Plant Encyclopedia

If you dare: growing Cypripediums in a hot climate


I have been growing lady slipper orchids, genus Cypripedium (here after called Cyps), in a warm temperate climate for 7 years in southern Japan (latitude 34 N).  In the spring, when the plants are emerging and flowering, I get a jolt of shear joy – mission accomplished.  Then, in June, the monsoon rains kick in and the rotting begins. Bugs get more active and slugs are a constant threat.

By late July (as I write this), I am positively sure that all is lost for the season since just about everything is worn, torn, ripped, rotted or eaten.  The oven heat of August and September seems to only emphasize the point – growing Cyps in a hot summer climate is madness.  Come November, with its cool rains and falling maple leaves, I dare to check the plants and am often surprised with what not only endured, but even grew well that year.  The following article is the method I’ve worked out over the years to grow these lovely terrestrial orchids.

Cypripedium kentuckiense in garden

Cypripedium kentuckiense growing in its 3rd season in a raised bed at the eves of a native woodland.

 

First of all, let me explain the climate I’m working with.  Kyushu, the southernmost main island of Japan, is a warm temperate climate sitting on the eves of the subtropics.  A typical day in January has a high around 7 C (44.5 F) and a low of 2 C (35.6 F), with an daily average around 5 C (41 F).  Why spend so much time worrying average temperatures?  The reason is simple – to vernalize adequately (have a proper dormancy period), most Cyps require at least 3 months of an average temperature at or below 5 C.  My town, situated just on the edge of a mountain range, barely fulfills that requirement.  In fact, the average temperature is a bit warm in the winter months.

If winter weren’t trouble enough, June and July create more problems.  This is due to the inordinate rainfall of the summer monsoon.  A meter or more of it can fall in just 6 weeks, something most Cyps don’t enjoy.  In mid July the rains stop and the oven turns on.  Average highs are in the low 30s C (90-92 F) with lows only down to the upper 20s C (80-82 F).  Fall comes late, with October drying off and slowly cooling off, but the true colder nights don’t come until late November or early December.

Cypripedium henryi

Cypripedium henryi is another potentially heat tolerant species that occurs in parts of SE Asia.

 

There is no equivalent climate in the USA that I know of, but the seasonal temperature range is similar to Atlanta, Georgia or Charleston, South Carolina.  Due to the moderating seas, extreme highs and lows are much less severe by comparison.  The coldest recorded temperature for the Fukuoka City area was almost -7 C (20 F) and the highest was around 38 C (100 F), but typical extremes are more like -3 C (26.5 F) and 36 C (97 F).  Overall, it rarely goes below freezing or above 35 C (95 F).  The consistency of the heat in summer coupled with a warm fall and too warm winter are what challenge the Cyp grower here.

From my observations and experiments I can tell you the following about growing Cyps in a hot summer climate:

1. They will diminish in pots even if they succeed for a few seasons. The main reason is soil temperature. Pots simply get too hot. I have tried many methods of cooling the pot down, but none have worked to their satisfaction. The only species I can maintain long term in pots so far is C. formosanum, but by far it prefers growing in the ground.

Cypripedium pubescens

Cypripedium parviflorum v. pubescens has been growing in my garden now for 4 years.

2. Seedlings will be a real challenge to get to adult size, if indeed you can even keep them alive. To date I have managed to grow only a handful to subadult size. For example, I grew C. parviflorum v. parviflorum from deflasked seedlings to almost blooming size and then they diminished this year.  Ditto with C. formosanum – they grow slowly bigger each season, but it is like watching paint dry. I have C. macranthos that grow year after year but so far have failed to put on any size. And so it goes.

3. Organic material in the soil needs to be kept to a minimum, optimally less than 10%. I use seedling grade orchid bark, fine grained pumice, and a material called kanuma that is something like perlite, but holds water a bit better. The ratio used is roughly 5 parts pumice, 1 part kanuma, and 1/2 part bark. C. japonicum and C. formosanum both like more organic matter, so I up it to around 30% with these only.  Since the bark and kanuma are acidic in reaction, a bit of lime can be added in the fall.

