Ask Burton

This week, some advice on home greenhouses.

Q: I want to get a home greenhouse – nothing too big, one of those little 6′ x 6′ houses, or maybe 8′ x 6′. Will they work? Will my plants be OK this winter? Do I need a heater? I’m just wintering over my patio plants.

A: All right. It’s time for some basic greenhouse maintenance recommendations.

• If the house you choose is too flimsy, it’s not worth much. Many set-ups have minimal bracing, and that’s not much to hold that much sail area together in a 25-mph winter storm wind, much less a truly powerful winter event. Make sure the bracing is strong enough to matter, and if it isn’t, consider sinking a 4″ wooden post or landscape timber in ground like a fence post to help brace each corner. If the plastic cover is solid, and you can prevent it from being popped off the ground or collapsing from wind, it’s fine.
• Prevent unintended air gaps. Up here, we’d just toss a few bags of mulch sandbag-style at the bottom edge of a greenhouse to minimize cold air sneaking in from below when it seems necessary. At home, you can sink the bottom edge an inch or two down in the soil, or do exactly the same thing we do and just use the mulch in the flowerbeds come springtime.
• On the other hand, intentional ventilation… When the sun’s out, a clear plastic greenhouse that size is a solar oven, and it gets hot very quickly! Either choose a model of greenhouse with a top cap that zips down to ventilate the house’s top thoroughly when needed, or carefully hold the door or zipper of your greenhouse open any time it’s not a freezing or cloudy day. Cool outdoor temperatures on a clear and sunny day can still build greenhouse temperatures high enough to harm your plants if you keep the house tightly sealed. Your greenhouse should generally be buttoned-down completely just for bitterly cold days or freezing nights. Which leads to…
• A quality outdoor all-weather thermometer is essential, as is your attention during the winter. Greenhouse thermometers we’d recommend have indicators that show how high or low a temperature has swung since they were last reset, and knowing how hot or cold your greenhouse environment can become is a great help for adjusting your ventilation properly and learning how your setup should be maintained. You aren’t looking for your greenhouse to be shirt sleeve comfortable all winter long just to overwinter plants unless they’re very sensitive or you’re starting seedlings – you’re just looking to make sure your plants stay warm enough to get through the winter. You’ll need to adjust the ventilation regularly, and try not to set your plants inside directly against the greenhouse plastic.
• Use caution when using heaters outside. For what you’re doing, a greenhouse seedling heating pad made for the purpose beneath the plants will help for the worst winter weather events, as long as the pad plugs in to a GFCI grounded outlet. Space heaters can tip and start fires, get wet from condensation and short, or possibly melt nearby plastic if they are too near greenhouse edges. None of those things are good! Greenhouse operators have heaters meant to heat large volumes of air with plenty of clearance to prevent disasters. We’d recommend against unattended space heaters for this small structure.