How deep can you plant
I'm new to rose growing. I'm in zone 4, and I've been pleasantly surprised at the success of my rose bushes so far. I'm in my 3rd year and have over sixty rose bushes, way more than I planned. Eventually I plan to reduce the lot down to 30 or 40. I'm surprised at how well they are surviving our winters, for example I have Oklahoma which was one of my first roses. I planted the union flush with the ground, the first winter it died down to a few inches from the ground and this winter I was left with a foot and a half of healthy canes.
Now I'm planting my rose bud unions four inches from the surface and they are all doing fine. I didn't lose any roses this past winter. I have a few that are rated zone 5. I'm wondering if I can go even lower than 4 inches.