Alfalfa is a cool-season perennial legume, fall planting is ideal though late summer can work. Average height is 3". You would likely find it easier to till under.

You can also buy bales of Alfalfa and till it in the soil. We have farmer bring us third cutting BIG bale. Till it in and also use as mulch. Can also add some chopped leafs.

You can fall-plant or spring-plant alfalfa, I prefer spring-seeded. If spring-seeded, the earlier the better - as soon as the ground can be prepared. They like cool conditions. Farmers often plant a companion crop, usually oats, to give some quick cover to protect the small alfalfa seedlings. A better more vigorous stand is obtained WITHOUT the companion crop. The first year it will reach 2-3' in height while it establishes very deep roots (if it can, it doesn't like hard soil like you're describing, but it might eventually bore through it). The second year is where it really shines, it'll make bigger roots and really grow if fertility and drainage are OK. For green manure in a small plot I'd probably cut it off and compost it separately or mow it into pieces so the stems don't wrap the tiller. Growing alfalfa on your plot, or tilling in a composted bale, would be good for your ground BUT I'd break up the hard pan mechanically. There's no guarantee of success with the alfalfa. I break up hardpan with a skidloader or a backhoe, depending on the size of plot and layout (I go 2' deep or so, watch out for underground utilities). I have, however, a slight advantage that I have both devices. Rent one? Then you can till it with some compost or manure and have a nice loose plot with excellent drainage right away.

thanks for all the advice. I may try several different approaches in different places this year, see what works best (and easiest!) and repeat that in the rest of the area next year.
Oh, I think spring is finally springing...
Must plant early spring bulbs next fall, I don't see any in this yard yet.

Other than a drawn out learning experiment, it would be a waste of time to try a lot of different methods. Bbriggs is exactly on target.

You really have to be careful taking some of the warm & fuzzy advice given on the some of these forums. While some of these theories sound nice and may even contain a fragment of truth, there is just enough truth to lead you down the wrong path -- because all of the other factors are ignored. Alfalfa is one plant that has had copious research done on it. It is well documented that alfalfa roots can eventually grow roots over thirty feet deep. (The word is can; not will.) It is also well documented that when alfalfa roots hit a hard layer of soil, they go sideways, not down. In fact, most farmers like Briggs know that they must mechanically (as he suggests) break up the hard pan for alfalfa to grow well. Translated, you must do something mechanical to break up the compaction -- spade, subsoiler, backhoe -- something.

This nonsense that some plant, worms or product will break up extremely compacted soil cannot be supported by any hard data -- lots of emotion and wishful thinking but no data because it just does not work. Natural factors may eventually eliminate compaction but it will happen long after you are gone from this earth.

The Oregon and Mormon trails are still visible over a hundred years after they were no longer used. This is an excellent example of compaction -- it just doesn't go away. Why do you think farmers have resorted to chisel plows that till so deeply?

Break up the compaction and add your amendments. Take a little time at the beginning to do it right or you will always regret it.

IronBelly

Well, pretty soon it will be thawed enough to go out and dig to see how bad it actually is. Ironbelly, your last line was right on-- do it right or regret it forever. The gardening budget is pretty small this year, and I need to do something in this area, so I was hoping for something less expensive than hiring a backhoe. Maybe some annuals just to cover the area this year, and a backhoe in the fall. We'll see.