No experience growing asparagus in containers, but over 30 years experience growing it in the ground. It is a heavy feeder and when planting, each crown needs to have its roots spread out in all directions and the roots from one crown shouldn't be touching the roots from the ones on either side. The crown is the part of the plant that is where all the roots meet - the center or top as it were. Most recommend planting the crowns at least 18" apart. You aren't supposed to harvest any spears for at least the first year, if not the first 2, so that all the energy can go into developing a strong root system. I'm sure they would love mushroom compost, but as I said, you need to hope someone has been successful with asparagus in containers and can guide you. Can't imagine putting more than 1 or 2 in each container, though, if you're talking the 18 gallon kind that's talked about here. Sure you don't have enough ground to put them in?
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