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What Good is a Greenhouse in the Summer?


You Bet Your Garden
Question of the Week © 2019 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 Good is a Greenhouse in the Summer?

Q. Anne in central Pennsylvania--"near Altoona"--writes: "We love the show and listen every week. I got a smallish greenhouse this past August to add to the raised beds that we have outside. It's a 'cold house', meaning we don't do anything to heat it--only what it gets from the sun. I planted spinach, arugula, and lettuce in the greenhouse in August, planted more runs in January and have been cutting greens continually since September. It's been great. From now on, however, we'll be working primarily in the outside garden. Any advice about what I could possibly grow in the green house over the summer? I was thinking about heirloom tomatoes and maybe eggplants or peppers. Or do you think it'll be too hot in there?"

A. Well, you have already discovered the perfect use for a greenhouse--growing cool weather crops like salad greens over the winter. As the weather improves, greenhouses can also be a good place for transplants that were sprouted indoors--as long as the greenhouse is decently insulated, and it doesn't get too cold at night. But then, as you note, they can quickly go from being greenhouses to hothouses in the summer--even in your relatively cool clime.

First question--is the greenhouse vented? Can you roll up the sides along the bottom as with a large-scale hoop house? Is there a vent in the ceiling that works automatically or that can be operated by you manually? And finally, there HAS to be a door so that you can get inside and out, right? So even if you don't have the preferred automatic heat vent in the ceiling or the ability to roll up the sides, you can always vent this puppy just by propping the door open.

If you can vent it, keep the greens inside for awhile. I presume they're up on a bench where they're much easier to reach than bending over to harvest from a raised bed. It's not a good place for tomatoes, but I would keep my pepper plants inside the greenhouse in pots, vented on a hot sunny day, but closed up and protected on the odd chilly night. And perhaps even more importantly, where they would be protected from the kind of pounding rains that we endured last season.

Keep moving the pepper plants up into larger pots and put a thermometer in there. If it gets above 95 degrees F. despite venting, rig a fan up to move the air around inside--ideally from the back of the greenhouse towards the propped-open front door. In my experience, peppers--especially hot peppers--can handle somewhat excessive heat better than almost any other plant; just keep them well-watered, as they won't be getting the benefit of any rain. (If you're on city water, collect rainwater in a barrel or bucket for your greenhoused plants.)

When the days begin to get noticeably shorter, start more runs of salad greens inside the greenhouse and keep an eye on that thermometer. Once the nighttime temps inside the greenhouse drop to 50, you'll need to either take the pepper plants inside your house or provide heat. This is well worth the effort. Peppers are perennial if protected from frost, and if you handle them correctly, a single plant can live and fruit for decades. That's why I'm suggesting growing them in pots, so that they'll be easy to move around. They'll HAVE to come inside the house and be under lights for the dead of winter, but with a little work you should be able to stretch their time in the greenhouse considerably.

The easiest way would be with heating mats. Studies performed with snapdragons in greenhouses found that the temperature around the root zone of the plants is more important than the air temperature. If you can run an extension cord out there, a good high-quality heating mat (which you may already have for indoor seed starting) might get your plants through the holidays. If you go this way, be sure to also securely weather strip any openings and repair any tiny tears to keep the heat inside.

Don't let the plants stay in the greenhouse after temps in there drop below 50 at night.

Other ideas: Your season is too short to grow sweet potatoes and most watermelons outdoors, but if you have the space and ingenuity, you might be able to do it in a greenhouse. Both crops love heat and an occasional mid-summer venting should be all they need.

Or how about this: Buy a whole bunch of Spring bulbs this fall, pot them up in good-size containers, water them once and then chill them in the greenhouse over winter. After twelve weeks of chilling, daffodils can be brought out into bright light to bloom; sixteen weeks for most tulip varieties. Now you're a bulb forcer!

And finally, there's always the option of closing it up tight for the summer to kill any nasties, like whiteflies and aphids. With no venting, the inside of the greenhouse will heat up enough on a sunny day to cook any bad bugs. You can even disinfect suspicious soil passively in there.

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