I have two raised beds that are covered with wood chips.  The top soil looks real good but about a foot to 18 inches down is a gravely clay mixture.  I have found local sources of cover crops to put around the fruit trees in one bed and between the rows of vegetables in the other. 
Would Rapeseed, Southern Giant Mustard, Sweet Clover and Danish Clover and Alfalfa be wise choices? 
I also have areas that I want to convert from Centipede grass to something more beneficial around my other fruit trees.

Any ideas on what works best in any of these situations?  I am slowly converting a lawn over to a food forest.

Thanks

I'd steer clear of clover.  It can be tough to get rid of.  It's great for pastures and places where you graze animals.  But you want to be able to clear a raised bed and keep it clear.

This time of year, I tend to use a standard mix of about 10 to 20 different plants, many of then nitrogen fixing.  I plant both cool season grasses and broad leaf plants (usually legumes) as well as some warm season grasses, grains and broad leaf plants.  I'll use the same cover crop mix (innoculated with beneficial bacteria) on my raised beds as well as throughout the entire garden and orchard.  I mix it in a 5-gal. bucket and then broadcast it into the mulch before lightly raking it in.  Usually, I'll buy my seed from https://www.greencoverseed.com/

No -- I don't work for them and have no affiliation with them.  They've taken too much of my money over the years.  I should own stock in them.

For cool season, I like Common Vetch, Hairy Vetch, Chickling Vetch or Purple Vetch, Spring Oats (or any other oats), Cow Peas, Pearl Millet or Proso Millet, Bell Beans, Chick Peas, Mung Beans, common alfalfa, Sunn Hemp, Buckwheat, Flax, Spring Lentils (or any other lentil—even a bag of them from the grocery store), Cereal Rye, Sorghum, Collards, and various radish seeds (whatever I have around).  Any mix of peas or beans that will germinate go into the mix.  Sometimes I'll sprout the legumes in a jar of warm water on the kitchen counter, and then gently till them into the soil with the rest of the cover crop mix, just to assure germination.

Terminate your cover crop before it goes to seed, or you'll lose a lot of that precious nitrogen that the plants have fixed.  This might be tough on a raised bed, but if you could find a way to tractor chickens over the top of the bed, you could let the girls do the tilling for you.  Maybe create some sort of mobile cage out of PVC and some inexpensive netting.  All that cover crop biomass either goes into the compost pile or is directly laid down as a mulch in places where I need some soil cover.  Nothing goes to waste. 

If I know that I'm going to be transplanting whole plants into a bed (like a tomato or pepper) and not planting seeds, I'll take my electric hedge trimmer and cut down the cover drop that way.  Zip -- it's done.  With a 4 foot wide raised bed, I just walk down one side, and then back down the other -- like a big green haircut.  That way all the roots stay in the soil where they belong and the nitrogen doesn't go away.  If you are planting seeds, it's better to yank that stuff out, as it's tough to plant evenly with all that root mass still in the soil.  After you've composted the cover crop, you can return that compost to the bed as a top dressing.

Best of luck.

Thanks Marco.  Do you buy those seed mixes or purchase the seeds you want one pound at a time?  I am in North Alabama so it will not be cool for long.  Early to Mid-June it starts to get real warm (90s).

thanks again.

I'll do both.

I look for value.  If they've got a seed mix that is inexpensive, I'll go ahead and buy 20 lbs.  Otherwise, I look for what is on sale to supliment what I'm growing myself.  If I don't terminate my cover crops in time, I'll go ahead and let them go to seed, and then I'll harvest by hand.  From year to year, I get quite a bit of seed that way -- particularly oats, radish and various wildflowers. 

When you terminate a cover crop much sooner, it's got more nitrogen in the plant itself, as it's still much greener.  It makes for a nice hot compost pile.  If I wait until after it's seeded, the plants are pretty leggy and woody.  I don't try to compost those -- they just get crop and dropped.  I'll cut it off at ground level and then take all that biomass and compost it.

Can you start a cover crop in the fall and let it overwinter?  That's my standard procedure.  Stuff grows year-round here, so I put my cover crop in after I've taken off the last of the summer crop.  Usually by the end of September or early Oct., I've got my cc planted.

What ever I plant in the fall would likely die off in January/February time frame.  I am also still learning and going full bore. 

I used the compost I made over the past few months around my fruit trees and put some brown sugar water on top.  I did not think of covering over this with plain wood chips to keep it from drying out.  So I will add some more sugar and cover in hopes of getting some strong fungal growth.

On my new raised bed that is covered with chips i have found some orange fungal growth in large quantities.  The Pluots and Plums seem to be real happy and have survived a freeze (30F) overnight.  I ran out of frost cloth and sort of let the fully leafed fruits endure the cold and concentrated on the newly planted Asian persimmons and jujube.  My grapes may have taken a hit but they are only partially leafed out so I am hoping they will recover.

Do you ever do the native wildflowers as companion plants>

Thanks