JARS v63n4 - I Love Greenhouses!

I Love Greenhouses!
E. White Smith
Portland Oregon

Greenhouses are great places to go to on a cold winter day, particularly to see flowering vireya rhododendrons! Nice and warm even if the greenhouse is at only 4°C (40°F) and it's freezing outside. At our nursery in Portland, Oregon, we have a big, old glass greenhouse full of vireyas, which can be in flower throughout the year but should be kept above freezing. The house has a big natural gas heater and is wired so if the power should go off during cold weather, we can run a gas-powered generator to supply electricity to the gas heater. Gas heaters might still provide heat without power but the fans will not run. We only used the power unit for two hours last winter. This old glass house is covered during the winter with clear poly sheeting which really helps keep the heat in and the rain out through all of the glass that has slipped down over the years. I am getting too old to get up on a greenhouse and recaulk glass and using the poly sheeting works very well.

We have a couple of other greenhouse type structures too. One is flat-roofed and covered with semi-rigid twin wall plastic, and two others are constructed from kits that use plastic fittings, 5 x 5 cm (2 x 2 inch) wood and clear poly as a cover. The flat roof structure is also heated with natural gas whereas the two kit units are heated with small "oil-filled electric" radiator type heaters. This all works well but the kit units are reaching the end of their lives and are just not strong enough to carry a snow load.

If I should build another greenhouse structure, I would build it strong. It could be of any type, a lean-to against another building, a free standing greenhouse or even a free-standing unit with the north wall solid. The north wall would be insulated with a work/potting bench attached to it. Very little useful light comes in to a greenhouse from the north side here in Oregon. I would build a foundation out of cement blocks up to bench height because you don't need light going under a greenhouse bench where nothing is growing. I would also cover the inside of the cement block wall with 5 cm (2 inch) Styrofoam sheets for insulation. The structural uprights would be treated 10 x 10 cm (4 x 4 inch) posts set in cement footings. Then the rest of the greenhouse would be constructed using 5 x 15 cm (2 x 6 inch) timbers. The reason for the latter timbers is so that I could cover the whole inside with clear poly sheeting, which would make a nice sized air space to really help with insulation. There are two hard parts in this scheme, i.e., fitting a good door and installing roof vents. Roof vents are very important for summer cooling (vireyas are temperate plants from mountain areas in the tropics, so like to be cool in the summer) and are a must in my mind. You can get roof vent openers that work automatically. Just opening a door on warm days might not be enough circulation.

During the winter I would want a fan up high in the peak of the greenhouse to push warm air back down, remembering that warm air rises and you are not growing plants way up there anyway. I probably also would put a window unit on the wall opposite the door for cross ventilation, maybe with a vent fan attached to it. Heating would depend on how big the greenhouse was, but because it is lined with poly it will be real tight so maybe an electric heater would not be too expensive, as the temperature just has to be kept above 0° C (32° F). You will need some electrical wiring anyway to run fans and heating cables for the rooting bed, and for lights. You also will need a source of water, and an old recycled sink would be nice to wash up in. Crushed rock on the floor works nice, as it both holds heat and allows water to drain away, and while a cement floor would really be a nice luxury, it is a lot of work and cost to install. A cement floor needs a slope and a good drain.

Would I use treated 5 x 15 cm (2 x 6 inch) timbers in the construction? Maybe, because in these bad economic times lumber has become relatively cheap. Treated lumber would last longer but cost more. I would cover the whole structure with twin-wall semi rigid plastic. This type of covering is sometimes opaque (not clear), so I might not need to have shade cloth put on during the summer months. You could also use corrugated plastic like the material used for some covered entrance way roofs, and it could last ten years or so, but it is harder to clean and can break much easier.

After you have the greenhouse built and water and power in it, you still need some benches to put your plants on. Benches can be built out of lots of stuff. I have always used cement blocks and cedar boards but next time I think I will go to a building supply store and get some of their painted or plastic covered steel shelving. Yes, I will have to probably cut it to fit but that's okay and easy enough to do. Galvanized expanded steel shelving would be better but I am not sure where to get it and it would cost more. Then you probably will want some lights, both to grow plants with and just to see be able to see them at night. I like florescent lights like the T8 units, or if you have lots of money, get the new T5 ones. T5s are said to be very good and efficient in power usage. You will want a couple of thermometers sitting around.

You might also want a special bed to root cuttings in. This can just be a cedar or plastic box lined with some good plastic with a few holes to let water run through. If you are going to root Rhododendron cuttings, you will need bottom heat, so you will have to come up with either a rooting heat pad or a heat cable with a good thermostat. Get good stuff and they will last a long time. You can get this equipment online at Charlie's (www.charliesgreenhouse.com - 1-800-322-4707) and Rain and Shine at (www.landscapeusa.com - 1-800-248-1981).

At Bovees Nursery, my wife, Lucie, roots Rhododendron cuttings in small 5 x 12.5 cm (2 x 5 inch) deep square pots over heat cables covered with about 1 cm (½ inch) of rooting mix. The rooting mix is just equal parts of plain fine peat and perlite. That is all, it's simple. Sure you will also need some kind of rooting hormone but there are lots to choose from. I have used just Root-Tone™ for years and it worked well and is available at most retail nurseries, but I also like Dip N' Grow™.

So now you have a greenhouse that looks okay and because you used timbers to frame it with and lined it with clear poly, it should be so air tight that it might be hard to close the door, like a new car is. Some people are now using opaque poly on the inside (which is white colored but passes light very well) and there is some research that the opaque poly is great because it tends to spread the natural light around much better than the clear poly. Do not get cheap poly or you will be sorry, as it will break down sooner and be a mess. Good stuff will last a very long time and will save you needing to replace it so often.

E. White Smith
E. White Smith is a member of both the Portland and Tacoma ARS Chapters, operates Bovees Nursery with his wife, Lucie, in Portland, and produces the Vireya Vine newsletter.