Asparagus... I'd only go as deep as you can reach in so you can maximize your growing and minimize the weeding - not like you'll be able to really rototill well once you get them planted, you know? My asparagus patch is 25'x4', and four feet is almost too deep to reach in and grab weeds. My caveat is that I'm 5'6" and have short arms, but still. There's only about 85-100 plants in there now - I pulled some up to put in pots when we thought we were moving, and a few just didn't like being transplanted a few times. They'll be... 4 or 5 years old this harvest season, I forget. I'll go with 4yo though. Not sure if the potted ones will make it through the winter - we've had more than our normal 2-3 weeks straight of 0-15* weather, so who knows.
Weed control... still working on that one. I've weeded and re-mulched with 1-2 inches of compost. #$^&I^% bermuda grass keeps coming in. Need to work on a better barrier between the lawn and the garden patches, that's on the 1-3 year plan when hubby's actually home during growing season(s). I can't put down anything like plastic because then the random places the new spears come out would be covered. I am still trying to figure out where I can plant another 50 purple asparagus plants though. I have issues.
Container... I grabbed some 5-7 gallon tree pots from a local nursery for fairly inexpensive. They're not the prettiest, but they work. Pain to keep watered as my climate's usually fairly dry during the summer, the black containers don't help, and asparagus is thirsty. Again, not sure how they'll overwinter in containers. I shall find out this spring if any survived.