You can wait as long as you want to harvest but if the plants are overcrowded and you don't thin them out all you get is thin roots. The plants need good spacing for the roots to develop any size.

Dave

I always harvested in spring, but then I had found in our hot weather, fall harvest was tougher roots and kinda bitter. Horseradish should be happier for you (farther north)

There was a really good discussion about horseradish culture & preparation here: Horseradish The photos are all gone now (no thanks to Photobucket >:-< ) but the thread is still very instructional.

Good point zeedman. There are many great past discussions about horseradish here that address this question. When Don asked this question last year I think the consensus then was Fall harvesting is far superior to any Spring harvesting.

Dave

https://www.gardenweb.com/gardenweb/query/harvest-horseradish/topic=cornucop