Mayber shape the top of it into a saucer, or dig a few holes that will tend to hold rainwater. Then you'll hav e some spots that will be more moist.
That would also be a great place to sheet-compost or spot-compost coffee grounds, kitchen scraps, grass clippings, yard waste, paper or anything organic. A few bags of dried manure. The saucer will hold anything of value until it can filter through the clay and start the soil-ification process.
Any amending you can do to the top few inches, even if only in one spot or strip, will let some plants get a toehold and break ground for fussier plants next year.
Sloping sides, especially the side that slopes down to the lower side of your pre-existing garde, will be drier.
If your yard slopes down towards the berm, the foot of the berm facing your yard may actually be a wet spot, especially if the berm has a convex curve on that side instead of conave. .If you want to conserve (or divert) rainwater, it might be worthwhile shaping the "wings" of the berm to either retain water or let it run away.
Then you might try low-cost low-effort "natural selection". Plant any old thing wherever the clay looks least bad.
- When you see something that looks hardy being discounted at a big box store, buy one small pot and dig a hole for it.
- Any overage seeds you might have thrown out, scratch the surface with a rake and scatter some there in a rainy season.
- Buy some cheap seed MIXES, or trade for them online, and scatter those on the clay berm.
- Buy a few pounds of a clay-tolerant cover crop (feed store or farm co-op) . Scatter some before every rain.
- If you see something tough-looking thriving by the roadside, or in a neghbors yard, dig some up and plop it in, but only IF you can ID it as not too invasive in your climate..
Whatever survives was the best choice! Anything that can force any roots down into that mess will gradually break it up for you.
