> Lettuce and Tomatoes don't keep well, which adds more seasonal variability

The more durable a produce is, the easier it is to transport over a large area, which will always smoothen price fluctuations. This alone could explain why lettuce and tomatoes have higher volatility than onions, which have higher volatility than potatoes, which have higher volatility than corn and weat.

At least that's the null hypothesis that the hypothetical effect of futures should be compared to.