Never made it myself so not sure how much help this will be but the site linked below is a good step-by-step instruction. Note it says the longer you let it sit/rest between grinding and adding the vinegar the hotter it will become.

From that point on it seems to be purely a "to taste" thing.

And I have read in other discussions on making it that the bitterness is due to the growing conditions - supposedly insufficient or inconsistent soil moisture levels just as that can cause bitterness in other root vegetables.

Hope it helps.

Dave

Here is a link that might be useful: How to make horseradish like Grandma

Usually horseradish is dug after the first frost and from then any time (depending upon condition of the ground) through the cooler months. Once the spring growth begins it shouldn't be dug.

For the hottest horseradish you have a window of 1-3 minutes. The longer you wait before adding vinegar the hotter the horseradish will be. Vinegar stops the release of volatile oils so at that point the heat level is fixed.

Carol

Here are some HORSERADISH THREADS mentioning the time when it can be harvested as well as what to add to it once its ground up. I dont add vinegar, and only a small dash of salt. Its done in a blender ad I harvest in the cold of winter about mid December. The soil its in is nice and loose and is right nect to my compost pile were it gets plenty of nutrients.

Here is a link that might be useful: SIMPLE 'HARVESTING HORSERADISH' THREAD