Building a Cold Water Disa Growing Tray
by Ed Merkle,







Disa is a genus of orchids native to Africa, and the Disas most often cultivated are those which come from the country of South Africa and occur along mountain streamsides, waterfalls, and seeps. While the plants can thrive in warm temperatures, their culture requires cool moist roots at all times.

I live in Nashville, Tennessee where we have long hot summers, often in the 95F to 100F range for several weeks at a time. The winters are very mild with an average day temperature of about 45F, but there are many nights when it gets down to freezing, and even occasional fronts of bitterly cold weather come through when temperatures can briefly go down to 0F or below. Growing Disas under warm conditions can be challenging and a way must be found to keep the roots cool. After some experimentation, I came up with the idea of using a chest-type freezer to chill rainwater which is used to flow through the Disa's growing tray. This has worked well for me and I am often asked specifically how to build a chilling unit. I thought I'd put together this webpage to explain my unit in detail.

For the past several years, I have kept my Disa growing unit in my greenhouse with all my tropical orchids. This year I moved the plants outside in January to see how they liked it, and they have done very well. The Disas are only brought indoors when outside temperatures are freezing, but they spent many nights outside when it was 35F, and even had some snow on them one day. During warm weather, the freezer unit is turned on. In June, however, my freezer compressor died, so I had to build a new unit. I buy my freezers in used condition at neighborhood yard-sales for about $50, brand-new they are about $175.

The photo at right is my new growing unit, which holds about thirty plants. The tray is made from sheets of "R-Max" foil-coated foam construction board used for insulating homes. The lid is removed from the top of the freezer, and the tray is made to fit the top of the freezer snugly. The pieces of R-Max are cut and taped together with foil tape. PVC fittings for a water inlet and a drain are put in opposite bottom corners. Then the tray is painted inside and out with several coats of epoxy resin to make the tray waterproof and hardened against possible damage.

As you can see, my Disas grow in full morning sun.

The inside of the tray is six inches deep, a little above the height of the pots I use for the Disas. I used three, thick coats of epoxy resin inside the tray, about a third of an inch thick total in the bottom, to seal in the PVC fittings so that they would not break out. The PVC fittings are spaced so that they do not obstruct when the tray is fitted to the top of the freezer.

Styrene ceiling grates made for covering fluorescent light fixtures are used to add drainage in the bottom. The plant's pots sit on top of these grates. These are of the "egg crate" design with half-inch openings. The pieces of grating are easily made to fit the bottom of the tray by using needle-nosed pliers to snap the brittle "bridges" of styrene. When doing this, use caution and wear protective goggles, because snapping the styrene can produce sharp pieces that fly through the air and can severely damage an eye. I've even had them lodge in my skin and draw blood. Ouch!

I used two layers of the grating in the bottom in order to make sure the plant's pots are completely drained when all the water is drained out of the tray. This is very important so that air can enter the bottom of the pots for good healthy roots.

 The "innards" of the unit look like this. The freezer lid is removed and the interior of the freezer space is painted with several coats of epoxy resin to make it able to hold water. Most freezers I've found do not have sealed spaces. Some have nice drain plugs to help when you defrost, but this one does not. I also painted the bottom side of the tray with resin to make it stronger. When the resin is dry, you can fill the unit with fresh rainwater or pure water such as reverse osmosis (RO) water, distilled water, or water condensate from air conditioning units.

An electric fountain pump is used to pump chilled water up into the tray and the overflow water drains directly back into the freezer to recirculate. The pump is on a timer to provide an "ebb and flow" periodic cooling of the plants.

The fountain pump has both an intake filter and an output that allows plastic tubing to be attached to it. I used an 85 gallon per hour electric pump. PVC fittings are used that allow the plastic tubing to be attached and which thread into the fitting glued into the bottom of the tray. The tubing is attached, the pump placed in the bottom of the freezer, the sensor wire for a digital thermometer is placed deep in the water, and the tray is set down in place.

