Year-Round Greenhouse Gardening Using Passive Solar and Microclimates

Like building a passive solar house, to make your greenhouse more naturally sustainable, you need to carefully consider your location and materials used. Your choices will vary by the conditions where you live. But make sure to think beyond just choosing a full sun location. Also, building materials will vary by what you can afford and what’s available where you live.

Here are a few things to consider as you start your planning.

Climate-Based Construction. In extreme cold climates, your shady side wall may need to be a stone heat sink rather than glass or polycarbonate panels. Or, perhaps, you might need to consider building partially underground such as in a walapini style structure. Alternatively, you may want to pre-plan to harness some geothermal heating and cooling techniques into your design.    

In warmer areas, you may need your walls to be flexible. Hoop houses with removable plastic sheeting or shade cover may work better than fixed walls. Or if you have more of a budget, then sliding glass or polycarbonate panelled walls might be ideal in warm areas.

Insulation, Heat Sinks, and Old School Technologies. Where I live in Northwestern North Carolina (USDA cold hardiness zone 7a), I can get away with a free-standing polycarbonate paneled greenhouse kit. Though, we did need to anchor it in concrete to keep it secure from occasional blustery winds. But I also need to make creative use of insulation and provide some artificial heat to make my greenhouse useful year-round.

• In winter, I use tarp-covered strawbales as exterior insulation on my windward Northwest corner to create an insulated cubby for my least cold hardy plants.
• I use strategically situated black water drums as heat sinks to regulate temperatures and humidity inside the greenhouse in summer and winter.
• Finally, I make hot beds with manure and straw and use strategically placed compost piles to maintain soil temperatures during our coldest months. (Note: Since I make compost year-round, this doesn’t add any work to my load.)

Thanks to these low-tech tactics, I can get away with using an inexpensive 120 square foot electric greenhouse heater (powered by solar panels) for our 432-square-foot greenhouse, even when we have weeks of nights below 20 degrees Fahrenheit.

Breezeways. Another technique, great for keeping greenhouses cool on hot days, is to use your door placement like a breezeway. This is basically like creating your own “dogtrot” greenhouse. By orienting your house so that your prevailing summer breezes flow naturally in one door and out the other, the plants to the left and right wings of your greenhouse breezeway are cooled in the process.

Harness the Power of Microclimates

Beyond good passive solar construction planning, you can also use the same microclimate creation strategies that work in an outdoor garden in your greenhouse. Here are some examples.

Flooring Options. The materials you choose as the floor of your greenhouse will have a huge impact on temperature regulation.

Mulch and Soil Carpet. All gardeners know that mulched, organic matter rich soil is better at maintaining a uniform temperature than a hardscaped driveway or compacted dirt. That’s why we constantly add compost and use mulch in our gardens.

So, rather than hardscaping your greenhouse with a lifeless dirt floor or dark gravel, use mulch and soil like carpet to help stabilize temperature. Even if you prefer to garden in containers, those container plants will be less stressed by the extremes of heat and cold when resting on a bed of soil than on hardscapes subject to extreme heat and cold swings.

Note: You’ll also need to water that soil and mulch carpet when you water your plants to maintain moisture since it doesn’t rain inside a greenhouse. However, by doing so, your plants will stay cooler and transpire less so you won’t have to water individual plants, even in containers, as frequently.

Pea Gravel. In some cases, though, you may want to reduce moisture in your greenhouse and allow plants to experience the extremes of external hot and cold days. Desert and Mediterranean plants, for example, are adapted to hot days and cool nights. 

So, for plants like that, a pea gravel floor might be a better option. The small stone size of pea gravel doesn’t act like a cold or heat sink the way larger stones do. Yet, they do help wick away moisture and provide a neutral backdrop to reflect the weather conditions outside your greenhouse.

Mixed Mediums. Since a greenhouse can be made beautiful just like an outdoor garden space, you may also want to mix your flooring mediums. Personally, I have nearly a foot of compost rich soil in all my planting areas. I plant directly in this, but I also set containers on top of it.

I also covered my foot paths with pine bark mulch. Plus, I reserved a quarter of my greenhouse as a human use space. There wood decking and pea gravel are my preferred flooring materials.

Grow a Garden

As gardeners also know, plants create their own microclimates.  This is why we use shade trees near an outdoor seating area to keep it cool in summer. It’s also why we use techniques like interplanting tall slender plants with ground cover to cool the soil and prevent moisture loss.

These same smart planting concepts help create comfortable conditions in your greenhouse too.  Here are some examples.

Plant Smart. Put your moisture loving plants toward the center of the greenhouse and your dry ones near the drier areas near the doors to reduce plant stress. Use larger plants, like dwarf citrus, to offer afternoon shade to heat sensitive plants.  

Start seeds in flats between more mature plants to keep them moist and protected. Then, as they mature, move those flats closer and closer to the sunny side door to get them ready for planting outdoors.

Plant in Pots and Plots. Plant some plants in the ground to take advantage of that soil carpet you installed. Then, keep others in containers and set them on the soil. The containers will act like a mulch to conserve moisture around the roots of the in-ground plants.

If you put larger plants like lemon, orange, or avocado in the ground and then use your containers for more seasonal things like ginger or turmeric, you’ll be able to grow perennial and annual foods in the same space with no difficulty. Also, when you water the containers, nutrients and extra water from the containers will flow to the soil floor. So nothing is wasted.

The more plants you pack in, while still ensuring that each plant has room for its root mass and good airflow, the more garden-like your greenhouse will feel. Also, when you reach a critical mass of mature plants, then just like a jungle or forest, the climate in your greenhouse becomes more naturally stable for all the life inside.

Personalize for Your Pleasure

My final bit of advice is that, like you would a garden, add some personal touches that reflect your personality, aesthetic preferences, and relaxation needs. For example, I’ve got a Grecian bench and a hot tub in my greenhouse.

They are both totally functional in supporting my use of the greenhouse. The hot tub increases humidity and acts as another heat source in winter. The bench is where I pot up and start seeds. I also have a folding table I use so I can sit at the bench and do some garden planning, tea sipping, and rainy-day reading. Yet, they also make the greenhouse a destination like my outdoor gardens.

In my experience, any garden that invites you to sit down and stay awhile is more likely to be well-tended. So, make your greenhouse a place you love to be year-round. That, my fellow homesteaders and gardeners, is what really makes the time and money you put into your greenhouse truly well-spent.

In case you need a little visual inspiration, here’s a look at my greenhouse in summer 2020. Like most people, I don’t have endless time or money. But bit by bit I keep adding more plants, making refinements, and growing more and more health-giving foods in my 432 square feet of paradise in the making!

Tasha Greer is an “Epicurean homesteader” and writer focused on simple, sustainable living. She gardens on about 1.5 acres and grows a large variety of annual and perennial edible, medicinal, and ecosystem support plants. Tasha also keep ducks, dairy goats, chickens, a pet turkey, worms, and (occasionally) pigs. She teaches classes in her community related to edible landscaping and organic gardening and is the author ofGrow Your Own Spices. When not growing food, cooking, or composting, Tasha is likely rocking on the garden swing with her partner, Matt Miles, surrounded by our two dogs and four cats. You can find the full list of articles she has written at her websiteSimplestead.com, and read all of Tasha’s MOTHER EARTH NEWSposts here.

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