After a few decades in the garden, I've noticed that roots are the key to many of the plant's needs and much of its behavior. While plant family and temperature preferences both are important, I find that root type tells a lot about when and how to plant. The photo shows a planting combination that works well because the squash and the lettuce have different kinds of roots and don't compete much in the soil.
Thinking in terms of root type has hugely reduced the time spent in decision-making, taking care of potted starts, and transplant. In my experience, there are four types:
Tomatoes, peppers, eggplant. (Plant outside after frost) Cabbage, broccoli, cauliflower, collards, kale. (Plant outside when trees begin to leaf)
Start these late winter/early spring, in pots or flats indoors. Movers have a fibrous, dense root system that is stimulated by transplanting. In studies at Cornell, the cabbages with the largest root systems at maturity had been transplanted not once but twice! I start tomatoes, peppers, etc. in March, transplant into larger pots in April, then into the garden in May.
While transplanting doesn't bother them, crowding does. Move them if needed to give them adequate spacing. Most are heavy feeders, needing fertility to maintain growth.
Beets, carrots, chard, cilantro, dill, fennel, peas, beans, parsnips, poppies, and radishes.
I direct-sow these. Or you can start in pots 2 weeks before expected transplant, and transplant extra-carefully (during cool moist weather or in the evening.) Divers make a few, large, succulent roots that are brittle, like a carrot or a bean sprout. They break when a tomato root would bend. Transplanting can send them into shock.
On the other hand, they are not worried by crowding. You can shoehorn them in among earlier crops. They find nutrients on their own, and do not need as much fertility as the rest.
Lettuce, spinach, Asian greens, and mustard.
Keep in pots only 2-3 weeks. Plant every few weeks starting as soon as the ground can be worked. Sprinters 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 won't get a second chance with these. Give them the water and fertility they need at planting time. They bolt quickly when under stress—such as getting pot-bound--so don't leave them in pots after they have about 6 pairs of leaves. And never buy old transplants.
Sunflowers, okra, corn, squash, melons, cucumbers, pumpkins, gourds.
Start these 2-3 weeks before last frost. For me that is in late April. Or direct-sow after frost, which has become my preferred method. Sprawlers are large plants with far-ranging, but fragile, root systems. Give them a sunny position after your last frost date. You can cut an opening in your winter cover crop, or plant the young starts among your spring peas and lettuce, but don't keep them waiting in the pot!
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.
Jamie Chevalier lives and gardens on a river in the Coast Range of Northern California. She has gardened professionally in Alaska and California, as well as living in a remote cabin, commercial fishing, and working with seeds. She is the proprietor of Quail Seeds.