Two nights ago was chilly. There was frost on the ground in most parts of my yard, but not everywhere. Which spots had less frost – or no frost – and why?

Have a look at this two-minute video I made on the frosty morning of December 16, showing four factors that create a range of temperatures out in my yard. These same factors are likely at work in your yard too.

Now for some brief explanation of the factors that create the winter warm spots:

Up hill

Cold air flows like water; cold air flows downhill. Because of this, areas higher on a hill are warmer.

South-facing wall

The fall and winter sun in Southern California is low in the sky and toward the south. Anything facing south, such as a wall or large boulder, therefore collects the most of the sun’s energy each day during these seasons. A planting site near such a south-facing side of a wall or boulder will be warmer than sites on other sides.

Under anything

At night, the warmth from the earth rises and disappears into the sky. If any sort of roof can trap this rising heat, then it will keep the area between that roof and ground warmer. For example, a house roof overhang acts as such a trap, as does a tree canopy, which is like a giant umbrella. (An actual umbrella can work the same.)

I have a thermometer under the eave of my front porch and I have another out in an open part of my yard. That morning of December 16, 2021 dipped to 32.5 degrees on the porch but down to 29.3 degrees out in the open part of the yard.

South- or southeast-facing slope

The sun shining on ground that faces it will gather the sun’s energy and warm more. So in winter, south-facing ground is warmer than north-facing ground (or east or west).

Also, the sun rises in the southeast in the morning during the fall and winter. The coldest time on a winter night is usually right before sunrise. If a plant is on a piece of ground with a southeast aspect, then it can get an immediate shot of warmth from the sun as soon as it rises, thereby cutting short the coldest part of the night.

How to make use of these four principles? Obviously, you ought to consider them when choosing planting sites for plants that don’t like cold nights. And you can consider them when choosing a place to temporarily protect plants that are in pots on cold nights (like my plants under the oak tree).

But don’t forget that some plants want cold nights and that you can use these same principles to find colder spots for such plants. For example, I planted all of my apple trees at the very bottom of the hill in my yard, as apples do best with as much winter cold as we can give them in Southern California.

Related post: “Protecting avocado trees from cold”

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