Here at Seeds from Italy, we grow thousands of plants every year that we start from seed. We grow most in our greenhouse, but we also start seeds in our basement when we don’t have enough to justify heating the greenhouse. In either case, here’s the process we use.
We buy a high-quality potting mix that contains a wetting agent but no nutrients. We use Pro-Mix, but there are several other brands that are as good. We recommend that you don’t buy the cheap stuff sold at most big box stores. We’ve done it and had plenty of problems. Go to a good local nursery and buy the best mix they sell.
We are firm believers in bottom heat for seed starting. We have a heat mat that accommodates three trays at a time. By starting seeds on the heat mat, we get germination very quickly. If you have trouble germinating seeds or have weak seedlings that die quickly, cold soil is the most likely culprit and you will really appreciate the difference a heat mat makes.
We wet the potting mix, then sprinkle the seeds on top. Nearly all vegetables need to be covered to germinate, so we lightly dust on more potting mix to about the same depth as the seed size. We moisten the soil gently by misting it, and we keep it uniformly moist after that by misting every day or more often if the soil starts to dry out. In the greenhouse, we have a misting nozzle for the hose, but in the house we just use a spray bottle. We use room temperature water to mist the seed trays.
We like to germinate our seeds in channel trays, which require very little potting mix and allow us to keep varieties separate. When we start seeds in the house, we put the channel tray into a tray without holes so we can water without worrying about dripping. In the greenhouse, we use trays with holes for quick drainage. We have clear plastic domes that fit over the trays to keep humidity around the seeds high while they are germinating. As soon as they come up, though, we take the dome off so the seedlings don’t get burned by too much heat.
After the seeds germinate, we let them grow in the channel trays until they have their first set of true leaves. Then they are ready to be moved into larger pots or cells.
We have different size pots and tray inserts for various kinds of seeds. We use 50-cell inserts, which have a large soil volume, for tomatoes and peppers. We use 72-cell trays for broccoli and cabbage, and 98-cell trays for lettuce. As a general rule of thumb, choose smaller pots for plants that you will plant outside within four weeks, larger pots for those that need 6-8 weeks before planting out.
For the growing mix, we use the same Pro Mix but we mix in worm castings, which are black, crumbly and odorless. Worm castings are an excellent source of organic nutrients for seedlings, with an NPK analysis of 1.5-0.7-1.5. They are becoming widely available at garden centers. Other fertilizers can be used; just be sure to follow the label instructions for seedlings or you may end up burning your tender young plants.
After we have mixed in the worm castings, we fill the trays to the top and water the flat until the potting mix is wet all the way through.
We remove a clump of seedlings from the channel tray and, using a pencil, tease apart the roots and separate the plants. Always handle the seedlings by the leaves, not the stem. New leaves will grow if you damage one, but new stems won’t!
Poke a hole into each cell or pot, and drop the seedling into the hole, tucking in the roots with the pencil eraser. Pot up all that you need, then gently water the seedlings to settle soil around the roots.