 

Cypripedium kentuckiense flower

Cypripedium kentuckiense is the most southern growing Cyp in the USA and likely is the most heat tolerant.

 

4. Sunshine is a no-no since the root run will get too hot. Bright shade, say that suited to Cattleyas is enough for most species.  Plants that require more light to thrive likely will be problematic, such as C. reginae or C. candidum.

5. Plants will grow or diminish regardless of your efforts. The following species and their hybrids are good choices: C. formosanum, C. japonicum, C. kentuckiense, and C. parviflorum v. pubescens. Others that might be OK include C. fasciolatum and C. henryi. Also tolerant of heat if grown correctly is C. debile, but it is a tough plant to maintain long term, as is C. acaule. Cool growers like C. parviflorum v. parviflorum (as well as v. makasin), C. macranthos, C. flavum, etc. are not likely to make it for very long.

6.  Plants grow best in elevated beds of the above mentioned compost that must remain well watered, especially during hot, dry spells in summer.  The native soil here is a sticky clay-loam that would kill a Cyp outright.  The area I grow in is wet much of the time in summer, so I first make a mound about 30 cm (1 foot) tall of the pure native soil.  One top of this goes another 30 cm of the Cyp compost.  Bricks or rocks are placed at the sides to hold the bed together.  In summer the root run stays cooler than the air due to evaporation off the top of the bed.  The wetter earth at the base wicks upward, thus keeping the evaporative cooling going.  The only rub is you have to keep the bed moist at all times.

Cypripedium Philipp

Cypripedium Philipp is the hybrid between C. macranthos and C. kentuckiense and so far seems quite tolerant of hot summers.

7.  Fertilizer should be applied every few weeks from April through June.  Fertilize often from the beginning of growth, through flowering, and until the real heat hits – say when the average daily temperature goes above 25 C (77 F). At that point stop fertilizing. If your fall tends to be warm through October you can give one last shot of fertilizer the last month of growth.  After that point stop fertilizing.  DO NOT fertilize plants that have gone dormant.  Any good liquid based fertilizer around an NPK ratio of 6-6-6 is fine, but use one that contains micronutrients as well.  I use a very dilute solution, perhaps 1/3 the rate on the bottle.

 

Cypripedium Aki Pastel

I'm trying Cypripedium Aki Pastel for the first time this season - so far it looks strong.

 

 

 

To summarize, if you are in a hot climate I recommend starting with healthy adult or near adult plants.  They should be heat tolerant species or hybrids.  They should be grown in elevated beds of nearly pure inorganic, free draining compost in bright shade. Fertilize often from the beginning of growth, through flowering, and stop when it gets consistently hot.  Also very important: never let the bed dry out because that will not only stress the plants, it will allow the soil temperature to increase dangerously, and will cause the plants to go into early dormancy or even die.

So, good luck with this endeavor, if you dare.  It can be thrilling and fun, but is also fraught with disappointment.

 

 

 

 

Here is a list of plants I have attempted using this method (species followed by the hybrids):

-debile (does OK, but isn’t reliable long term)

-formosanum (grows, flowers, and increases)

-japonicum (grows, flowers, and increases)

-kentuckiense (grows and flowers, but has good and bad years)

-parviflorum v. pubescens (grows and flowers, doesn’t increase)

-parviflorum v. parviflorum (grows slowly, but isn’t viable)

-macranthos (same as parviflorum v. parviflorum)

-Gisela (parviflorum x macranthos) (grows well, increases slowly, but doesn’t flower)

-Philipp (kentuckiense x macranthos)  (grows and flowers, doesn’t increase)

 

New this year (fingers crossed):

-candidum

-fasciolatum

-flavum

-henryi

-reginae

-Aki Pastel (pubescens x macranthos)

-Lothar Pinkepank (pubescens x kentuckiense)

-Michael Alba (henryi x macranthos)

