What do I plant on a hill (with clay dirt) to prevent erosion?

Contact your County Agricultural Extension Cooperative. They are so knowledeable about local conditions and can assist you best. It's free.

Try white clover. Perfect for erosion control, adds nutrients to the soil, never needs mowing, is soft and fluffy for pets. I added in white clover when I reseeded my backyard lawn. Love it!

Best way to reduce clay in clay soil is? Flamethrower and a hoe or screen to sift it. You turn the clay into pebbles and they hoe or sift away from the soil. Once you do that the Clover should take like mad, plus you will have some red stones for decor.

It's possible you could swing some hardy herbs in that space.

Clay varies so if it's rock solid gumbo clay you growing anything in it could be difficult without amending it first.

I live on black land clay and my knock out roses do well

https://www.gardenupgreen.com/2017/08/easy-growing-knock-out-roses.html

My favorites are the coral drift - they are very forgiving.

https://www.gardenupgreen.com/2017/08/beautiful-coral-drift-roses.html

Hillsides are wonderful opportunities to create knockout gardens and

Grasses and clover don't seem to hold up well on hillsides. So I cut in several rock gardens into my hillside (as well as waterfalls, see post). I terrace the hill and amend the clay soil slightly when I drop in plants.

Plants that take well are

Mint

Thyme

Lambs ear (a few plants will grow to hundreds when happy)

Soapwort (gorgeous mounds)

Lillies

Iris

For starters you can even just buy a pack of wildflower seeds for your region, and throw them onto a roughly prepared area.

The fastest way to do that is to turn the soil with a hoe working in a stair-step fashion up the hillside. Then cast the seeds in the spring.

Here are MORE IDEAS:

https://www.hometalk.com/diy/grow/flowers/creating-a-colorful-oasis-30469269

Here's how we handled a few areas of my mother's sloping site.