How to Make It
Combine vinegar, water, sugar, and salt in a medium saucepan. Bring to a boil over medium-high, stirring occasionally, until sugar is dissolved.
Place kohlrabi, apple, cucumber, garlic, dill sprigs, pickling spice, black pepper, and crushed red pepper in a large bowl or 8-cup measuring cup. Pour hot vinegar mixture over kohlrabi mixture. Let stand and cool to room temperature, about 1 hour.
Transfer mixture evenly into 2 (1-quart) glass jars, covering kohlrabi mixture with vinegar mixture. Seal each jar with lid, and completely cool in refrigerator, about 4 hours. Store in refrigerator up to 2 weeks.