seed requires light to germinate. Generally speaking, the larger seeds can be covered while the very tiny seeds need to be sown on top of the mix. Check the seed packet to be sure.
Seeds that need to be covered should be placed about 2 or 3 times their diameter below the soil. While small surface sown seeds should be lightly pressed into the top of the mix so the seed makes good contact with the soil.
You can choose to sow your seeds into rows in the container, in which case you will need to transplant them later. Or you can sow two or three seeds per cell in your flat (or pot). Later you will save the strongest seedling and discard the others.
As a guide, sow large seeds about an inch apart; medium seeds about 1/2″ to 3/8″ apart; and tiny seeds about 1/4″ apart.
Since germinating seeds need high humidity, cover your container with clear plastic of some sort. Be sure to keep the plastic propped up off of the surface of the soil. For greenhouse flats, clear plastic “domes” are available.
Put your newly planted container in a warm spot to hasten germination. Placing them on a seed starting heat mat is ideal. These electric mats usually come with a built in thermostat to keep your tray at the ideal temperature for germinating most garden seeds.
Some seeds will emerge from the soil within a day or two while others take longer. After they are up, they need to be cared for, we’ll discuss how to do that and how to avoid potential problems in part six of our series.