"I live in the city of San Francisco and grow peonies in containers on our sunny south facing patio. Have had a great deal of success with blooming. Many varieties have done well.... both herbaceous and tree. However, this year, the peonies won't go to sleep like they are supposed to. I cut down the leaves and stalks of the herbaceous and cut the leaves off all the tree peonies three weeks ago. Now, the red eyebuds are swelling and many are putting out new growth and leaves in late October. It is in the high 40s at night, but we won't get into the 30s in the City. My question is what to do. Should I let them grow? Or prune and force them into dormancy... if possible? This did not happen last year, and the peonies came out in mid January. Thanks for any advice you could share."
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My concern would be that after you trim the foliage, more would start to grow provided the temps are the same or warmer.
In future years I would recommend leaving the foliage on a lot longer into the late fall/ early winter which should help prevent this premature leafing out. From your email it sounds like you cut back the foliage in early October which is much too early for your growing zone 10b. I am in growing zone 7b, and about half of my peonies that are planted in the ground here at this time of year (early November) still have their foliage. So next year I would recommend waiting until at least the beginning of December or even late December to cut back the foliage in your growing zone. I know the foliage will probably start to look quite ratty, but if it can die back naturally it is best to let it attempt to do so. Also the fall season is when the plant is absorbing the nutrients in the leaves back into its roots. So the longer you can leave the foliage on the peony during this time, the better.
As for this year's early leaf out, I cannot say what will be the best for the plant in the long run. However if the leafing out is not too much, you may attempt to trim the foliage to force the plant into dormancy, but if you select this method I would recommend some other aids in helping to force this dormancy. Do you have a dark, cool place in your home, perhaps a garage or basement? If so you may want to move the potted peonies to a location such as this to limit the temperature and light the plants receive. Also I've heard of other growers in CA putting ice on top of their peonies to simulate a winter chill. This may be another option for you. However I'm not sure how long you would need to maintain an ice covering. So this may be something you'd want to research.
If the leafing out has already developed into a large growth, you may just want to leave the plants as is this year (as long as there is no danger of frost). If for some reason you do get some freezing weather, you'd want to bring the pots in overnight only. Sometimes it is best to let Mother Nature do her own thing. I have ordered some peonies from vendors in cooler areas then mine, and had the peonies leaf out in late fall. I usually just leave them as is. Since I do have frost here in the winter, any shoots that have leafed out will die back when we get a cold frost. This does seem to set the plant back a little in terms of growth and development, but they do always seem to survive. They may just take a little longer to get established. Since you don't usually have any danger of frost, the foliage may grow fine for you.
Good luck with your potted peonies this winter. Please keep me posted on what you decide to do with them and how they fare in the spring! :-)
