Passive Solar and Plants

Show passive solar home in sunny setting in southeastern New Mexico.
Passive solar is an energy-efficient way to live and grow plants in the Southwest.

Passive solar design for homes relies on many of the same principles as for placing plants; likewise, landscaping can affect a home’s passive solar efficiency.

We live in a passive solar home in the high desert/intermountain area of southeast New Mexico. The home is designed to maximize the warming effects of the sun in winter and the cooling effects of our valley on summer evenings. Solar gain to heat the home comes from sunlight that directly strikes the living space and indirect gain comes from materials that store thermal mass, such as trombe walls. Read more about our house and how passive solar works on my blog post from a few years ago.

See how much more sun strikes the southern windows in winter.

Passive solar saves lots of energy; we just have to adapt and work with the home’s features to make the most of its heating and cooling abilities. Likewise, all gardeners can learn more about choosing optimal planting locations and conditions based on the same understanding of the sun’s path, heat gain and cooling.

sunlight from passive solar
Winter sun comes in at an angle, and adobe walls, adjoining walls, floors and plants absorb the solar gain.

Passive Solar Basics

First, passive solar design relies on orienting the home to maximize heat gain. So, southern-facing windows gather the most sun and heat in winter. If you choose a plant that thrives on the edge of hardiness for your area, it stands a better shot at surviving if given a southern exposure in winter. Likewise, place plants that might be sensitive to heat on the east or north side of your home or where they get good afternoon shade from a tree or structure.

Summer shade from the mature apricot tree on the right helps cool plants in the shadow, as well as the patio and house.

Plants and Heat Gain

Like the tree pictured above, plants can help make a passive solar home function better. A classic design concept is planting of a deciduous (loses leaves in winter) tree for summer shade. In summer, the apricot on the southwest side of our home shades our patio, part of our garden and our master bedroom. In fall, the leaves drop, letting more sun strike the patio and all the walls and windows. An evergreen would shade the house in winter; having no tree would add to overheating of the house in summer.

The north side of your home is darker and cooler, an important concept for passive solar cooling and for plant placement.

It helps to use the same ideas when choosing or placing plants in your landscape. Consider sun exposure and whether a shrub or tree is deciduous or evergreen. Then think about how that might affect the plants that fall in its winter and summer shade. And plan ahead – trees grow and cast their shade further after several years. 

Light hits the trombe wall, which absorbs heat.

The room I used to have for an office faced due south all day and all year, with little to no shade. We removed a large cotoneaster bush that was overgrowing the path. But at the time, I didn’t consider the shading effects of the large shrub on the passive solar window. The office got hotter in summer than it once did. As the replacement plants mature, I hope to get back some cooling effects.

Indoor Plants

Consider passive solar concepts for your indoor plants as well. For example, passive solar home overhangs are designed only to shade windows when the sun is at its highest summer peak. In winter, they let more sun in as the sun’s path dips lower. Your houseplants’ sunlight also changes inside from one season to another, which means you might have to move plants or give them artificial light to help them through low-light periods.

These houseplants get perfect winter light from the sun’s angle, length of day and small overhang on the windows. Many get too little sun in mid-spring through mid-fall.

The same goes for plants in your landscape – if you take plants out for whatever reason, think about how the bare spot or size and features of a new plant might affect other plants nearby. For example, consider shade, sunlight exposure and even water flow.

The apricot tree also shades our potting bench in summer and early fall.

To get the most benefit from passive solar design, homeowners should understand basic concepts and help maximize the home’s features by opening and closing windows or shades. Plant lovers can apply the same concepts and attention to their plants – both indoor and outdoor—to keep them healthy all year long.

Teresa Odle, Southwest Gardening contributor

Teresa Odle is the editor of African Violet Magazine, a freelance editor for BobVila.com, and author of a blog on low-water gardening in the Southwest.  Teresa trained as a Master Gardener in Albuquerque, N.M. She grew up in the Phoenix area, and has lived in the mountains of northern New Mexico. Teresa and her husband now attempt to manage four acres of land in zone 6B of southeastern New Mexico. The land includes a large xeric garden, herbs and vegetables, and a small orchard that borders the Rio Ruidoso in Lincoln County. Teresa’s blog, Gardening in a Drought, won a 2016 national award for best writing in digital media from the Association for Garden Communicators.

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