The plants are at least 5 years old and have no heat at all.
Why Isn’t My Horseradish Hot?
Certified GKH Gardening Expert
Most often, this is a problem with the processing rather than the plant itself. First, in order for horseradish to have heat, the chemicals in different kinds of cells in the root have to mix. If you just cut off a small piece and eat it, the root will have little heat. But, if you take that same root, grind or chop it fine, you damage the cell walls and this releases the different chemicals in different cells and then this creates the hotness associated with horseradish.
Another common mistake in processing is that people often use vinegar to preserve the horseradish after it is ground/chopped. The vinegar stops the chemical process that makes the horseradish hot. The chemical process that makes the horseradish hot develops over time. The longer you leave it without vinegar, the hotter it gets, but if you add it early on, you will only get mild horseradish.
If neither of these are true for you, then likely you are being too nice to your horseradish. Herbs produce there flavor compounds in response to stress. All herbs, horseradish included, will be less flavorful if they get too much fertilizer and water. The more comfortable an herb is, the less flavor it will have. Do not fertilize your horseradish and only supply water in times of drought.