Clean 7 pint or 12-oz jars, along with the lids and rings well with soap and hot water. Keep the clean jars warm by placing into warm canner water, filled with hottest tap water in the sink. **See notes below for refrigerator pickling.**
Prepare asparagus: Put one spear in a jar and cut it to 1/2" below the jar top, then use that as a measure for cutting the remainder of the asparagus. Have all the asparagus cut before proceeding.
Fill a water-bath canner 1/2 to 3/4 full of water and bring to a low boil.
Add water, vinegar, canning salt and sugar to a large, non-reactive pot. Stir well and bring to a slow boil.
Pack the jars: place a garlic clove and the optional peppercorns and red pepper flakes in the bottom of each jar and fill with asparagus spears point-side down. Pack them in tight, as they'll shrink when heated.
Fill the jars, one at a time, with the hot vinegar mixture, leaving a 1/2-inch headspace. Remove air bubbles with a non-metal spatula, replace any brine as needed to keep headspace, and wipe rim with a damp towel.
Attach lid and ring, tightening to just fingertip tight.
Lower each jar as you fill it into the canner (or set them on a rack resting on the canner) using a jar lifter. Continue filling each jar and placing them in the canner until done.
Bring the canner to a rolling boil over high heat. Set a timer for 10 minutes and adjust the heat so the canner continues at a soft boil.
When the timer goes off, turn off the burner, remove the lid, and let jars sit for 5 minutes (USDA recommendation). Use the jar lifter to remove each jar and set on a towel-lined surface as gently as possible.
Leave to sit undisturbed for 24 hours. Check lids and seals, store any unsealed jars in the fridge and store the rest, without the rings, in a cool, dark, place. The jars are best used within a year to 18 months (but we've eaten 2-year-old jars and they've been fine).
Notes
To Make Refrigerator Pickles (no canning needed):
Clean jars, but don't worry about keeping them warm, and fill with vegetables, spices and vinegar mixture as outlined.
You can fill the brine all the way to the top, since headspace doesn't matter.
Attach lids as outlined, let sit on the counter for a bit until cooled and place in the refrigerator for storage. They will last 6 months to a year.
High Altitude Adjustment: If processing at an altitude of higher than 1000 feet above sea level you'll need to adjust the processing time according to this chart.