My freezer is on a timer to come on and chill the water for several hours each day, which keeps the water in the 55F to 65F range. The freezer thermostat's highest setting is about 30F. An alternative would be to add a thermostat which will keep the water at a specific temperature...say, 60F or so. Perhaps adjusted to 50F during the winter. The rainwater is replaced several times a year.

I found it necessary to find something that allowed the water to fill the tray to a depth of about two inches, but also allowed all of the water to drain out when the pump is off. My answer was a piece of PVC pipe with two slots cut into one end.

The slotted end of the piece is placed down into the PVC fitting. It is not glued in so it can be removed and cleaned. When the pump comes on, the water begins to fill the tray and starts draining out the slots. If the pump is large enough, and the slots are the right size, the pump volume exceeds what is draining out the slots. After a couple minutes the water level reaches the top of the piece of PVC and the overflow drains back into the freezer. When the pump shuts off, the water continues to drain to the bottom of the slots. You may have to fiddle with the height of this piece of PVC in order to provide the height of water you want around the base of the Disa pots. I let my tray fill to about two inches deep. You may want to check these slots often so they don't get clogged with debris or algae. I loosely wrap pieces of window screen around this tube to screen out debris and this works really well.

This is what the tray looks like when it is functioning.

In the upper right corner you can see the little mound of water entering the tray from the pump. Down in the lower left corner the water has reached the top of the piece of PVC drain and the water is overflowing back into the freezer. During normal running I have plants spaced closer to the inlet and the drain. You can see how the plants stand in the chilled water and the entire pot becomes wet from the wicking action.

There are some species of algae that love this set-up as much as the Disas, and I find it necessary to clean out the tray when the growth gets too heavy. Because the Disas are fertilized only very lightly, the algae is not a huge problem.

I use a soil thermometer to monitor the temperature of the Disa roots. As you can see, on this warm summer day of 85F, the roots are a chilly 55F. They really don't require to be quite this chilly, its best to keep the roots below 70F. This soil thermometer has a metal stem that is poked well into the potting medium.

I have switched to terra cotta clay pots for this kind of growing because the terra cotta allows more air into the pots, which is necessary for healthy roots. Also, the water wicks through the clay bringing moisture up through the entire pot, and the clay helps to chill the roots through evaporation. My Disas are growing in a mixture of chopped New Zealand sphagnum moss and perlite.

A digital thermometer with a long sensor wire is a good idea to monitor the temperature of the water in the freezer. This thermometer has an eight foot sensor wire, so it is easily mounted nearby and the sensor can reach deep into the water. The thermometer itself is not weather resistant so it should be given some protection from rain.

My Disa growing unit is outside on the east-facing side of our car-port. There are wide eaves that shade the plants after noon. Up until that time they are usually in full sun. Most Disas like bright light but should be in partial shade. The flowers are at their their best when they form in very bright light. Species such as Disa cardinalis prefer very bright light and may need full sun. The wide eaves partially protect the plants during rainstorms, but blowing wind will shower the plants with rain.

If your Disa plants are happy, they will look like this come late Spring. This is a hybrid Disa Kewensis (uniflora x tripetaloides) which is in spike in a four-inch pot. Notice how wet the pot is. These pots are nice and clean now, but soon they will get algae on them. A little Physan 20 algaecide helps prevent this and doesn't harm the Disas.

If your Disa plants are not happy, they will look like this. This is a Disa uniflora that didn't take well to the hot spell in the 90F's while I was replacing the freezer unit. The nice new green leaf may indicate the plant will be alright after all. Truthfully, most of your Disas will look just like this in the fall anyway. Each autumn, right after blooming season, the plants decline severely. The year-old growth dies and new growths are produced around the base of the plant, often filling the pot. Repotting is done in late fall or early winter. The plants should be divided, old dieing growths thrown away and everything given new potting media. By mid-winter, the new growths should be well up and they should begin to receive a little fertilizer.

This is a first bloom Disa uniflora of fairly good quality blooming in the middle of June 2004. The flowers are about three and a half inches across.

If you want further details on my unit, or want to discuss Disas in general, feel free to email me at: