Gardening and Horticulture - Asparagus harvesting

I live in upstate SC and planted Jersey asparagus four years ago.My problem, I don't know when or how long to harvest. I finally gathered about three dozen spears at the first emergence of spring, but was afraid to gather more because I was told that the crowns needed to develop. Should I wait until several canes from each crown have grown, Harvest at the first sign or what? confused. Tom

Snap off 7-9" spears with tight tips. The stem that is left will dry up and disintegrate. When harvesting, pick the plant(s) clean, and do not allow spears to fern out. This is where asparagus beetles like to lay eggs.
You do not harvest during the year of planting. After that you can harvest several times through a three week period, depending on temperature. Spears start popping up when soil reaches 50 degrees. Growth depends on air temperature. You should be able to harvest every 2-4 days. As temperature increases, you can harvest 5-7" spears once or twice a day. After two years, plants tend to produce for 6-8 weeks. Harvest season varies based on air temperature. Stop harvest when spears are smaller than about 3/8". When harvest is complete, snap all spears off to ground level. Apply a little ammonium nitrate fertilizer.

I thank you very much for the info. I want to make sure that I have one thing clear.
When the spears emerge in the spring, leave nothing, pick all of them until they start to get slender then apply the ammonium nitrate and let them grow. Tom

Pick all that are ready to be picked before the tips fern. More shoots will pop up. And, more and more until they get too skinny to fuss over.

Thank you very much twelvepole. I have been going about it all wrong. Tom