Freezing is the best way to preserve asparagus, much better than the mushy product produced by canning this choice vegetable.
For the best result, use locally grown asparagus when it is in season in your area. Out of season asparagus that has been imported from other countries tends to be less tender and have a bland flavor. Choose spears that are at least as thick as a pencil - thicker spears hold up better in the freezer.
You need to blanch asparagus before freezing it. This step prevents discoloration and preserves a better texture.
You can blanch asparagus in boiling water or by steaming it. Have a large bowl of ice cold water ready to transfer the vegetables to when they are done blanching (this stops them from continuing to cook from residual heat).
Before blanching your asparagus, hold each spear at either end bend it until it snaps. Compost or discard the tough bottom ends (or peel and save to make asparagus soup). Leave the tip ends as spears, or chop into 1 to 2 inch pieces.
Boiling Water Method
Bring a large pot of water to a boil. Add up to 1 pound of asparagus at a time to the boiling water. Let the asparagus blanch for 2 - 5 minutes depending on the thickness of the spears.
When the time is up, immediately drain the blanched asparagus and transfer it to the cold water. Leave it in the cold water for the same amount of time that you left it in the boiling water. Drain the asparagus in a colander.
Bring the water under your steaming basket to a boil. Place the asparagus in the basket and cover. Steam for 3 - 6 minutes depending on the thickness of the spears.
When the time is up, immediately transfer the blanched asparagus spears or pieces to the cold water. Leave them in the cold water for the same amount of time that you steamed them. Drain well in a colander.
2 Ways to Freeze Blanched Asparagus
1. The simplest way is to place the blanched, chilled and drained asparagus spears or pieces in freezer bags or containers, label with the date and freeze.
2. If you want the asparagus spears or pieces to remain loose and not stick together (so that you can take out just what you need), flash freeze them first. To flash freeze, spread the blanched asparagus in a single layer on baking sheets. Freeze for 1 to 2 hours, then transfer to freezer bags or containers.
Do not thaw frozen asparagus before cooking it.
Blanched asparagus will keep in the freezer for 1 year. It is still safe to eat after that, but the quality will start to decline.