Cut them back after they turn brown. You let them grow because they are feeding the root crown by photosynthesizing. With healthy crowns they should get about 5 feet tall (or more!) Shorter ferns in fall may indicate that they need a fertility boost, but starting to fern while still short in spring is nothing to fret about.

Yep just as Bill said. Exactly when they will turn brown - fall or winter - depends on where you live gardening zone wise. For me that is usually early winter. Then mulch the bed well after cutting them and feed it well come early spring before the new shoots appear. Be sure to keep it well watered this summer and fall.

Dave

Cut the ferns off near the soil(even if you have to pull the mulch back to do it),when they turn brown.
Do not wait till Spring, mulch the bed at this time.
The cutting the stalk close to the ground, is to stop the Asparagus Beetle(Criocerus aparagi) from finding the old stalk & laying her eggs on it. The eggs hatch & they go to an all night & day lunch.You should burn old ferns if you have a problem with Asparagus beetle, if not you can compost it.The beetle can over winter in the mulch.

Here is a link that might be useful: This may help.

Thanks everyone!