Peonies in warm climates, what works for you?

Summary: Gardeners living in warmer regions sometimes have difficulties growing peonies. This is due to the fact that peonies need a Winter chill to grow well. However if they take into consideration some small adjustments to average peony gardening, peonies have a good chance of growing and blooming well. Starting with a larger bare root division planted at ground level, protected from afternoon sun, irrigated during Summer and stems cut down (herbaceous peonies) or foliage removed (tree peonies) in Autumn. Through choosing the right peony varieties, chances of success will improve even further. Tree peonies do best, intersectionals are also much rewarding and if herbaceous ones are to be grown, then earlier flowering less double hybrid ones are best.

A short while ago someone inquired about growing peonies in regions with warm Summers and only a short and mild Winter. That is an interesting question I thought, and I suggested I could write something about it, so… here it is. We must mention that we ourselves live and grow peonies in a climate that is far from being ‘warm’, Belgium may be getting warmer due to climate change, but it hardly is comparable to beautiful Italy for example. We have no personal experience in that respect, except for the peonies we grow in our greenhouses. We’d gladly take your advice or recommendations if you happen to live in a warmer regions and have peonies growing in your garden.

Why is it a problem?

Peonies are plants that require a cold period and are thus essentially confined to the temperate zone. Why? After their foliage has died in Fall, the plants go dormant, meaning there is nothing happening above ground, although underground the new buds may slowly develop further. This dormancy is only broken through a certain amount of cold. If during early Winter there are some unusually warm days which would be favoring growth strongly, it will be noticed that peonies will not start growing. That is because their dormancy requirements have not yet been met. It protects them from growing during such an early warm spell which could be followed by severe cold again during the latter part of Winter, as this would otherwise kill the new growth. Only when sufficient cold has been received will they start growing at a certain threshold temperature. The more cold they have already received, the lower this threshold temperature will be. More cold received will also result in faster growth in Spring and more numerous and longer stems. That is also the reason commercial cut flower growers may not start forcing (providing heat to) their peonies too early. Because if they have only just received the minimum cold requirements they will grow more slowly and with less stems that will be much shorter. That explains why in some regions that are borderline for peony growth, some years with colder than usual Winters will give far better results than others where Winter was warmer than average. It also explains why in such borderline regions the plants don’t seem to increase very fast over the years, despite the fact that they have a much longer growing season compared to more northerly colder regions.

Where can they still be grown?

For an answer to that we could start by looking how far South wild peonies grow in nature (species only grow naturally in the Northern hemisphere)? On the American continent wild peonies, Paeonia californica, grow into the North of Mexico in Baja California. But that is truly an exceptional species and no cultivars or hybrids of it exist that are being grown commercially. Most of the lower USA states are somewhat borderline for peonies. In North America the United States Department of Agriculture (USDA) has a classification for hardiness zones and generally speaking peonies grow well in zones 3-8, with 8B being the absolute border, although a very few may succeed in 9.

In Europe/Africa the most Southern species grow in warm countries like Morocco and Algeria around the Mediterranean Sea, but they only grow high up in the mountains where temperatures are lower and where they will also receive more cold during the winter again. Some that grow at lower altitudes are P. cambessedesii on the Balearic Islands (Spain), P. clusii on Crete (Greece) and P. peregrina in Italy, the Balkans and Turkey. Thus in Europe they are reaching out towards the whole Northern part of the Mediterranean (P. broteri, P. coriacea, P. corsica,…) but South of it only in the coastal mountains. Further East some can be found in the coastal mountains again of Lebanon and Syria (P. kesrouanensis) and some in the Easternmost part of Israel (P. mascula ssp mascula). In Persia there’s one particular species which is very interesting in this respect and this is P. wendelboi, growing on a stony, dry and sunny location in North West Iran. However it also grows at a higher altitude and does receive cold during Winter. Further into Asia along the Southern frontier we find P. tomentosa and P. archibaldii, both of them along the South shores of the Caspian Sea higher up the mountains in Iran again. P. emodi follows in the Hindu Kush of Afghanistan, Pakistan and Kashmir (India) going further into the Himalayas, so yet again: the mountains. In china you would notice that some of the wild tree peonies are the ones growing in the warmest locations (P. delavayi, P. jishanensis, P. ostii). You will also find wild peonies as far South as South-Korea (P. lactiflora) and Japan (P. japonica). That’s more or less as far South as you could go. 

