Harvest when they're the size you'd like to eat. No need to wait until they're 10 or 12" tall. If the buds open the flavor isn't the same. It's just remembering to look on those busy days that's important. Don't let any go to fern (except the newly planted that need another year) until you're done harvesting for the season. Those ferns grow wide and tall very fast and could prevent you from seeing good spears to enjoy.
We harvest here until the end of June and extend that for emergency asparagus needs knowing time is short. You just don't want to deplete the plants energy by cutting spears into the heat of summer. At the end of the year, after you cut back the ferns as low as you can get, add compost and mulch. They will love you for it.
Even your first few trips to your asparagus beds may not yield many spears but cut them up into the size pieces you'd like and add them to a salad. They are so good. Do try one raw!
