mostly clay

topping a small hugel bed with mostly clay, suggestions on what to plant in the mostly clay the first year..to help break it down and feed it..it was dregged from a pond and has cattail roots and other pond life in it, and it has dried for 2 years so it is breaking up into smaller pieces quite readily..under is mostly apple wood...horseradish leaves..cardboard at the bottom...edged in wood

Daikon Radish and buckwheat are what comes to mind for me.

I work on heavy clay and basically any green manure is good if you want to turn it under.

If you don't want to do any further soil work, yes, radish and buckwheat are good. I'd add sunflowers - they grow very well for me, while other things are so very slow.

The biggest thing for clay AFAIC, is patience. If you're starting with an unworked heavy clay it'll take ten years of cover cropping to have anything resembling tilth. If you want to churn it up with something like a rotary plow, it can happen in half that time. I'd rotary plow for 3 years and then let the roots take over. By rotary plow I don't mean rototill, btw. It's a vertical shaft 4 share plow that kicks the dirt out the side. It doesn't turn the soil over and doesn't chop up worms, yet it separates the roots from the soil and aerates it well.

My yard was all red clay 17 years ago.  Now it is all black rich soil that is about 10" deep before you find clay.  That clay is not as dense though.  I used lots of green manure and lots of compost and lots of unwashed sand.  It was a long process but not anything I plant in the yard grows like my dendelions and plantain.  Weeds that my rabbits love.  I was going to suggest sunflowers or buckwheat.

Buckwheat is a good suggestion IF the clay bed has been worked (loosened). If not, it will have a difficult time penetrating packed clay thereby stunting its growth. This has been my experience when comparing its growth between a tilled and untilled plot.

I find that dandelion does an incredible job breaking through my hard soil, so you may want to try that as well.

magicdave wrote:
I used .... lots of unwashed sand. 

Careful when mixing sand with clay, you can create a very dense layer to work in. Addition of organic matter seems to be the preferred solution for mitigating heavy soils.

I wasn't specific about how I did it just what I used.  The property manager here kept asking me why I was digging up my entire yard every winter before the frost.  I have to admit that I did use quite a bit of grass seed.  Thankfully at the time it was provided for free by the property owner.  I added lots of mulch for the first few years and then started adding in sand too.  It seems to have worked out well.  I agree about dandelions.  I read an article almost 40 years ago in Mother Earth News about using dandelions to bring nutrients to the surface because of the deep taproot.  It made sense to me.  I now know that it works well.  Makes for good eating too for both me and my rabbits.