A Controlled Environment Box For Sprouting Rhododendron Seeds
David C. Purdy
David Leach1 identifies the optimum temperature for sprouting rhododendron seeds as 75°F (24°C). He also identifies the optimum light level for growing rhododendron seedlings as that given by a fluorescent bulb 10 inches from the surface of the growing medium. He further states the need for high humidity both for sprouting seed and for early growth.
Starting with these rules, I have, after several years experience and uncontrolled experiments, determined that, for me at least, the following conditions lead to the best germination results:
♦ Temperature: 74°F.
♦ Light level: two fluorescent bulbs 10 inches above growing medium.
♦ Type of bulb: daylight or mixed daylight and growth.
♦ Light duration: 14-16 hours per day.
♦ Humidity: as maintained by a plastic cover.
With these conditions, one must be careful to control fungus by judicious application of Benomyl. After germination, the seedlings are best removed to a location with lower temperature. My experience indicates that a temperature in the range of 55°F to 60°F yields the best results.
I have developed a seed sprouting box which maintains the sprouting conditions automatically. Figure 1 is an outline drawing of the box. Figure 2 shows the box in operation but with the doors open to show the interior.
| Figure 1. An outline drawing of the Purdy controlled environment box for sprouting rhododendron seeds.
Drawing by David C. Purdy
| Figure 2. The Purdy controlled environment box in operation but with the doors open.
Photo by David C. Purdy
The structure of the box is made of ¼-inch plywood (1 x 2s and 1 x 3s). The box is equipped with:
♦ Fluorescent fixture with two 40-watt bulbs to provide light.
♦ Heavy duty timer to control the fluorescent lamps.
♦ Two 100-watt incandescent bulbs mounted on ceramic bases to provide heat.
♦ "Muffin" fan to circulate air.
♦ Line voltage thermostat to control incandescent bulbs and fan.
The end walls of the compartment containing the incandescent bulbs are made of glass rendered opaque by paint. Glass was used instead of plywood because of concern with overheating surfaces close to the bulb if the fan fails to operate. The fan removes air from the upper part of the plant compartment. The air is heated by passing downwards around the incandescent bulbs and then passes through a duct formed by a space, 1¾ inches tall, between the floor of the plant compartment and the bottom of the box. The air then re-enters the plant compartment by rising though an opening in the floor of the plant compartment at the end of the box opposite the heater compartment. This circulation path provides a degree of bottom heat to the seed boxes.
The box is currently operating in a room where the temperature is 60°F. To maintain a temperature inside the box at 74°F, the heater lights operate about 18 percent of the time if the fluorescent bulbs are on and almost continuously if they are off. If the box were used in a cooler environment, it would require some insulation. I do not recommend larger incandescent bulbs because they might overheat the wood structure.
This type of sprouting box has given good service over the past several years. By using it, I am sure that my seedlings are given the best possible start in life.
1 David G. Leach, Rhododendrons of the World. Charles Scribner's Sons, 1961.
David Purdy is a member of the New York Chapter.