Add vinegar, water, salt, and sugar to a medium saucepan and bring to a simmer. Cook until all of the sugar and salt has dissolved.
Trim asparagus spears so they fit inside a quart jar, then tightly pack them into the jar. Add the clove of garlic, then sprinkle in the pickling spice and crushed red peppers.
Once the sugar and salt have dissolved, pour the hot brine into the glass jars, leaving about a half inch of headspace.
Wipe the rims clean, then seal the jars with canning lids and place them in the refrigerator to pickle for at least two weeks for the best flavor. Alternatively, you can process sterilized mason jars in a water bath for 10 minutes for long-term storage.
Notes
Scale this recipe up as needed depending on how many bunches of asparagus you wish to pickle.