How We’re Heating Our Greenhouse

We have been absolutely loving our greenhouse—but the first winter with our greenhouse means that we have to figure out how we want to heat it! We’re in Maryland zone 7A, which means it can get very cold during the winter here.

We’re approaching heating the greenhouse conservatively—I don’t want a huge power bill, nor do I want to keep tropical temperatures in there. I want to focus on growing cooler weather crops over the winter and then seed starting in late winter.

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Base-level heat retention & issues

So let’s get started by talking about base-level heat retention and issues we’ve addressed. The greenhouse has excellent base-level heat retention. And since I don’t want to keep the greenhouse at tropical temperatures, I don’t have to run a heater during the day. The “greenhouse effect” does it all for me, even on cloudy days, and even when it’s in the 20s.

To improve our baseline heat retention, I did an audit of the greenhouse to determine where large cracks were. I didn’t want to permanently caulk those closed, so I just used a small shovel to shove foam backer rod (like you’d use when caulking) in and fill them.

We also addressed the ventilation system. It’s great in the summer—but even when the vents are closed, it leaks heat. I tackled this by grabbing some scrap foam insulation from our basement finishing project and cutting that down to size. Then I covers that with thick plastic and taped it in place.

For the fan, I foamed and taped from the outside. I used duct tape, and it has held up fine through a couple nasty rain storms and one snowfall. Then on the inside, I covered the fan with plastic and taped in place.

Heater setup & thermostat

But just insulating the vents and cracks doesn’t keep the temperatures high enough at night. So I invested in the BioGreen Palma Twin heater. This is specifically designed for greenhouses, and the temperature range is -58°F to 210.2°F.

It has two separately adjustable heating modes, too—750 watt and 1,500 watt modes. We only need the 750-watt mode for the smaller size of our greenhouse. With the coordinating thermostat, the process of keeping the greenhouse at a certain temperature is largely fuss-free.

I just set the thermostat to whatever temperature I don’t want things to get below, and the heater kicks on and off accordingly. At first I thought I could manage heating things without the thermostat, but it’s definitely a worthwhile investment in my opinion.

Additional bluetooth thermometer

Another thing I’d recommend investing in is a bluetooth thermometer. Unfortunately the thermostat that comes with the heater can’t link to your phone. But this little gadget allows me to monitor temperatures in my greenhouse whenever I want! (Well, as long as I’m within bluetooth range, I didn’t want to pay for a WiFi one.)

The app can be a bit glitchy at times, but it mostly fits my needs. I love how it allows me to watch trends over time, too. And it’s really cool to see that the greenhouse can get up to 60 or 70 degrees on a sunny 30-degree day, too! It also lets me monitor night temperatures—I can see exactly when the heater kicks off and back on based on the line graph.

Seedling heat mats

And the last thing I’ll touch on is seedling heat mats. I got a bunch of these for free in my local plant trade group and will be using them for seed starting. But I’m also using them to help warm up soils a bit and germinate some of the cooler-weather crops in the greenhouse.

And I’ve also got some seedling heat mats going inside of this smaller greenhouse that I’ve got some tropicals propagating in! This is not something I’d recommend as the ideal growing conditions for hoyas, but I wanted to test out how the seedling mat would help with heat. And the plants have been doing great.

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