Asparagus should be harvested when it first comes up and the heads are tight. In good weather, it may need to be harvested every day. As you discovered, if you let them start to fern, they're not worth eating.

After you've finished cutting for the year, you HAVE to let them fern so the roots can grow for next season. They'll get very tall. After they've gone brown in late fall, you cut them and get rid of them if you're worried about the asparagus beetles. They can overwinter in the old stalks.

You can tell you've got them if the tops of the new spears curve like a shepherd's crook.

I have family members in cold climates who have already harvested their asparagus, much to their surprise.

How old were the crowns you planted last year (or was it seed you planted last year)?

You MUST let them fern at some point, as ltilton pointed out, so the plant can gather energy for next spring.

If your crowns aren't producing nice fat spears yet, the crowns are either too young still (let them fern and grow for another year or two) or you need to give them more nutrients (a layer of compost or whatever a soil test tells you they need).

Patience is a virtue with new asparagus beds.

Macky asks good questions. I started a new bed with Jersey Knight seedlings two autumns back, and the plants are just now producing a smattering of moderate spears. I think they need another year in full fern before I do any serious harvesting. Had I started with 2-year-old crowns, they would be farther along.

They were crowns when I planted them, not seeds. They all grew and ferned last year. This year, some of them are nice and thick, but several are thin. Could I harvest the larger ones and let the thin ones fern?
Thanks for your help, everyone!

You can do that. After the plant starts to fern, it may produce new stalks, but not so many. If all the stalks from a given crown are thin, it probably needs more time.

They're heavy feeders. Be sure to feed them as they grow.

I also planted my asparagus bed last year. I want to harvest some this year since some new studies actually show moderate second year harvest encourages better growth later. How do I determine whether a stalk is thick enough to cut though?

I read stop harvesting when your stalks are no thicker than a pencil. I have a bed that's 3 years old this year, so I am no expert. Just starting harvesting some of mine this spring.

Don't obsess about the size of the stalks. They often sprout in various thicknesses from the same crown.

If the stalks are thick enough to be worth eating then i would harvest them. there is no need to leave some. If the roots system is strong enough, then they will keep sending up shoots until you stop harvesting in a few weeks. if you let some go to fern, it will trigger a response in others and they will all get a bit woody in my experience. ltildon is right, they are very heavy feeders, its a new england tradition to fertilize them with your wood ash from the winter, but this is nowhere near enough unless you burn some serious wood during the winter, use a balanced organic fertilizer.

Once they have started to fern DON'T cut them back, especially if they are newer plants. That is how the plant grows bigger and stores energy for next year. Only cut the dead fern off in the winter.

You can pick lightly on two year old plants, just don't cut them for more than a week or two. After that let them fern out. Keep them weeded.

We had snow predicted and so I covered up all the little ones peeking out with more compost. It is to early for asparagus here.