If your native soil drains well, I would not add anything else to your hole but more of your native soil. You are after all planting a native plant into the native soil, should be all you need. This first summer you will need to water as needed until your plant gets established. Al

Thanks Al -- but if I can't scrounge up enough native soil from other parts of the yard, what would be a suitable alternative?

i've used Topsoil Plus from Armstrong Nurseries. I would mix it thoroughly with the native soil. The other alternative is to dig a hole from somewhere in your yard in order to get the native soil you need, then fill the hole you dug with Topsoil Plus.

Thank you buddyben, I'll look for that. We have several new plants that need planting (late, I know... life happens) and I'll be able to use some of the soil from those holes as well.