Lettuce and Tomatoes don't keep well, which adds more seasonal variability and reduces the ability of a futures market to smooth prices.
Compare with potatoes, which keep about as well an onions, and are farmed in the same areas. Look at onions and potatoes over a 5-year time span (because the default all-data time span is silly for this). It doesn't give enough control of the Y axis to make the comparison easy - for potatoes it shows me Y=40-320, for a range of 280; for onions it shows me Y=120-440, for a range of 320. This means that potatoes are more "zoomed in" and it's graph will exaggerate volatility relative to onions, but qualitatively, I'd say the potato graph looks smother just the same. This is exactly what economic theory says a futures market should do to the price.
Compare with potatoes, which keep about as well an onions, and are farmed in the same areas. Look at onions and potatoes over a 5-year time span (because the default all-data time span is silly for this). It doesn't give enough control of the Y axis to make the comparison easy - for potatoes it shows me Y=40-320, for a range of 280; for onions it shows me Y=120-440, for a range of 320. This means that potatoes are more "zoomed in" and it's graph will exaggerate volatility relative to onions, but qualitatively, I'd say the potato graph looks smother just the same. This is exactly what economic theory says a futures market should do to the price.