I just checked my copy of 'Square Foot Gardening'. What it says is that the early varieties of corn reach 4-5 feet and should be spaced 8 inches apart. Midseason vaieties reach 5-7 feet, and late varieties, which should be spaced 12 inches apart, reach 7-8 feet. With this spacing, you get a smaller yield per plant, but a larger overall yield. It's standard stuff which you'll get in any book covering bed planting. My view of this is that it's bed planting packaged for people with very small spaces.