Your grandfather was happy if his corn was knee-high by the Fourth of July. He probably didn't plant it until late May. It became a catch phrase, and for decades farmers figured they would have a good crop if it was knee-high by the Fourth of July.
That phrase is so antiquated now that most people talk about 'tasseling' by the Fourth of July. In many years in the past two decades that would describe many fields. A much earlier preferred planting window and hybrids that reach maturity a bit quicker while still yielding well account for the difference.
This year many fields were past knee-high by the Fourth of July, or will be, but only a few around the Midwest will be tasseling. Planting was delayed across the Corn Belt enough that 'waist-high' by the Fourth of July is what many people are hoping for this time.
Some areas have had so much rain, especially in Iowa and in pockets of Indiana, that small corn may still be under water by the Fourth of July, or there may be spots where the crop didn't make it. In some cases there may be no corn because rather than plant so late the farmer opted for prevented planting of his corn acreage.
The corn that is waist-high or taller should be OK, says Dave Nanda, long-time plant breeder and director of genetics and technology for Seed Consultants, Inc. With normal weather patterns from here on out, the crop should mature and should also have the opportunity to reach average to above average yields.
One factor favoring corn planted in late May or later is that it requires about 200 fewer heat units to reach maturity than the same hybrid if it had been planted in later April or very early May. Bob Nielsen at Purdue and Peter Tomlinson at Ohio State University documented this phenomenon several years ago.
There may be some limit on yield potential that goes along with it, however.