This is everything I know so far :)
Dallas, Texas is a hot growing climate (warm is just not accurate, it's HOT). So, can you really grow peonies? And of high enough quality to be used as a cut flower? The answer is, probably. Can you grow them in your garden as a beautiful specimen? Definitely.
I want to be clear that I'm only growing two 60ft rows of peonies, so I'm growing them on a scale that isn't really production farming. I've only had two successful years of harvests in a row: 'successful' defined as 3-4 quality blooms per plant with stems over 24" in length. As I continue to add varieties and harvest year after year, I'm sure I'll learn more and update this post eventually. But for now, this is everything I know.
First, early blooming varieties are the best. While Karl Rosenfield is known for tolerating the heat, it's not an early bloomer, so the buds set after our heat has arrived and the buds fail. Since we've recently hit 90F in February two years in a row, an early blooming variety is a must. Even though Sarah Bernhardt is routinely recommended for the south because it requires the least amount of chilling hours, it is a very late season bloomer and the buds consistently fail. The heat just comes on too quickly. One of our most reliable performers is Coral Charm and it's one of the earliest blooming peony varieties ever. Duchesse de Nemours also performs well and is an early season bloomer.
Pay attention to where you plant them. They don't like being moved. We have boggy clay soil here and I planted them in a terrible place without mounding up like I did all my other plants, but I got a little lucky. The roots do not tolerate poor aeration of the root zone. I happen to plant mine near several trees. The tree roots help aerate the soil and drink up the excess water in a heavy rain event. They also provide some summer shade which is essential to peonies here. Hollingsworth Peonies has this excellent summary of all the things peonies hate and most of them are factors here, but I've accidently been able to mitigate them with a planting next to trees. I'm not yet sure if I would recommend it. I think other types of natural shade and a raised bed or loamy soil would be much better. :)
Peonies need roughly 400 hours of temperatures below 40F and even in a mild winter, we have enough chill. But if you wait for a killing frost to trigger dormancy, you could be waiting until Christmas. My peonies start sending up shoots mid-January so that's a super short dormancy period. We force dormancy by cutting back the peony plant the first of October and tarping over them. The peonies then experience all their chilling hours while dormant and this could partly explain why we're able to grow them.
The most important thing I ever learned about Peonies, I learned from Dave Dowling during an Ask the Expert session of the ASCFG. Peonies store their food for spring blooms at the end of summer. This means in September, when we're typically ignoring our peony plants, is the most critical time to water them :) This causes them to have more stored food for spring than they would otherwise as they're typically stressed from heat. They set more buds for more flowers if you give them extra care right before Autumn arrives. Hollingsworth warns "...prolonged moisture stress (as with heat and drought) can be expected to bring on early die-off of the foliage and the end of food storage for the season." this is why summer shade and late-summer watering is so essential.
At some point, you will have to protect your peony plant. Whether it's frost cloth over a low tunnel due to a late-season ice-storm or shade cloth due to an early heat blast, you'll need to be prepared to erect some sort of temporary structure tall enough to cover your peony bushes. I've never covered my baby peony buds from a late freeze, because they're surprisingly hardy, but I have covered them with shade cloth in February!
Don't over fertilize. I have clay soil and it's pretty fertile. I don't add fertilizers on my farm except for homemade calcium sprays. But peonies benefit from occasionally being fed, so I top-dress the spring growth with compost.
I've read (but can't prove) that double-bomb type peonies don't do as well in the heat as single-form flowers. Since this is partially true for dahlias, I wouldn't be surprised if it's true of peonies. Both Karl and Sarah B. were double-type peonies that did not do well for me. As I try out more peony varieties, I'll let you know what I find.
I don't do anything crazy like throw ice-cubes on them. The most intense maintenance we do is to add shade cloth in early spring. We don't add support for our peonies but it could be because they're still young plants relatively speaking. In another year or two, they may be mature enough to need some support.
In short, growing peonies in the south isn't as complicated as it may seem. Most of your success will hinge on variety type, planting area, and late summer care. Perhaps because they seem so elusive, the work required to grow them feels well worth the effort.
Happy Growing!
Sarah Jo