Especially when working with a small garden, growers hope to use every inch of ground efficiently. The spacings noted on seed packets may seem extreme, but nearly always turn out to be accurate. Spacing plants closer than recommended only succeeds for some types and only if special techniques are used. Some plants respond well to intensive gardening if soil fertility is high -- others need even more room in optimum growing conditions. What looks like wasted space early in the year should be filled to capacity late in the season when elbow room matters the most.
Planting small seeds, like carrot, at precisely the right interval may seem impossible. If that distance isn't respected, plants compete for light and nourishment and don't reach usable size. Either space the seeds properly or thin the row to the correct interval after plants emerge. Two carrots planted in the same spot don't add up to one carrot planted alone.
Some plants like corn, squash or melons respond well to the hill planting system. Individual plants in the hill may be only inches apart, with several feet dividing between hills. Fertilizer can be concentrated in the hill rather than the row. Double the optimum number of seeds planted to ensure a good set, but be sure to thin out the weakest plants. If directions say three plants to a hill, don't leave four. Crowding reduces the yield of all plants in the hill.
Even when seed spacing is correct, many growers yield to temptation and reduce the space between rows. Normal row spacing provides room for mature plants at the peak of growth. Without that space, leaves don't dry after dew or rain and fungal diseases flourish. Rows can become so crowded that pollinating insects don't even reach hidden blooms. Diseases set in so quickly that a mature planting vanishes in only days.
Heavy feeders like corn need plenty of growing room because of high nutritional needs and high water use. Close spacings create stunted plants and small ears. At least four rows of corn at the proper row spacing is absolutely essential for good pollination. Without efficient pollination, even full sized ears yield only a few mature kernels. Spacing affects pollination rates even when insects pollinate the blooms--closely spaced squash plants may produce only male flowers, yielding pollen but no squash.