Prepare the jars: In each jar, add ½ tsp dill seeds, ½ tsp red pepper flakes, 2 sprigs of fresh dill, and 1 clove of garlic. Clean and trim the asparagus to fit in the jars, leaving ½-inch of headspace.
Prepare the brine: In a saucepan, combine the vinegar, water, salt, and sugar. Bring to a simmer and stir until the salt and sugar are dissolved, 2 minutes.
1 ½ cups distilled white vinegar (5%), 1 ½ cups water, 1 tbsp kosher salt, 2 tsp granulated sugar
Add the brine and store: Once the jars are filled with spices and asparagus, add the prepared hot brine to cover the asparagus. Cool to room temperature and top with a tight-fitting lid. Store in the refrigerator for at least 24 hours or up to one month.