I don't think asparagus in a raised bed is advised. I tried strawberries in a raised bed (bed about 8-10 inches off ground level) and had problems with the edges dying out. Asparagus are supposed to have deep roots and I would wonder if the roots would penetrate the subsoil.
I do have asparagus planted in a bed above a retaining wall, but it is next to the house and 4 feet away from the wall. It has been doing okay there. IMHO.

Hoghaven, thanks for the info. If the soil under the bed is thoroughly cultivated, will that help the root issue? The main reason I want to do a raised bed is for grass issues. Maybe there's a better way to keep lawn grass out of the asparagus patch? Would edging work? And is there a type that is better? Thanks again for the advice :)

Most garden experts would recommend that the prospective asparagus bed be under clean cultivation (or heavily mulched) for at least a year. But since I am not an expert and am very impatient, here's what I would do: I would dig out the sod and replace or fortify the top soil to at least 18 inches deep--do not try to grow asparagus in heavy clay. I would keep the bed at ground level, not raised, for the frost protection. Then I would edge the area with 2x6 cedar boards placed deep enough that I could mow the edge. Because my Duluth subsoil is basically ledge rock (penetrable only by dynamite), I have purchased top soil twice--one time the soil was relatively weed free and the second time I spent most of my time the next season pulling out quack grass roots. So try be careful to get the soil as clean as possible. Since you will have the asparagus bed for many years, you want to be diligent about keeping the weeds out. Some people I know have used concrete blocks for edging and of course, there is the purchased edging. The rubber stuff that comes in rolls has frost heaved when I used it in heavy clay soil--so I guess it will depend on your soil conditions and what you have available. Home grown asparagus is worth the extra time and trouble. Good luck.

Thanks again. I will take this all under advisement. The potenial asparagus plot is not sod, I grew sunflowers and cucumber last year, so at least I won't have that struggle. I'd never thought about a cedar edging, I like that idea. And the lawn mower suggestion will be appreciated by my kind neighbor who mows my lawn for me. I'm glad I'm not the only one too impatient to wait that extra year for the soil to get just right. Thanks again, I really appreciate the advice. And I think my asparagus will appreciate it too! P.S. my husband and I are UMD alum and we miss Duluth. So does my 3 yr old, she loves the "Duluth boats". Amy

7 years ago I planted asparagus in a raised bed and they are doing great! My box is 3'x16' and hold 18 plants.
My box is made out of 1"x12" lumber. I did not till the soil underneath it and my soil is that nasty gray clay.
I did make my own soil mix.
Hope this helps. I'd do it again in a heart beat.

"Asparagus plants have a deep root system that will penetrate at least six feet."

Here is a link that might be useful: Growing asparagus in MN

10' deep & 15' long, does not mean you can not put it in a raised bed!
Asparagus has been grown in large containers too. Yet everyone I have asked & thing a have read says it can not be done, but medontdo on this site saw it done & spear harvest.

Here is a link that might be useful: jon's raised beds for asparagus.