When starting seeds this month, remember that the most important part of the plant is invisible--the roots. How the roots develop will shape everything else about the plant, and will determine your success or failure in the garden.
Once you know what kind of roots the plant has, you will know many of its needs. Like how long it should stay in the pot or flat. And once you know that, you'll know when to sow the seeds.
Here is a guide to root types, and how to treat them. We recommend heat mats or a warm surface (top of a refrigerator?) for seed germination and root development. Of course, the little plants also need plenty of light. See the links at the end for more on that.
Plant these now.
Movers have a fibrous, dense root system that is actually stimulated by transplanting. In studies at Cornell University, cabbages had much larger root systems at maturity if they had been transplanted--and those that had been transplanted twice had the largest of all! These slower-growing, transplant-loving crops can stay in the pot for about a month, then can be transplanted outside or to bigger pots. While transplanting doesn't bother them, crowding does. Move them if needed to maintain adequate spacing. Most are fairly heavy feeders, needing fertility to maintain growth. Prime Movers: tomatoes, peppers, eggplant. (Plant outside in May.) Cabbage, broccoli, cauliflower, collards, kale. (Transplant outside in April.)
Direct-sow these, or start just 2 weeks early, and transplant carefully.Divers make a few, large, succulent roots that are brittle, like a good carrot or a bean sprout. They break when a cabbage root would bend. Divers do not like transplanting, and should be either direct-sown, or transplanted when quite young, before their roots get too big. On the other hand, they are not so worried by crowding. You can shoehorn them in among earlier vegetables or a cover crop. They find nutrients on their own, and do not need such heavy inputs as the rest. Popular divers: beans, beets, carrots, chard, cilantro, dill, fennel, peas, parsnips, poppies, and radishes.
Plant every few weeks as soon as the ground can be worked. Keep in pots only 2-3 weeks.Sprinters are the juicy, leafy, fast-growing plants that mature very quickly. They have been bred for crisp juicy leaves, mild flavor, and fast growth. They can be sown in place or transplanted once--but then they need to get down to business and finish up. You wont get a second chance with these--give them the water and fertility they need at planting time. They bolt quickly when mature, or if they get pot-bound, so don't leave them in pots more than a month. Classic sprinters: Asian greens, lettuce, spinach, and mustard.
Start these in mid to late April. Or direct-sow after frost.Sprawlers are large plants with far-ranging, but fragile, root systems. Give them a sunny position after your last frost date, even if it means cutting an opening in your winter cover crop, or planting the young starts among your spring peas and lettuce. They love organic matter, and want a lot of it. The edge of a compost pile, or the area where one was, is a favorite situation for them. Some sprawlers: sunflowers, okra, corn summer and winter squash, melons, cucumbers, pumpkins, watermelons, and gourds.
More on seed starting: https://www.bountifulgardens.org/articles/7
A quick selection of seeds to start indoors now: https://www.bountifulgardens.org/departments/295
Seeds to start outdoors now (or soon, in very cold places): https://www.bountifulgardens.org/departments/288
The 5 gardening seasons: https://www.bountifulgardens.org/articles/40
This work by Bountiful Gardens is licensed under a Creative Commons Attribution-NoDerivatives 4.0 International License.