My patch is going on 30+ years and still producing just fine every year. I recall reading how it should be replaced every 15 years or so...haven't seen the need so far. I always wondered how the plants would naturally bury themselves 12 inches or so deep. I let mine go to seed, leave the ferns to winter over, then burn off the patch early in the spring. Make sure you plant twice as much as you think you'll need-you can thank me later. Oh, deer and rabbits will not bother your patch at all. Asparagus beetles, on the other hand, will give you trouble.
I started a compost pile, because I gardened. Now I find myself gardening, so I have someplace for my compost!!