The Southern hemisphere is of course something else. Peonies are being grown succesfully in Southern Chile and Patagonia in Argentina. Remarkably they can also be grown in South Africa at higher altitudes. They also grow well in Tasmania and parts of Victoria (Australia) and New Zealand, although the South Island is much more hospitable to peonies compared to the warmer North Island where they sometimes struggle.

Of course as has been noted, if you live in a colder microclimate (higher altitude) you could always give them a try. It has been noted that if apple trees, also needing a cold spell, can grow in your region, peonies should also succeed. Going into the numbers: What are the minimum dormancy requirements? It is generally assumed that on average a good two months (some 70 days) with temperatures below 7°C at night are required. Fluctuating temperatures are better than constant temperatures because the same ‘cold’ period below 5°C then seems to be more efficient.¹ The lower temperature the better, at around 1-2° C it goes fastest; at 4° C is goes a bit slower, but at 7°C the duration of cold required nearly doubles. On the other hand temperatures below 0°C don’t really add anything extra.² It is also possible to fulfill the cold requirements chemically, at least partially, by giving the plants some gibberellin GA3 at the end of Winter (when flower formation in the bud has completed), but those are not easily available for an average gardener. If you can obtain it, then some 250 ml of water with some 100 ppm GA3 in it applied directly onto the buds may result in your peony plants giving more stems and flowers.³

Copyright Liberto Dario. Paeonia peregrina on Lefkada island (Greece). Full sun and limestone scree for this redder than red species that also appears on the mainland and in less harsh conditions.”

It’s not only the cold

It’s not only the cold requirements that are important. Also the growing temperature during Spring is a matter of concern. If temperatures are moderate then more stems will deliver a blooming flower. Especially the night temperatures are of utmost importance, they should preferably be rather cool. An experiment conducted with constant temperatures is given in the table below. The best was treatment 1: some 22°C during day and 10° C during night. If the nights were warmer (treatment 2), many more flower buds aborted, resulting in stems without a flower. Higher daytime temperatures of some 28° C (treatment 3 & 4) also resulted in less flowers. A normal spring however starts with lower temperatures and gradually increases towards higher temperatures (treatment 5). Here again, the higher the temperatures the worse it is for the plants because many buds that did look good initially nevertheless still aborted when the heat set in. You may also note that the best temperature still gave about half of the flower buds aborted at an early stage. But of course if they start growing at lower temperatures than the constant temperatures you see in the experiment, it is to be expected that less flowers will abort and results will thus be better. Treatment 6 is another interesting one, although not of much use for an average gardener. It shows that during the first 30 days of growth they can handle hot temperatures better without any noticeable damage, which is interesting if you grow them in containers or greenhouses, but again it shows that cold nights are essential (treatment 7). ⁴

Source: Kamenetsky

How to grow them in warmer climates

This all leads to the following advice if you want to try growing them in a warmer region. 

1. Plant a larger than average bare root divisions in Autumn. Because they will probably not grow as easy in your location as in colder regions, it is best to start with a larger 3-5 eye division. A small division will simply take too long to grow into a mature plant. As usual the specialized peony nurseries are the best source for these. Keep in mind that you can only tell the second year your peony has grown quite well, because the root you’re planting may already have received some cold prior to arriving at your place and that will only happen once obviously. 
2. Plant them shallow. Soil is a good insulator, but you don’t need insulation against the ‘cold’ as gardeners in cold climates do. When it freezes the top soil is frozen first, this frozen soil then acts as a barrier for the cold above it and it never reaches below it. So in your region you don’t want to plant your herbaceous peonies the usual 2-5 cm below soil level, but instead at ground level. Shrubby peonies still need to be planted somewhat deeper to avoid the rootstock sending up its own shoots which you don’t want in a grafted plant. Perhaps a division on its own roots would be better as there is then no risk of that, but we have no personal experience with this. 
3. If you can provide some shade during the hottest part of the day, that might also be beneficial as it will result in lower temperatures then. So planting them on the east side of your house will give them sunshine during the morning till noon and will shade them in the hot afternoon. 
4. They will probably need additional water during warm dry spells to grow well, even after the first growing season. Otherwise they will die back too soon. 
5. Contrary to the previous point you must also stop giving them water in September/October somewhere so that they will die back and can start their dormancy cycle. If the foliage remains green, then cut down the stems of herbaceous peonies to force them into dormancy. For shrubby peonies you could remove the foliage.