-Sabine Alba (kentuckiense x fasciolatum)

-Victoria (pubescens x fasciolatum)

 

13 Responses to “If you dare: growing Cypripediums in a hot climate”

  1. Darcy Gunnlaugson says:

    It’s just this side of a miracle that you can grow those charmers outside in your southern Japanese clime, no mean feat by any standards, and a testament to your skills. This difficult genus is not for unskilled nor the novice, no matter how enthusiastic they might be. I have great difficulty with this beautiful bunch here on Vancouver Island with our mostly mild wet winters and dry summers, but from your discription of climate there, it is just this side of miraculous that you can grow them successfully at all. My rule of thumb is that you have to grow any of the species for a minimum of 7 years to claim success , as nature runs in 7 year cycles and you will have experienced the gambit that in that period. I have lost colonies which increased over 5 or 6 years into lovely communities, and then had nature wipe them out. Only those gardners living where snow is standard in winter are likely to have assured success, and then only with the cold blooded species. Species like segwaii, subtropicum, and the warmer blooded spectrum of the genus will not survive in such cold conditions, nor will the near impossible Mexican spectrum of the genus, but these all may well adjust to your atypical environment, were some availed to you. It would be interesting to see how they did as compared to those which you grow successfully there now in atypical conditions.

    • tommy says:

      Hey Darcy – yes, I am amazed sometimes that anything makes it through the summer here. Right now it is 28 degrees and it’s only 8:30 AM! I’d love to try segawai, but this plant is very rare nowadays. I heard that the species as a whole was wiped out in its native habitat and pretty much finished off by just one collector. Subtropicum is coming to market more and more, but all are wild sourced and very expensive. Hopefully someone will be successful propagating them. I agree about your 7 year theory as well. Terrestrial orchids love to wax and wane with no apparent cause – most likely they die off from microflora in the compost getting out of wack.

  2. Darcy Gunnlaugson says:

    I totally agree with your notion of microflora. Sub-soil pathogens of the fungal sort are a bane here, especially in the springs following wet winters (which occur too often) where both dormancy and the temperatures for dormancy are marginal . Making accomodations to combat these pthogens by employing progressively more lean and non organic mixes, means becoming a slave to watering in the summer, or even with automatic systems, means becoming a slave to monitoring the effects of coverage or lack thereof of systems which do not have human exactness. Cyps are just, in plain language, a very, very difficult bunch, but somehow worth all the worry and care for those enraptured with them…read endentured to them.

  3. Steven, 8b Germany says:

    Hi Tom!

    How did your Cyps develop?

    Right now, my Calanthe reflexa is in full flower, it becomes nicer and bigger every year!

    Greetings from Germany,
    Steven

    • tommy says:

      Hey Steven,

      The new hybrid Cypripediums I got this year were a mixed lot. Cyp. Aki seem to be doing the best, followed closely by Cyp. Sabine. Unfortunately, the one I really wanted to do well, Cyp. Victoria didn’t do very well – they all went dormant early. Luck of the draw…

      As for the Calanthe, this was a good year for me too. Lots of flowers and surprisingly less rot problems. These plants just love to rot!

  4. Hi guys, planning to relocate from madly variable Ireland… (2011 being the coldest winter in more than a century with many venerable trees freezing out, while previous years allowed cool subtropicals to grow all winter long)…to the Mediterranean coast and a new set of challenges. But I’m enamored and ensnared by cyps since many years in New England.

    Here’s my thought on the above thread, particularly regarding the nasty microflora. Have either you Darcy or you Tommy tried incorporating charcoal (activate, ordinary hardwood, or biochar)into the gritty substrate? It’s often used to grow epiphytes on its own, or in soil mixes to deter bacteria. I wonder would it help with hot season wilt?

  5. tommy says:

    Hello Erik,

    I have used natural charcoal intended for use in the BBQ with potted Cyps, but not in open ground beds. It seems likely that its activity would be limited to just a season or two. In the case of pot grown specimens that is not a problem because I usually repot every winter, or at least every other year. Currently I don’t have many Cyps in pots due to overheating of the substrate – Cyps absolutely hate high root temperatures.

