Mine agrees with MoBot. Also, the species doesn't color up well in northern Virginia.

Cecily, how old are your plants and are they in full sun? I wonder why the plants would be so much larger in Chicago than in Missouri. One would expect the reverse, no?

Mine came from Bluestone five years ago and are planted in full sun, clay soil. The size discrepancy does seem odd.

eta: at Meadowlark gardens (about ten miles from me) there is a nifty hedge of amsonia hubrichtii alternating with baptisia australis. Both plants are the same height.

So at least 3 ft tall, right?

Yeah, plan for 3 ft x 3ft.

My mature specimens of A. hubrichtii here in upstate NY in all-day sun top out at about 3.5’ tall and wide.

As far as sourcing, Dancing Oaks is well reviewed on Garden Watchdog and seems to have ”Butterscotch” available:

https://dancingoaks.com/products/amsonia-hubrichtii-butterscotch-butterscotch-bluestar

I have 'Blue Ice', which I believe is quite different. Small. Slow to establish and increase. No fall color so far, I think next year will be season 3. It's very neat and easy care and the color of the flowers is very nice. It's not spectacular and a short bloom period gives you a short window of interest. It's compact and to me makes a good edger even. I'm still waiting for this year to be a bigger impact from the plant. I'd almost want to get the larger Amsonia that gives you bloom and fall color. I just don't have room for it

.Amsonia 'Blue Ice'

'Blue Ice' is thought to be a hybrid between A. tabernaemontana and another unknown Amsonia parent, but the parentage hasn't been fully resolved yet (if I recall).

Regardless, I have several straight species A. tabernaemontana, and while the flowers are certainly more impactful than A. hubrichtii, the fall color has never been anything to write home about, and even in a "weak year", A. hubrichtii is much more attractive come autumn.

Err, here hubrichtii is native and when I see it in plantings it's typical an annual shrub (ie it looks too big to be an annual stemmed forb even though it is). I haven't taken measurements but it seems 3 foot is quite conservative, I'd say at least 4.5 but it's purely guess. That may well be with supplemental fertilizer. I have one in my yard but only planted last year, so it's still fairly small. I'm quite sure that mature specimens look like a forsythia, and spread at least six foot wide. That may require about five years growing, but it's quite impressive. Fall color is great.

I do know A. illustris and I know that on my prairie when I lived in Oklahoma, it would reach 5 to 6 foot at maturity and typically had two flushes of flowers, the primary, and a more sparse secondary after it sets another flush of new growth.

From what you say, it may be strongly affected by the zone/growing season, and thus the farther south the larger (to some degree). That does fit what you've found.

Actually, it was the reverse with Mo. Botanical reporting much smaller than Chicago Botanical Trials.

Was it? Well I found at least one place while googling quickly yesterday that reported 4.5 foot as the expected height. Maybe Prairie Moon was it ?

My three footers receive no fertilizer or supplemental water.

Perhaps the Chicago Botanical Garden is located in an urban heat zone.

I would think the length of the growing season might be more important than heat. It has certainly proved true for the Persicaria polymorpha I had at my old place. Late spring, smaller. Early spring, larger.

That could be....just pretty sure I've seen much larger than 3 foot ;) I admired them so much that I finally decided...yes I guess I do want another species of amsonia.