Patty has it down right.
Make sure you leave a few pieces of root(with eyes) in the ground throughout the winter. It is very harty and will bounce back in the spring. Mine even ends up being under a foot or two of flood water, off and on, throughout the winter, and still bounces back.
When you grind her up for your raddish, make sure you have your vinegar handy. You probably already know this but, after grinding, the heat is determined by how long it sits until you stop its action with the vinegar.
When I first started I didn't know exactly what to expect. I quickly divided my grindings into three equal portions. I flooded the first portion with vinegar in about two minutes, the second portion in another two and the third portion in still another two. This resulted in Mild, Medium and Very Hot horse raddish, relating to the two, four and six minute intervals.
I also found out I was a pansy. I love the flavor, but am definitely a two minute man (No comments about that, please)