When I was growing up, we were taught that if the corn crop was knee-high by the Fourth of July, then the crop would turn out pretty well.
Over the past decade or so, Ms. Eclectic and I have been keeping tabs on the corn crop in our area, and pretty much every year, the corn has been noticeably taller than knee-high by the Fourth of July.
This year has been different though. There were a few fields where the corn might have been that tall, but for the most part the corn in this area was not knee-high by the Fourth of July. We have had a colder-than-usual spring and summer so far. The strawberry crop in our area is smaller than usual, and Ms. Eclectic's tomato plants have been very slow to flower. This is in sharp contrast to my dad's telling us that on hot summer nights in Missouri, he could hear the corn growing --- we haven't had any hot summer nights in our area yet (nightly lows in the range of 10 to 15C, where C stands for Canadian and F stands for Foreign).
If this colder summer persists, as is forecast, and if it is widespread, then I might recommend buying corn futures. But I expect others more knowledgeable and smarter than I have already taken the unusual weather into consideration and have driven the price of corn futures to an appropriate level, whatever that might be.
Update: Phil Miller says the corn crop around Mankato was well over knee-high by the Fourth of July despite the cooler weather.