Sterilize your jars, lids and rings. This can be done by placing them on a tray in a warm oven (200 degrees F) for 5-10 minutes while you prepare the jelly. Or, you can place them in the water bath canner and bring the canner to a boil and allow the water to sterilize the jars as you prepare the jelly.
In a tall stockpot, combine the chopped radishes, granulated sugar and water over medium heat. Stir constantly until sugar dissolves.
Bring to a rolling boil. Add pectin and stir until dissolved.
Bring to a rolling boil for 1 minute longer (until the temp reaches 220 degrees F - the setting point for jelly).
Remove from heat and skim off foam. Stir in horseradish.
Prepare a hot water bath, and process (pint jars process for 10 minutes, but see notes below). The time starts once the water starts a hard boil. Once the jars have processed for 10 minutes, remove from the water bath; allow them to rest on the counter - jars will seal with a "pop" over the next 12-24 hours.
Or, opt to forego the hot water bath and store in the fridge for short term use.