JARS v46n2 - RSF Installs New Propagation Benches

RSF Installs New Propagation Benches
Dennis Bottemiller
Federal Way, Washington

Heating the root zone for propagation of rhododendron cuttings is nearly essential in a commercial operation. The cuttings root faster and more uniformly allowing a higher crop turnover and better quality plants. At the Rhododendron Species Foundation we have used bottom heat in our propagation benches for many years. Until last year our benches were equipped with electric cable heaters which do an adequate job when they are working properly but are prone to breakdowns and a short life-span. In October 1990 one of my two propagation benches had a temperature flair to 110 degrees F. Needless to say I was forced to pull the plug on it. Within a few weeks of the first disaster, the cables in the second bench failed, and I was left with cold propagation benches through the rest of the winter. We had been having intermittent problems with both benches the past couple of years, so we decided that the time had come to look for a replacement system.
There are a number of bottom heat systems on the market and of course every company claims theirs is the best, but when I started asking other growers about their preferences, circulating hot water was far and away the first choice. It is also the least expensive in the long run, although initially expensive. The manufacturer I chose claims a 30-year plus life span for their systems so initial cost spread over that period becomes quite reasonable.
The system I selected was sold as a package and included all the technical plumbing, electrical controls, and a custom designed blueprint drawn to fit the specifications of our growing area. It did not include a water heater, wiring or the PVC to pipe the system. When all the parts arrived I was sure I had more than I could handle. Having never been a plumber I was a little baffled by things like expansion tanks, safety aquastats and pressure reduction valves. After some coaching from the sales representative I dove in and discovered it wasn't so difficult; it just took a long time. First, I built new benches and then laid the circulation tubing in them. The hot water enters and returns at the same end of the bench through manifolds which direct the water to the small tough rubber tubes. These tubes are stapled to the bench at two inch spacing, so on our 54 inch wide benches there are 26 tubes to carry the water the whole length of the bench. Next, the main plumbing was assembled, run underground and connected at the end of each bench. The plumbing went fine until I went to fill and pressure test the system. No...it didn't leak like a sieve, in fact it turned out that I had plumbed it without a single leak. After a good deal of blueprint consulting and head scratching I discovered that the back-flow preventer valve had been installed backwards. At least I know for sure that it works. Once this problem was solved things flowed quite smoothly.
The next step was to install the heater and electrical system. The heater is a simple residential water heater with a 75-gallon capacity. We already have propane space heaters so it made sense to fire the water heater with propane as well. Once this was complete we wired the system. Each bench is thermostatically controlled to maintain a specified temperature. The drive pump and the solenoid inlet valves at each bench are controlled by individual thermostats so the benches can be regulated separately. This is all wired to a confusing set of distribution relays. I was able to get through this part of the installation only with the help of a very dedicated volunteer who also happens to be a retired electrician. When the four full days of electrical work was done, the week that it was supposed to take me to install this system had lapsed into four months doing little bits at a time. At any rate it was finally complete and we were ready for start up. With my heart racing we threw the circuit breakers and opened the gate valves and the system was operational. Over the following days I observed the system closely and it worked without a single hitch. Three new propagation benches 54 inches by 54 feet each fitted stem to stern with state of the art bottom heat worked on the first try!

The heat system has been up and running continuously for four months now and so far it has worked flawlessly. There is no discernible difference in temperature from one end to the other in a run of over 50 feet. Another benefit I have noticed is it does not dry the mix out below the surface like the electric cables did. Previously when we are taking rooted cuttings out of the benches I often noticed that the mix around the cables was very dry. This reduces the heat conductivity of the mix, effectively reducing the amount of heat that actually reaches the cuttings. I have not yet noticed this problem with the water circulation heat; however, I cannot yet explain its absence.
To summarize the system: It is a closed loop water circulation system with a 75-gallon heating tank, driven by a single pump which pulls the hot water from the tank to the benches. As the temperature drops in the rooting medium of each bench the thermostat energizes a solenoid valve which opens to let hot water flow to the bench. When the temperature of the medium reaches the preset on the thermostat it closes the solenoid valve and turns off the pump. This cycle repeats as often as necessary to keep the bench at the prescribed temperature. The water flows in at one end of the bench, travels to the other, does a U-turn and flows out through a second manifold and back to a common return and then on to the heater where it is recycled.
If any readers are in town for the 1993 ARS Annual Convention, I would be happy to show off our new benches and answer any questions you might have. I hope the results of our new addition will be evident in the plants we ship to you in the future.

Dennis Bottemiller is a graduate of the horticulture program at Washington State University and has been the Horticulturist at the Rhododendron Species Foundation for the past three years. He is a member of the Tacoma ARS chapter.