I usually have plastic containers that are 24 inch, 20 inch, 16 inch and three "very large black nursery tree size" that are plastic and I leave all outside during the Winters.
I've grown Agastaches in 16 inch and also in one of the "very large black nursery tree size". I make sure they all have excellent drainage. I add extra perlite in the potting soil. Sometimes the Agastaches don't survive the Winter and sometimes they do. Can't remember if I added gypsum in the containers, but starting this year, it's for sure going in.
For planting in the ground: I always make sure when I plant newly purchased Agastaches, to make sure the hole and soil has excellent drainage. I add all-purpose sand (inside Lowe's in masonry isle), gypsum and make sure the soil isn't fertile and rich. Then add small crushed gravel (I'm using limestone gravel) on top and in the hole too.
