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Journal American Rhododendron Society

Current Editor:
Dr. Glen Jamieson ars.editor@gmail.com


Volume 48, Number 1
Winter 1994

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The Lofthouse Pressurized Propagator
John G. Lofthouse
Vancouver, British Columbia, Canada

        If, as they say, "a glass of wine isn't every one's cup of tea," then propagation isn't the "elixir" of all gardeners. But to some of us, creating and sustaining plant life quenches the "thirst" in our souls! This propagator, an improvement on the original Lofthouse propagator of 1970, was developed in 1990. It was introduced to ARS members at the 1991 fall conference in Olympia.
        The original Balanced Closed Case Propagator (see Quarterly Bulletin, April 1970) has obviously been satisfactory. Convention and local chapter papers have been presented on its ability to root cuttings better than most other available methods. It has been known for many years that extra air in the compost promotes better rooting. My father told me, as a boy, that chrysanthemum cuttings rooted better around the edge of a clay pot, compared to the center. He had caught my attention when he left the centers unplanted.
        Long after, when I tried to root cuttings myself, I remembered his revelation. But how could I get more air into the compost? Laying the compost on an open meshed grid would help. Also heating the air under the compost would increase air circulation, because heat rises as convection currents are formed. The Lofthouse propagation unit of 1970 was built on this open grid theory. Several years later I developed a method to heat the air and get additional aeration. (I was on my way to Great Rooting!)
        Figure 6 shows a drawing illustrating the original theory of operation. Air enters at the bottom and is heated. (I'll cover "how" later.) The air rises, heating the compost and adding needed aeration. Thin poly covers the cuttings. Small openings at the top allow for circulation of air. Actually, this simple method, with proper compost heating, will root cuttings. Of course, proper scion preparation and environmental control must be observed.
        The preceding description was of the original Lofthouse Balanced Closed Case Propagator. What is the Lofthouse Pressurized Propagator? The main difference is that air is fan-forced into a sealed chamber under the scions, heated, and then circulated through the cuttings. This extra heated air increases rooting percentages even further, and provides a better rooting environment. Scions hold their leaves better, and there is less die-back from stem rot.
        In the summer of 1990 I replaced my small propagation greenhouse. My propagator was designed to be placed on the south side of a 13 ft. x 9 ft. greenhouse, the long side running east and west. I constructed, of 1-inch fir plywood, a box 84 inches long, 30 inches deep, and 24 inches high. The sides and ends were assembled first, using waterproof glue before nailing. Screws might be a stronger alternative. A piece of 3/4-inch plywood is fitted into the base, slanted down to front, and also slanted from right to left. Three inches of decline from right to left and an extra 2 inches from back to front should be enough. This is to get the drainage to gather at the left front, where the drain will be installed. This bottom board has to be fitted quite well, because when finished it has to be sealed to be completely air and waterproof. Proper operation requires that the air pressure built up under the rooting compost goes through the compost. Any leakage will negate its proper operation. I mentioned a drain! Some of you might say, "It won't be pressurized if it has a drain." And you'll be right.
        To have a drain, and yet retain air pressure, was a problem in my early designs. I thought of many ways to achieve this, but most were too complicated and, surprisingly, subject to possible problems. The simplest and best way finally came to me: "use a plumbing trap." The air pressure required under the compost is less than that required to blow the water out of the trap. It worked perfectly. Install a 1-inch trap in the front left of the propagator. Seal this so there are no leaks!
        The next problem was the fan. I knew that too much pressure would blow the water out of the drain. I obtained a small two-speed fan from Valu Village and used the low setting. I see a lot of them around. I believe they come from home air purifiers or humidifiers. They have small fans available at Radio Shack. I can't give you the exact specifications but get a small one that will take continuous service. Too much air will reduce compost temperature, especially during the winter months. I cut a hole and mounted mine 3 inches up from the base on the right end of the propagator. Mount it in a waterproof sleeve. I used a one-gallon plastic pot with the end removed. It did quite well. If you use a small fan get a smaller sleeve. Large spaces reduce the pressurization. Use only as much clearance as needed. An added thought here. It might be a good idea to install an adjustable baffle at the rear of the fan. You could then reduce the air pressure if required.
        Now how do we provide bottom heat? Soil cables have always been a nuisance to me. They give uneven heat. If not in a protected lead sheath, they can be dangerous and cause fires. (A break in media can cause arcing. This can ignite dried peat. A friend lost part of her garage in a cable caused fire.) If they burn out, all cuttings have to be removed. To get away from these problems I now use three 150W pyrex spot lamps mounted in reflectors. Heat is controlled by a greenhouse capillary thermostat. Voltage to globes is made variable by a 450W lamp dimmer. Lowering the voltage extends life of lamps, up to many times their normal expectancy. Figures 1 and 2 show construction and lamp mounting details. An article on the lamp and the electrical installation of a previous propagator appeared in the Spring 1987 Journal.

Figure 1. New propagator construction
Figure 1. New propagator construction: Three 150W Pyrex
lamps in weatherproof socket, focused on aluminum
reflectors provide heat. Three smaller cross boards support
grids that hold compost. Wide cross board makes a partition.
 
Figure 2. Detail on heat lamps and 
sockets
Figure 2. Detail on heat lamps and sockets: Aluminum
reflectors are cemented to base joints and seams are
sealed to prevent air leakage.

