Tomato transplants may be tender, but there's no need to baby your broccoli. The big danger with broccoli is "buttoning." That's when stressed plants start to make a premature crop, producing tiny heads no bigger than a button.
It's well known that exposure to extreme cold during the seedling stage predisposes plants to buttoning. But do other sources of stress on the seedlings, such as being potbound and leggy, affect the quality of the crop?
Not a great deal, according to W. J. Lamont at North Carolina State University. He tested three varieties in three pot sizes, in a spring and fall planting. Both times the results were about the same. For a fall planting he seeded some plants on January 15 and others on August 4, to be planted outdoors on August 20. For a spring planting he seeded on September 4 and January 29, to be set out on March 14. The plants were watered regularly and fertilized once a week.
When Lamont transplanted, the old broccoli looked like what you would expect for transplants that had been held over for more than six months! They were tall and leggy with just a knot of leaves at the top and a clogged, contorted root system. But by harvest time he couldn't tell the plants apart unless he studied the stems near the ground. Given the same soil fertility, regular watering and black plastic mulch as the "ideal" transplants, the old plants produced nice big heads, indistinguishable from the others.