    With regard to keeping Cyps long term in hot climates, I think it is imperative to treat them with fungicide while in growth. I don’t mean when you see trouble either, but as a part of normal maintanence. The other option would be to inoculate with various mycorrhizal fungi products as a protectant against nasty microflora. Either way, keeping Cyps long term in any garden setting is an ongoing process and requires regular maintanence to ensure plant health.

    Good luck with your move to the coast – a far cry from New England to be sure! I wonder though your success at growing Cyps in any Mediterranean climate. Here the only thing that saves my plants is the just cold enough winter followed by a relatively late and cool spring. As far as whacky weather goes, I think that is becoming a world wide phenomenon.

    Tom

  6. Frank says:

    Hey Tom! I don’t grow any Cyps yet but am thinking about it now that I’ve moved to a new house. Surprised that C. kentuckiense doesn’t grow well there. I live in north Louisiana where it is native and the temps are about the same, even a bit hotter. It must be all that rain! Unbelievable! It often dries out here terribly during the summer and that’s when it gets hot. It is telling that the natives work the best for you. C. kentuckiense is the only one I’m planning to attempt. Love your whole site! Frank

    • tommy says:

      Hey Frank,

      Good to hear from you. I agree, it is surprising that C. kentuckiense doesn’t do as well as I had expected. I suspect it is more an issue of soil microbes rather than temperature. Also, the summer monsoon is just nuts here – for six weeks we see little relief from the rain, up to a meter or more can fall in just that short time. This gives rise to all kinds of fungus growth, and that can overwhelm plants in bad years. Typically we get a dry period in late summer and of course the temperatures hit the roof then – that too is not good for any Cyp.

      I think you have a good chance with C. formosanum as well. It is not only tolerant of hot temperatures, it is also one of the most resilient Cyps for warm climates. Conversely, in cool, northern climates it can be difficult. You may also want to try C. japonicum, but you will no doubt find it far more difficult than its Taiwanese relative. Good luck!

  7. Hein says:

    What about C. californicum?

    • tommy says:

      I never tried this species here, mostly because it doesn’t tolerate heat around its roots – or so I’ve been told. Still worth a try. In nature the plants are subject to very high temperatures during the day, but with a substantial cool down at night. Also, their roots are always bathed in cold mountain water. Here I cannot replicate that situation since summer nights are very warm (over 25 C often) and the temperature of the pots, even with the cooling system, is too high. In drier climates this species might be worth a try using this method though.

  8. Jon says:

    It’s been a few years since you wrote this. Do you still grow Cypripediums? I’ve ordered C. gisela to try and grow in southern Australia. It can get up to 45 degrees in summer here but some years (like last year) we don’t go over 38. I may not get very far with those kind of temperatures but it’s worth a try (expensive try). Do you still think the wicking pot in pot is the way to go? We have cold wet winters and warm – hot dry summers so I have to grow in a pot to stop the winter wet. I do grow other plants that require a dry winter so I’m used to moving pots in and out of the weather.

    • tommy says:

      Hi Jon. Yes, I do still have a few Cyps, all in pots except C. formosanum. I cannot speak for growing them in your climate, though the high temps are a bit much for most. As for cold, the key issue is consistency, not depth. Averages around 6 C or lower seems just enough to keep most dormant, but if there are lots of spikes above 10 degrees you may have a problem. The wicking pot idea sounds perfect for your climate due to the dry heat. Where I live relative humidity is never much below 70% in the growing season, so evaporation is a lot less, thus defeating the point of these setups. BTW, wet winters are not a big problem for most plants except those derived from SW China.

      As for me, I’ve pretty much finished with trying new Cyps here – it is just too much work and keeping them alive and flowering a tricky business. Nevertheless, good luck with your attempt. C. Gisela is a good starting choice!

      Tom

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