Understand Thermal Mass To Be A Climate Zone Hacker!

Picture yourself on a beach. It’s late August and you’re sinking your bare feet into the sand. You can feel the warmth of the sand on your toes. It’s not just warm, it’s downright hot. It’s starting to be uncomfortable, so you wiggle your feet a few inches down into the sand where it is damp and cool. Much cooler.

You’re still hot, so you reach your hand over to a large granite bolder that’s placed behind you, part of a large breakwater. It’s warm – cooler than the hot, dry sand but warmer than the damp sand working it’s way under your toenails. You lever yourself up and walk down to the ocean, which seems completely unmoved by the full power of the sun above. The water is cold and you splash around just enough to cool down before finding your way back to the hot, sandy spot you’d staked out.

Why is the damp sand cooler than the dry sand? Why is the large boulder a temperature somewhere between the two? Why is the ocean water the coldest of all? The sun is beating down equally on all these things – why do some respond by warming up and absorbing that heat more than others? Most importantly, what does any of this mean to gardeners, and how can we use it to hack and manipulate our microclimates?

It all comes down to thermal mass, the shorthand term for the ability of an object to hold heat and even out temperature fluctuations, and specific heat capacity, the related measure of how much energy is required to change the temperature of a given weight of a substance by one degree.

Dry sand heats up quickly when the rays of the sun beat down upon it because it has relatively low specific heat capacity. It just doesn’t take a lot of energy to warm that sand up. Water, however, has the highest specific heat capacity of anything we are likely to deal with in a garden. So damp sand – a mixture of sand and water – takes far longer to heat up than dry sand. Granite boulders…well they have a little bit higher specific heat capacity than sand but not quite as much as water.

Specific heat capacity measures how much energy is required to change the temperature of a substance, and that shows up both on the warming side and the cooling side. A pound of sand, warmed up quickly to the ambient temperature of the day will, with its fairly low specific heat capacity, cool off quickly come the evening. You can think of this as the “Easy Come, Easy Go” principle of heat storage, and it explains why hot, hot daytime deserts become cold, cold nighttime deserts nearly as soon as the sun sets.

On the other hand, a pound of water is quite slow to warm up, but will hold heat longer into the evening, releasing it to the environment and warming anything planted nearby, perhaps by a crucial degree or two. This is why the ocean is basically the same temperature day and night, summer and winter, and why seaside towns tend to be more moderate in temperature than inland ones. A large mass of water finds the temperature average and just holds it.

In this capacity, water is amazing stuff – it has a specific heat capacity several times that of its nearest realistic competitor (liquid ammonia and liquid lithium not being particularly useful in our backyards).

So what does this mean to us, as gardeners? Can this help us understand our soil and ripen peppers or stave of frost-kill of tomatoes? Resoundingly, the answer is yes!

First, let’s look at soil.

If you want your soil to warm up in the spring faster, dry it out. Cover it, keep the rains off, increase drainage. Obviously you need enough moisture to get your seeds up and growing and to keep your plants happy, but if you’ve got heavy clay, moisture-retentive soil that gets rained on every day in spring, you’ve got soil that may not really warm up until August. Pacific Northwest people, this is probably the path you should follow.

If you want your soil itself to be a better heat sink – to hold a more even temperature throughout the day because you live in a place with big day-to-night temperature swings, keep it moist. The added water in the soil will mitigate radical soil temperature swings. Add moisture-retaining compost, mulch the heck out of your beds and water as necessary. If you live someplace like Texas, where you can easily see a 50-degree temperature swing from day to night, this is probably the path you should follow.

Second, let’s look at season extension opportunities.

I imagine most of my Pacific Northwest readers are familiar with the season extension product, the “Wall-o-Water.” Local folks swear by these products to get heat-lovers going earlier and protect against temperature dips.

Specific heat capacity is why products like the Wall-o-Water work – surround a tender plant with water and you’ve just created a top-notch heat-sink. The solar radiation absorbed by the water during the day is slowly radiated out at night, creating a pleasantly warm microclimate for plants that would otherwise be hurt by overnight lows.

The more high heat capacity stuff you can put around your tender plants, the more microclimate hacking you can do. That’s where thermal mass comes in. If a pound of water buffering overnight lows, five pounds is even better, and fifty pounds is great! Thermal mass is a fancy way of saying “More is Better.”

Let’s say you set up a black 50-gallon rain barrel on the south side of your house. It absorbs heat all day. 50 gallons is 400 pounds of water. That’s a lot of thermal mass! In fact, it’s enough water to warm an adjacent 10-foot cube of air by 21.5 freaking degrees for every degree the barrel cools off overnight.

Of course in a real garden we don’t confine our air to ten-foot cubes, so what you’ll actually find is a gentler but broader warming effect! But if you could increase overnight lows in one piece of your garden by 10-degrees, what would that mean for the plants you could successfully grow? That’s huge!

Getting that rain barrel to absorb some extra heat directly from the sun – by painting it black and putting it in a sunny spot – will increase not only the amount it has available to radiate out at night but also raise its temperature up above the daytime ambient temperature and contribute a bit to warming the air in its vicinity when the sun is out.

Added mass will always help to even out temperature fluctuations. Putting heavy things like water (think ponds, rain barrels, wall-o-water, etc.) or dense rocks (think rockeries, “bench rocks” and rock borders) near your plants will level out the difference between daytime highs and nighttime lows and help tender plants survive chilly nights. And if you can put them in the sun – then so much the better!

If you look at the greatest climate hackers out there – people like Sepp Holzer, who grows citrus in the Alps, and Ben Falk, who’s growing rice in the hills of Vermont, they do a ton of work with ponds. They place masses of water into their landscape strategically, to capture and hold solar energy and mitigate low temperatures. These ponds also reflect light to the surrounding edibles, like a giant mirror placed to boost the sunlight in a garden.

Sepp Holzer’s Farm:

Ben Falk’s Farm:

Those guys and others like them are doing crazy interesting stuff, but all gardeners, at any scale, can benefit from understanding specific heat capacity and thermal mass. Backyard growers can:

• Use Wall-o-Water type products for season extension
• Place rain barrels where they will do double duty as heat sinks and water catchment.
• Use rocks in your garden – from big boulders near a garden border to a pea-gravel mulch in a pot, use rocks to help heat-lovers in cool climates
• Pay attention to soil moisture as it relates to soil temperature
• Install small water features, ponds or swimming pools in places where they will create a desirable microclimate.

How do you hack your climatic limitations?

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Related Stuff and Resources…

(These are affiliate links. Purchases made through these links cost you nothing extra but help me buy Advil after a long day spent digging a pond and moving rocks. Full financial disclosure here. Thank you for your support, guys!)

Wall-o-Water type things. I don’t use these but it’s not because I don’t think they are great, I do! It’s just that I am way too clumsy to fill them without crushing my tomato plants. More nimble-fingered gardeners swear by them.

The Resilient Farm and Homestead – The very detailed and beautiful handbook Ben Falk (the Vermont rice guy) wrote about how to assess sites, create earthworks, work with water, etc. It’s a bit of a textbook in terms of information density, but it’s wonderful. This one’s more applicable to folks with acreage.

Sepp Holzer’s Permaculture: A Practical Guide to Small-Scale, Integrative Farming and Gardening – Sepp Holzer’s book on the techniques he used to build his farm in the Austrian Alp’s, the Krameterhof. I haven’t personally read it, but I hear good things. If you are interested in this kind of stuff, you might watch this documentary on the Krameterhof (YouTube link).

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