I'm a long time grower of agastache in medium clay.

In my regular bed, what I do is dig out the clay, cover the bottom of the hole with chicken grit (tiny pebbles), then use a 1/4 sand, 1/4 composted manure and the soil I dug out mixed together. That seems to work fine.

However, last year I created a new bed using the lasagna method. Very rich soil but also very good drainage. I planted three tiny agastache plants (acapulco orange) and, as you can see, the rich soil poised no problem.

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Wow - beautiful!

I'm concerned about long-term performance of lean-soil-loving Agastache that is grown in rich amended clay. The first year they will look great, but I'm concerned they will decline with each passing year. About 10 years ago we grew A. cana but it succumbed to the amended clay after a few years.

Lean soil is not the issue - drainage is. As long as one provides good drainage, especially during the colder times of the year, agastache should be fine.

In fact, one of the largest breeders/hybridzers of agastache suggests this: "Best in well drained, fertile soil. Needs excellent drainage to overwinter."

I have alkaline clay and have amended it with peat moss and cotton bur compost and my agastaches have done well and have overwintered well. After the first year I rarely water them.

I have horrible clay with no organic matter. When I dig down (actually, I can't dig; I have to chisel), it's yellow rocky clay with no worms, no bugs, no life at all.

I sheet mulched one area using mostly wood chips, coffee grounds and half-finished compost and planted agastache. By this spring, I couldn't dig in there without getting a scoop full of worms. The agastache there are going nuts.

I'm gonna guess that the area has about 6 inches of sheet mulching on top of the clay. I didn't have to do any supplemental watering at all this summer.

As others have said, the problem is drainage, particularly in the winter when the beds are covered in snow, which in your zone you aren't likely to encounter.

Here, where winter snow and drainage on clay can be a problem, I only have luck if the agastache are planted in south-facing, raided beds. I deal with clay as well - but I've found they really like a rich soil, so in the spring, when I clip them back, I spread a few inch thick layer of compost all around, and in the summer, use grass clippings as a mulch.

I plant two dozen or so 'Ava" with 'Desert Sunrise' (high country gardens, see link) about a foot apart for a very densely planted row, with mums in front. The mums come and go, but from about the last week of July until a killing frost, the agastache are spectacular.

Here is a link that might be useful: high country gardens, great source

Thanks everyone. David: I know about HCG and already have half a dozen of their Agastaches in my shopping cart. :-)

Here's another clay gardener. Though my southern red clay is brick hard, it does drain well. My agastache thrives with little fuss. I add a layer of mixed wood chips annually but that's about it.