Peonies need 400 hours with ground temps at 40 degrees Fahrenheit or colder. If they don't get this, they will be less likely to bloom. People in warmer climates actually take ice out to their bushes and allow the ice water melt to chill the soil.
I am growing a little more than a dozen peonies in 5 or 12 gallon grow bags. I have them out on a covered porch where they get around 6 hours of overhead light and the rest of the day in shade. They require watering about every 4 days - the bags let a lot of moisture go. In the winter, I will bring them into a garage where temps will be around 52 degrees - they'll continue to grow, but probably slower. Once it gets cold enough, I will take the bags outdoors and allow them to get around freezing and will eventually get my 400 hours done.
I am reluctant to leave the peonies outdoors in bags. I've read about peonies dying due to cold during droughts - something about the lack of moisture making them more vulnerable, I guess. Anyhow, if I can't water the bags because of the cold, then the cold temps will rob the bags of moisture and I'm worried that I will end up with a dead peony. Standing water / ice can also cause problems in bags or containers, so I just lug them back into the garage.
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