Plants for dry, deeply excavated soil

Hi all-

I've got a big mounded row of dirt excavated from a bunch of fence post holes that I'd like to get something growing on.  I'm not ready for much in the way of time, water, or expense.  I'd like to find something I could plant as a seed that would tolerate these conditions.  The soil was pulled from anywhere from the top 2" to about 3' down.  It's quite clayish, with little organic matter.  Its sitting on a few layers of cardboard, which are busy smothering a weak layer of grass.  It's all below a bit silver maple, so the soil is quite dry and relatively shaded.  I'd like to not have to water the mound, with the exception of germination.  I live in the Willamette valley, and our summers can be 100+ for a couple of days, with a practical drought for July and August.

Any ideas?

Thanks..
Ryan Lenz

Edited subject line for clarity

Buckwheat was reccommended....but I was under the impression that buckwheat would need watering through the summer.  Right? 


There must be some plant out there that would love these conditions.  I should also add that the chosen plant should be easily turned under/smothered/mowed.  I'm planning on making this a shady native garden.

Do you not want to invest the time and water at all, or just this year?

What you're looking for is an annual weed that tolerates poor soil and drought conditions.  Domesticated plants just won't take that kind of abuse.

Let the weed grow, then mow it when it flowers and let the mulch lay in place on the ground.  If you let it go to seed, it will spread.  If that's what you want, fine.  If you have plans for the spot next year, you won't want to let it go to seed.

Sue