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Question of the Week © 2017 Mike McGrath

Note: Hundreds of Mike's informative articles are available (in alphabetical order!) right here at the Gardens Alive website. To find Mike's answers to your specific garden problem, Click here and find your topic (like Weeds, Worms, Rhododendrons...) in our complete alphabetical archive of Questions of the Week.


What can an Inexpensive Greenhouse Do for You?

Q. I find your show to be interesting and helpful. I recently read your article at the Gardens Alive website about greenhouses and was wondering if you had any recommendations for a specific greenhouse I could buy in the two-hundred-dollar price range. I realize this is a super tight budget, but it's all I can afford, especially considering that I'll still have to buy a lot of other supplies.

---Sallie in Richmond, Virginia

A. There's no way to point anyone to a perfect 'two bill greenhouse'; especially when we have no idea what they expect to get out of it. So I emailed Sallie back to ask what she intends to use it for. Her response: "I am looking to be able to start my seeds outside."

Now, this is great timing, as seed-starting for plants like tomatoes typically begins on or just before March 1st in the Richmond area (March 15th in my Northeastern PA locale), and there is honestly no better place to germinate seeds and grow out young plants than a greenhouse. By the time we get to March, the hours of daylight are much longer, and the sun is getting to the right angle for really good photosynthesis. And the open-to-the-sun-all-around structure of a greenhouse allows plants to get as much—maybe more—natural light than they will when you plant them in the ground.

But that's a real greenhouse; one that can protect plants on cold nights. The "last average frost date" in Richmond is April 15th—essentially a month before mine in PA (which is May 15th). But as I have been trying to hammer home lately, that's just an average. As noted in the 'frost dates' section of the 'Dave's Garden' website, such a region 'will almost certainly receive frost through April 2nd; and it is almost guaranteed that you will not see frost after April 28th."

So plants started at the beginning of March in an unheated greenhouse are almost guaranteed to be (fatally) exposed to some below-freezing nights—and also a lot of just-plain-too-cold-for-tropical-plant nights. And research shows that exposing young tropical plants like tomatoes and peppers to temps in the low 40s can set them back quite a bit, delaying the eventual harvest by weeks.

Bottom line: an unheated greenhouse supplies the necessary light but will lose any of its daytime warmth right after the sun goes down.

And it takes a lot to keep an unattached greenhouse heated. I had a big, professional-sized one installed in my driveway back when "I" (Organic Gardening magazine, of which I was the Editor-in-Chief) was a major exhibitor in the Philadelphia Flower Show in the 1990s. It was well insulated, the growing bench was completely covered with heating mats, we had rolled one of those portable 'oil filled radiators' inside, and the high Intensity lights we used gave off an enormous amount of heat—and they were timed to go on at 10 pm and stay on until sunrise; and we had two small fans mounted at the top of the ceiling to force their warm air back down.

So we're talking around two hundred a month just in electricity. And one prolonged power failure and it would all go to waste.

Now, a greenhouse that sits flush against a heated structure like your home into which you can run a portable heater on a timer on freezing nights could work really well—especially if the 'glazing' material is solid, like glass or heavy plastic panels. But a stand-alone structure requires a lot of supplemental heat. And a late heavy snow could damage or collapse one made of stretched plastic. (My farmer friends have had industrial grade hoop-houses severely damaged by high winds and/or ice storms. [And coincidentally, we are currently in the third day of sustained 45 mph plastic-shredding winds as I type these words here in rural PA. I just watched another big branch drop in the backyard.]

So I strongly suggest you take that money and buy a nice indoor seed-starting system with stackable trays and multiple lights and utilize the free heat of your home instead. But after the starts reach a decent size—say at four weeks or so of age, when the nighttime temps won't be as dangerous—then you should definitely think about moving them outdoors to a little greenhouse-like structure for their hardening off period.

Or move them out to cold frames built against the side of your home and angled South; one of the best gardeners I know starts his plants under lights in the basement and then grows them out in cold frames towards the end.

Just be aware that closed structures under glass or plastic can heat up dramatically on a sunny day, and you either need automatic venting systems or the ability (and inclination) to prop cold frames open or vent your greenhouse personally on sunny days and them seal them up tight at night.

I have found that experienced gardeners can handle these kinds of systems well. But it takes the right personality; one unvented day or one uncovered night and you're buying your tomatoes pre-started.


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