Wash and rinse canning jars and keep hot. Prepare lids according to the manufacturer's directions.
Wash asparagus gently under running water. Trim stems so the spears fit the jars with slightly less than 1/2-inch headspace. Peel and wash garlic.
Place 1 garlic clove in each jar and tightly pack asparagus with blunt ends down.
Combine water, vinegar, optional hot peppers, salt, and dill seed. Bring to a boil. Place 1 hot pepper in each jar if using.
Pour boiling pickling brine over asparagus, leaving 1/2-inch headspace. Remove air bubbles, adjust headspace, wipe rims, and apply lids.
Process 12-ounce or pint jars in a boiling-water canner for 10 minutes at 0–1,000 ft, 15 minutes at 1,001–6,000 ft, or 20 minutes above 6,000 ft.
Cool undisturbed 12–24 hours and check seals. For best flavor, allow jars to stand 3–5 days before eating.