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Greenhouse Gardening 101


Q. I have been considering getting a 'hobby greenhouse' for the backyard. Do you have any recommendations as to type or features?
    ---Robert in Elkton, MD
I searched your website and couldn't find your thoughts on backyard greenhouses. It seems that they could give you a few extra months of growing time in a semi-controlled environment. What are your thoughts, tips and concerns? Thanks,
    ---Wayne in Phoenixville, PA
A. Well, thank YOU, Wayne—I couldn't believe that we didn't have a full-fledged greenhouse discussion in the A to Z archives, but you are correct—and thanks to youse guys, I will correct that lack of info under the letter G right now!

There are, of course, many different kinds of hobby greenhouses. But to me, one of the most important differences is attached vs. free-standing. If you have a South or East facing portion of your home that could accept the addition of a greenhouse-like structure, I would strongly suggest you consider putting it there rather than build a free-standing separate structure—especially in the North, where supplemental heating is going to be a major issue.

Let's say your home has a brick or stone wall you can use for the back side of your greenhouse; the sun would heat that wall during the day and radiate it back into the greenhouse at night. If everything is facing in the correct direction, this could provide a monster amount of free, passive heat. And you wouldn't need (or really want) to connect the structures with a costly walk through; the door into the greenhouse can just be on the new structure itself.

Free standing greenhouses are much more challenging to keep warm—as I learned when we built one at the bottom of my driveway when 'my' magazine, ORGANIC GARDENING, began mounting major displays in the Philadelphia Flower Show. (I was the mag's Editor-in-Chief during those wonderful years.) Probably the smartest thing we did to conserve energy and reduce costs was to vent the hot air from our electric clothes dryer into the greenhouse through a system of well insulated pipes and do all our drying late at night, when the need for winter heat was the greatest. (But you can't do this with a gas dryer; it's just not safe.)

I had also read a fabulous study that found the root zone temperature of greenhouse plants to be much more important to their survival during cold weather than the actual air temperature inside the greenhouse, so the entire bench on which my plants sat was covered with a heating mat that kept their little rootsies warm at night. It was MUCH less costly than trying to heat the air.

And because any hot air will naturally rise to the top of a peaked roof, we installed powerful fans in the peak to keep pushing the warm air back down to where the plants were. (The added air circulation the fans provided was also very helpful in preventing disease.)

And on really cold nights (we had some worrisome record setters that first year), I also turned on one of those oil-filled portable radiators inside the greenhouse, which I probably would not have had to do nearly as often if I had the insulating wall of a house helping out.

But equally important as creating heat on a cold night is having an automatic method for releasing too much heat on a sunny day. You would be amazed at how hot a well-sealed greenhouse can get in the middle of winter if it's receiving full sun all day. And, of course, during the warmer days of Spring and Fall, heat can build up rapidly, making an automatic vent an absolute necessity. These are typically a panel in the roof or at the top of one of the peaks that opens up when the temperature triggers a gas filled cartridge to expand and push out a little arm that opens the vent. No electric; it's a passive automaton—although you'll need a real exhaust fan as well if you live in the South and/or hope to use the house during the warmer months.

So Wayne in Phoenixville hit it right on the head; the best and simplest use for a greenhouse is to get you started a couple months early and protect your plants for a few months longer at the end of the season—which greenhouses do really, really well. There's no better way to start seeds and/or to coax small plants up to a bigger size early in the season, or to ripen up last runs of peppers and such in the Fall.

In most climes, a greenhouse is also ideal for growing lettuce, spinach and other cool-weather crops right through the winter. In my part of PA—about an hour colder than Wayne's part—it would also be a perfect place to overwinter plants like rosemary that are just shy of being cold hardy enough to make it outdoors here. But the further North you are, the more expensive it would be to keep tender plants alive all winter, as I learned the hard way the year I filled mine with pepper plants for the Flower Show. If my company Rodale hadn't been paying for the extra electric, I would not have had any tropicals in there; it's much easier and less expensive to keep smaller numbers of those kinds of plants alive under lights inside our actual house house.

And summertime, in most areas, is when a greenhouse rests, with the inside just too hot for anything, even with the door propped open and the vent at full mast. Besides—its summer! Get outside! Blow the stink off yourself! (Note: This doesn't apply in very cool climes, like parts of Alaska and Washington state, where gardeners need a greenhouse to successfully grow 'ordinary' plants like tomatoes and peppers in the summer.)

Bottom line? I like greenhouses a lot; they are fun, addictive and move you up to the Ph. D. level of gardening.

Some final advice: Don't skimp on the door; it's where you can lose a lot of heat in the winter; give yourself a full season or two to learn how to make the best use of your new friend; give a lot of thought to placement and construction before you start…

…and you'll have a place to garden when others are trapped inside until Spring.

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