Not all peonies are created equal

Now it is also important to know that there are differences in cultivars and peony groups. Some simply 1/ require less cold, others 2/ bloom earlier before the excessive heat of Summer and 3/ still others are more easy openers.

1. Some peonies that require less cold are shrubby peonies (aka tree peonies), especially the Chinese suffruticosas and the Japanese Moutans. Also the lutea hybrid shrubby peonies might do better at your location. The P. rockii cultivars are more adapted to colder climates so will probably do less well. It also follows from this that the intersectional hybrids are good candidates for your region as they are partly shrubby. Garden Treasure for example seems to do very well. Within the herbaceous cultivars some double whites seem to need less cold. Duchesse de Nemours, a very widely available double white cultivar, needs much less cold to grow well compared to the most famous double pink Sarah Bernhardt.
   
2. Early flowering cultivars are far better than late flowering ones. The most common herbaceous peony cultivars are lactifloras. These however flower late in the season. It is better to go for some hybrid peonies that flower earlier and which may also start growing at lower temperatures thus earlier in Spring. Coral Sunset is a prime example of a hybrid peony doing well in warmer regions. Early flowering hybrid peonies are nowadays widely available. Some species peonies, they have been mentioned already, may also be worth a chance if you can obtain them.
   
3. Some varieties open better than others. The double flowered lactifloras often have difficulty opening well. When the buds are swollen it often happens that the excessive heat ‘freezes’ the buds (an ironic description of the phenomenon) so that they don’t develop any further. Semi-doubles and singles open much better. So it’s best to pick a variety with a less double flower if you want to avoid disappointments. If you want to stick with the lactifloras, you might consider Miss America. That is a semi-double white which opens well and flowers early in the lactiflora season. Also note that some colours simply absorb more heat than others and are thus more prone to bud blast. A dark red will always absorb more heat than a white flowered cultivar which reflects heat much better.
   

There are also always some varieties that are simply better than others, not even following the rules above. From personal experience in our greenhouses we have found that Old Faithful is also a very reliable cultivar, despite it being dark red, fully double and flowering later in the season. So that is another recommended variety. Cytherea is yet another peony cultivar that will probably do quite well, it is a large semi-double that blooms quite early. Contrary to most peony cultivars it also grows well under some shade, and that might protect it from most of the heat during the day, further improving its chances of doing well in warmer climates. It is derived from P. peregrina as are many others, for example the very pretty Lovely Rose, and many of those are in the peony trade known as ‘Saunders lobata of Perry hybrids’ for the famous hybridizer who introduced them. We might also note that the aforementioned Coral Sunset, hybridized by Sam Wissing, is derived partly from P. peregrina.

Many new cultivars being registered nowadays are hybrids and thus it can be expected that amongst those several will be good candidates for growing in warmer climates.  We hope this advice will set you on a course towards success with peonies in your warmer climate. If you happen to have some experience growing them in such a climate and have any advice or recommended cultivars please feel free to add some comments below this article. 

Footnotes:

1. Cohen, M., Eitan, R., Din, G.Y. & Kamenetsky, R. “Effect of constant and alternating temperature regimes on post-dormancy development of herbaceous peony.” In: Acta Horticulturae, 2017, no 1171, pp. 89-98.[↩]
2. T.A. Fulton, A.J. Hall, J.L. Catley. “Chilling requirements of Paeonia cultivars.” In; Scientia Horticulturae, Volume 89, Issue 3, 2001, pp. 237-248.[↩]
3. Halevy, A. “Evaluation of methods for flowering advancement of herbaceous peonies.” In: HortScience, 2002, vol 37, no. 6, pp. 885-889.[↩]
4. Kamenetsky, R. et al. “Temperature requirements for floral development of herbaceous peony cultivar Sarah Bernhardt.” In: Scientia Horticulturae, 2003, no 97, pp. 309-320.[↩]