        Figure 3 shows the unit finished, painted, and ready for compost. The fan sleeve is mounted at the far end. The three lamps are mounted behind the near long side and out of view. The voltage adjuster is on the bench at the bottom left. I installed a meter to record the voltage; it is convenient but not absolutely necessary. The long pipe is part of the misting system. It allows one to root at any time of year. There are three smaller boards that hold the plastic screens. Two only can be seen; the other is behind the larger board which is part of a partition. It is supporting the white screen that's under the valve in Figure 4. My propagator was built as a three partition unit. The first partition, which is delineated by the deeper board on the left in Figure 4, can be used when a small number of cuttings need to be rooted. It is shown being used in Figure 5. The second and third partitions are shown to the right in Figure 4. They can be shut down or used as a pair or together with the one on the left. It's up to you. I usually use them all. As should be apparent, this unit could be built to any size, after proper modifications.

Figure 3. Partially finished unit 
installed in bench
Figure 3. (Bottom left) Partially finished unit installed in
bench: Fan input is on box at far end. Long pipe is my mister.
Electronic leaf for mist control is below window on left.
Box at bottom left includes voltage control and voltmeter.
 
Figure 4. Compost support detail
Figure 4. Compost support detail: Two layers of rigid 5/16-inch
white plastic grid, top and bottom, with a sandwich of flexible
3/16-inch "gutter-guard" layered in between. Center unit has bottom
plastic and "gutter guard" being placed. Left partition is now finished.
 
Figure 5. Left area of propagator 
being planted
Figure 5. Left area of propagator being "planted": Electronic leaf
that controls frequency of misting is mounted on back left of unit.
Voltage control and meter on bench at left. And a new yellow hybrid
being forced into bloom. Unit roots big leaved varieties such as R.
macabeanum
, at left center. R. sinogrande, of which I have a very
good form, roots easily.

        The pictures are self explanatory. One thing that you think should be easy, isn't. And that is the screen that holds the compost. In the construction I allow 6 inches above the screen for compost. The white plastic should be strong and yet fine enough to stop the peat and perlite from falling through. The only plastic screen that I've found that is strong enough has about a 5/16-inch mesh, which is much too coarse for the purpose. On one of my addresses to an ARS chapter, a member who had built the earlier unit said he applied a fine mesh hardware cloth on the coarse screen. I had tried that years ago, but it didn't work. I told him that water elasticity blocks the mesh and inhibits air movement. He said, "So that's why my unit doesn't work properly!" This is an example of what can happen when not following instructions, implicitly! Up to now, my best solution is to use two layers of the rigid plastic grid, staggered, with a layer of soft plastic gutter guard in between. This works well but I'm sure someone will find a better way!

Figure 6. Operation of original Balanced 
Closed Case Propagator
Figure 6. Operation of original Balanced Closed Case
Propagator: Air enters from openings at bottom. New
propagator has lower part sealed and air is forced
in by a fan.

        You will need two service ports conveniently located between the lamps on the front panel. These are needed to replace the lamps and hose out the inside when required. Cover with plywood panels and make pressurized seals with silicone cement. Before painting all seams should be sealed with fiberglass cloth and resin. Also several coats of the resin before painting is a good idea for the entire inside of the box. One- or two-inch sheets of Styrofoam insulation should be applied inside the box. Approximately 5 to 6 inches of compost is required for rooting. There are many recommended mixes. I use about a 70 percent perlite to 30 percent peat mix, which I find quite satisfactory.
        This unit, properly made and with the correct air flow and compost temperature, will propagate nearly all plants from cuttings or seeds. There will be, of course, some exceptions, but very few. Used as a greenhouse unit, block up the box so its top is about 6 inches higher than the benches. It is easier to work with scions if they are elevated. As mentioned before, I use mist. I have not tried it as a "closed case" (polyethylene cover), but it should work in the cooler months of the year.

Figure 7. Electrical circuit
Figure 7. Electrical circuit: Rough elevation to show detail explained in text. Air enters chamber through fan at bottom right.
A slightly higher than normal air pressure is created under the compost. Air is heated by the Pyrex floods and forced through
compost. A NE2 neon lamp and 150V meter can be wired in if desired. Electronic parts can be obtained at Radio Shack.

        It can be used as a regular Lofthouse propagator "unpressurized" if desired. Simply do not cut the hole for the fan on the right end. Leave it uncut. Cut a 2-inch hole where the drain was to be located and omit the plumbing trap. This will be the air intake and drain for the regular propagator. The unit will still work well. If used outside, without mist but with a poly cover, locate in good light but away from direct sun, which could cause heat damage. In a cold climate, increase insulation and in very cold weather, translucent sheet Styrofoam could be placed over the poly top. Leave a little space for air circulation, however. The unit should be above ground for air intake and drainage.
        Included is a diagram of my electrical circuit. It should be installed in accordance with electrical codes in your area, as safety should always be a major concern! If I have overlooked some construction details, reference to the photos should answer most questions. Good propagation!

Jack Lofthouse, a member of the Vancouver Chapter and well-known hybridizer, has often contributed articles to the Journal.


Volume 48, Number 1
Winter 1994

DLA Ejournal Home | JARS Home | Table of Contents for this issue | Search JARS and other ejournals