Finely grate the radishes using a box grater or food processor, then wrap them in a clean kitchen towel and squeeze firmly to remove excess moisture—this step is crucial because radishes are very watery, and removing that liquid prevents a thin, runny jelly. While preparing the radishes, fill your canning pot with water and bring it to a boil for the water bath, then place your jars in a 200°F oven for 5-10 minutes to sterilize and warm them.

In a large, heavy-bottomed pot, combine the squeezed radishes, sugar, and water over medium heat. Stir frequently until the sugar completely dissolves, which should take about 5-7 minutes—you'll know it's ready when you don't see any granules on the spoon. Once dissolved, increase the heat to high and bring the mixture to a rolling boil, stirring occasionally to ensure even heating.

Once the mixture reaches a rolling boil, add the fruit pectin while stirring constantly to prevent clumping. Continue boiling and stir frequently, checking the temperature with a reliable candy or jelly thermometer—I find it helpful to clip the thermometer to the pot side so you can monitor without constantly lifting it. When the mixture reaches 220°F (about 1-2 minutes after adding pectin), remove the pot from heat immediately; this temperature ensures proper gel formation.

Quickly skim off any pink foam that has accumulated on the surface using a spoon or skimming tool, as this improves the clarity and appearance of the finished jelly. Stir in the prepared horseradish and lemon juice until fully incorporated—the horseradish adds a subtle peppery bite that makes this jelly unique. Working quickly while the jelly is still hot, ladle it into the warm, sterilized jars from Step 1, leaving about 1/4 inch of headspace at the top.

Wipe the jar rims clean with a damp cloth, then place the lids on the jars and screw the bands on finger-tight. Place the jars into the pot of boiling water from Step 1, ensuring they're covered by at least 1 inch of water, and process for 10 minutes. After processing, remove the jars with a jar lifter and place them on a clean kitchen towel at room temperature—allow them to cool undisturbed for 12-24 hours while the pectin sets, then check that the lids have sealed (they should be slightly concave and won't flex when pressed).