The “pot in pot cooler” – potting Cypripediums for a hot climate

For those of us who simply won’t be stopped from having plants we shouldn’t be growing given the local climate, the “pot in pot cooler” method is a perfect solution. Well, not really, but it is a good excuse to try plants we should pass over. In my case that includes most of the genus Cypripedium, to which I have a hopeless attraction. This article explains the method I’ve used with a modest amount of success in growing Cyps in a climate that nearly approximates Charleston, South Carolina, U.S.A.

When I moved to southern Japan from Florida 10 years ago I had high hopes to be able to coax some of the warmer growing Cypripediums to live in this nearly subtropical climate. In the beginning the failure rate was high, but over time I managed to figure out a method that seemed to work pretty well – the so called “pot in pot cooler” (a variation of a zeer pot refrigerator). What follows is a step by step explanation of this method and materials needed for this approach.

The problem at hand are Cypripedium roots – except for the hot growing outliers, C. subtropicum and the Mexican species, all Cyps are temperate slipper orchids and need cool roots. Any sustained temperature above 25 C is detrimental to Cyp roots, opening them to infection or simply stopping their growth. It is important to note that it is the roots that need to be cool, not necessarily the plant itself.

C. californicum for instance grows in seepage bogs where the substrate never goes above 20 C, but the plants themselves often endure temperatures well above 30 C. Similarly, cool loving species such as C. arietinum and C. reginae are found in bogs and wetlands in the southern reaches of their ranges. While people have mistaken this to mean they like wet conditions, what they really need are cool roots, and bogs make great evaporative coolers. So, let’s see how we can artificially create our own cooler.

The idea of a pot in pot cooler is to create conditions so that the evaporation rate off the pot containing the plant is as high as possible – the more evaporation, the cooler the pot. This necessitates the use of pots made of porous material such that water can migrate from the inner wall of the vessel to the outer easily, thus evaporating once hitting the air. Common, unglazed clay pots fit the bill quite nicely. Choose ones that are a bit larger than necessary to accommodate the plant – Cypripediums like to be over-potted. Other materials you’ll need include horticultural wicking tape (usually sold in rolls and commonly used for growing African violets), a fairly deep (6-8 cm) dish, and chunks of natural charcoal (optional).

The next important issue is what compost should be used – it has to be proper for the plant, wick water easily, and be airy in structure. Porous inorganic materials such as pumice, baked clay, perlite, as well as any number of soil amendments used to lighten mixes (e.g. Soil Perfector, Sermis, etc.) can be used in blended or pure form. Organic materials that don’t break down quickly and conduct water easily can also be used.

Here in Japan a material made from the shredded bark of Japanese cedar (Cryptomeria japonica) is commonly available and known as Kuriptomosu (Cryptomoss). Many Cypripedium growers use this product exclusively since it promotes healthy roots and can be used in pot in pot coolers. Do not use chipped wood products, peat moss, or any typical soil. If you live in the USA, cypress mulch made from bald cypress bark (Taxodium distichum) should work provided it doesn’t include wood chips, just the fine, shredded bark. Coir (material made from the husks of coconuts) is also another possibility and has the added bonus of being eco-friendly and available world wide.

So here we go, step by step (steps 1-7 illustrated in Figure 1 above):

1. To allow water to migrate upward into the growing medium, horticultural wicking tape is cut into three strips from 12-16 cm in length (depending on the size of the pot used). Lace the strips through the drainage hole with at least 6 cm of the strip below the hole (Figure 2). Cover the hole with mesh if desired.

2. Drainage material is put on the bottom of the pot with the wicking tape projecting out from it and evenly spaced.

3. A layer of natural charcoal is placed on top of this – activated charcoal is probably best, but I use a product intended for BBQ use. This may help to keep the compost clean of toxins.

4. In the middle of the pot form a small mound of your desired compost. I usually use a mix of Cryptomoss and pumice (perlite is fine too) in a ratio of 2:1. Put a bit more compost around the edges of the pot and be sure the wicking tape is still sticking out around the base of the mound.

5. Place the root stock over the mound, splaying the roots out in an even manner. It is OK if the wicking tape makes contact with the roots – some of them will even grow through the tape.

6. Carefully fill in more compost around the root stock and tape. The tape should be positioned around the root stock such that it points upward toward the top of the compost. That will allow the water to wick up around the plant and towards the uppermost part of the pot.

7. Put a layer of pure inorganic material light in color such as pumice or river sand as a top dressing. White colored gravels intended for aquarium use are fine for this too, but don’t make them too deep, a centimeter or so is all that is needed. This will keep the compost below protected from drying and reflect sunlight as well, thus helping to keep pot temperatures cooler.

8. A second pot of the same size is placed in a relatively deep dish of pure water (Figure 3). The pot with the plant is placed into this pot (Figure 4). The wicking tape should be free to dangle into the water. It is necessary to keep the dish filled with water at all times. If the water level gets too low or dries out, the inner pot will dry since the wicking process will stop, and pot temperatures rise.

The pot should be placed in a proper growing area – bright light, but avoid sunshine since pots in sun heat up fast. Humidity levels should remain high to ensure the plant’s health – Cyps do not like dry air, especially hot air. Fertilize from above with liquid applications every week or so. First, I take the growing pot out of the outer pot to fully flush it with lots of pure water – this helps reduce the build up of salts in the compost. Then I water with the fertilizer water. After the pot is fully drained, it is placed back into the outer pot. It is best to use a very dilute rate, but to fertilize weekly, from flowering time through early July, and discontinuing during the depths of summer’s heat.

For a video tutorial of the process, check this out:

So does this really work – well, yes, it does, but just barely in Japan’s hot, humid summers. Evaporative cooling is of course most efficient in dry, hot conditions which are antithetical to Cyp health, a fact diametrically opposed to the way this pot works. I’m not sure how to balance these two issues, but a balance must be reached. In the hottest weather I place the pots under azaleas in the open garden with ferns as companion plants. This seems to work pretty well. Plants in exposed positions, especially near stone or concrete, have suffered more during hot weather.

In the heat of southern Japan it is also necessary to repot every fall. This is done in November when the plants are fully dormant. Any dead tissue is carefully removed and if a plant looks stressed, it is given a fungicide bath for at least 10 minutes before replanting. The Cypripedium macranthos shown in the article on Cypripedium health was grown using this technique, so I think this method is a viable way to grow Cyps, albeit not perfect.

There are less strenuous variations on this method, for instance placing a plastic pot inside a larger clay pot with sphagnum moss filling the void between the plastic and clay pots. This not only keeps the inner pot cooler, it also adds to the humidity level near the plants. I’ve used this method to grow C. macranthos seedlings for many years.

For a while I had abandoned trying to grow Cyps in pots simply because I’d had more success with elevated beds in a woodland garden (see the article on growing Cypripediums in a hot climate). I’ve recently been moving back to pots due to problems with controlling fungus and bacterial rot in the open garden however. Cyps aren’t easy plants to grow even in good conditions, and they are a super challenge in the heat of southern Japan.

The pot in pot cooler is one way to try to coax cool-loving temperate plants into growing in a hot climate. If you are one of those “zone challenged” folks who have to grow plants not exactly suited to your conditions, by all means give it a try. It is an interesting journey, but don’t expect it to be labor free nor completely successful.