In most vegetable gardens, plant spacing is determined by what sort of things you plan to grow and how many plants of each variety you plan to put in. Conventional gardening wisdom advises that vegetable plants need ample space around them for air to circulate, and also to avoid crowded roots, and that’s true up to a point. The planting info on the seed pack or plant tag is a good general source of advice on spacing individual varieties. But there are other factors to consider as well.
One year, I goofed when I planted a row of brussels sprouts down the middle of one of my raised beds. What appeared to be ample space around them to grow other things vanished by midsummer. Their massive size—and the shade they produced—prevented me from sowing secondary crops of lettuce, radishes, and other shorter vegetables near them, and probably hampered the yield from a nearby patch of beets as well. They needed a lot more space, because of what I wanted to plant around them.
It’s not always your own crops you need to be concerned about. If you have a plot in a community garden, it may be wise to chat with your immediate neighbors about what they plan to put in on the edges between your plots before you plant. Last year, a friend of mine who’s in a community garden was dismayed to see a stand of corn on the edge of her neighbor’s plot quickly grow up to cast long shadows over her plants.
Another thing to think about is the plant’s habit, or how it grows. With plants that grow in a vertical manner it’s easy to estimate planting distances. Vining plants that sprawl like melons, squash, cucumbers, and tomatoes can be more challenging, even if you have a lot of space. Left to run, most will quickly grow into and over one another, making it difficult to harvest the produce. One solution may be to train them up a fence or trellis, which can be efficiently done with all but the heaviest vegetables—thus enabling closer planting.
When it comes to tomatoes, which except for the container varieties have vines that not only sprawl but are also incredibly dense, I believe training them on a support of some kind is essential to managing their growth in a limited garden space. If they are allowed to fall on the ground the vines will quickly root along their length, and once that happens it’s nearly impossible tie them back up and they’ll quickly become an impenetrable tangle. Check out our post on Tips for Growing Tomatoes.
Finally, if you plan to grow heirloom vegetables and intend to save the seeds from the harvested produce to grow the same type next year, you may want to consider increased planting distance as a way to insure that different varieties of the same species don’t cross-pollinate. If they do, you’ll end up with plants the next year that aren’t true to type. However, in most home gardens that might prove to be difficult, if not impossible, as the distances required can be great. For example, tomatoes are said to need 30 feet between other tomato varieties. On the other hand, cross-pollination is how we get wonderful new heirloom types, so if that happens in your garden perhaps you’ll end up growing something new and remarkable.