Speculation

There are no onion futures but volatility remains high.

The onion conundrum: no futures market, high volatility - Jun. 27, 2008:

"And yet even with no traders to blame, the volatility in onion prices makes the swings in oil and corn look tame, reinforcing academics' belief that futures trading diminishes extreme price swings. Since 2006, oil prices have risen 100%, and corn is up 300%. But onion prices soared 400% between October 2006 and April 2007, when weather reduced crops, according to the U.S. Department of Agriculture, only to crash 96% by March 2008 on overproduction and then rebound 300% by this past April"

Thanks to Marginal Revolution.