Companion+planting+2
Garden vegetables can be grouped by their root structure and their tolerance for transplanting.  This system tells you which crops like to be transplanted; which don't want to be disturbed; and which can wait the longest to get to their final position in the garden. There are four major groups. Below you'll find simple strategies for sowing and transplanting each group.

Root type is important when choosing companion plants as well. If plants are going to be growing in close quarters, they will get in one another's way much less if they have different types of roots. For example, carrots or parsnips will burrow down into levels below a lettuce plant's matlike root system.


Movers

These are mostly the traditional European brassicas--cabbage, Brussels sprouts, kale, broccoli,  cauliflowers, and so on. They have been selected over the centuries for transplant into the garden. Tomatoes, and to some extent peppers, are  summer crops that can also be handled in this way. They grow fairly slowly, (which makes cabbages more frost-hardy and gives tomatoes the framework to support fruit).
All 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 flat or pot for about a month, then can be transpanted to a nursery bed (or in the case of tomatoes, ever-larger pots) until space opens for their permanent homes. While transplanting doesn't bother them, crowding does. So don't hesitate to move them as often as need be to maintain adequate spacing.  Just make sure that they get plenty of compost and dependable moisture. A mulch can be helpful as well.


Divers

What sets these apart is their roots--most are tap-rooted and send down one main root fast and deep. Root crops like carrots and parsnips are divers, but also some leaf or stem crops like Fennel and Chard, and some summer crops like beans and corn. Even when they have more than just one taproot, divers tend to have a few, large, succulent roots rather than a mat of small flexible ones.  These succulent roots 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. In the garden, that means you can direct-sow them while other crops are still in place. In spring, you may be able to sow them into openings in your winter cover crop. An example would be sowing beets under the shade of caged tomato vines, or chard under spring peas. Most legumes, like peas, favas, and lentils, should be treated as tap-rooted divers.


Sprinters

These are the juicy, leafy, fast-growing plants that mature very quickly--Asian greens, Lettuce, Spinach, and Mustard. They have been selected over the centuries 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. They bolt quickly once they are mature.
Traditionally, the farmer sows and harvests multiple quick crops a year, which are  cut and used right away (or pickled as in Kim-Chee).  For example, heads of lettuce  or bok-choy that are cut and refrigerated properly will often stay in good tasty condition longer than similar heads that are left in the ground.

Sprawlers

These are the cucurbits--summer and winter squash, melons, cucumbers, pumpkins, watermelons, and gourds. They are vines with a need for a far-ranging root system. (Bush squashes are a mutation of the original type; they are a less vigorous vine, not a true bush.) While they are not tap-rooted, they do need a large, fast-growing root system, and their roots tend to be somewhat fragile.
They can be transplanted (carefully) once when small, but have traditionally been direct-sown. All of them love organic matter and can use vast amounts of fertility if it is available. So they are best given a final, sunny position early, even if it means cutting an opening in your winter cover crop, or planting the young starts among your spring peas and lettuce.
I like to plant cucurbits by hoeing openings in a bed of winter vetch, which enriches the soil.  The vetch can be cut, stacked to one side to dry, and later used as a nutient-rich, weed-smothering mulch for the squash.



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