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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Greg, at what temperatures do we generally have to worry about damage to avocados and citrus? I had some frost the other morning too, but I am further west & coastal so definitely not a hard freeze here.
Hi Rob,
For the smallest avocado trees I’ve only seen light damage starting around 30. Larger avocado trees don’t notice cold until it’s maybe 28. But much depends on the duration of the cold and some other factors.
Citrus is tougher. I’ve only seen damage on citrus leaves at about 25. But it must get even colder to damage fruit.
I put the bulk of my tropical stuff in front of my house which is against a south facing wall. Stone fruits and such are down the bill on the north side. Seems to work well. We get mangos in September, have one star fruit (carambola) hanging on the cherimoyas, papayas, passion fruit, etc in front.
Hi Bob,
Your place has so many microclimates. You’ve made great use of them. Don’t forget the coconut palm in the front too!
Good memory. Interestingly enough, of the three cocos I planted up front, the two that are under the papaya canopy are doing the best with the one closest to the wall and papaya doing a little better than all three. Also, we had a lavender gem fall so I opened it and it was really good. They’re only gonna get more colorful and flavorful from here on out. Let me know when you’re in the area.
Under anything –
would creating a pvc structure and then placing a cardboard box over it be considered enough? not around the entire area..but just the top portion.
Hi Johnny,
Yes, absolutely that would make a difference.
Hello Greg,
I have a Fuerte starting its 5th year in the landscape. It experienced a rough start with a heat wave in the summer, followed by a cold winter in 2016. fast forward to the present. Our Avcocado tree is in much better health at this time. About 13 ft. high and 6 ft.wide with a nice looking profile. 80% of the leaves are lime green in color even with annual fertilizing.Leaves have been the same color since 2016. Few blossoms and no fruit. Hoping for blossoms and fruit this spring. Any suggestions? Leaves are yellow to light lime green. Large and fully developed.
Alan
Hi Alan,
With regard to leaf color, it can also be affected by rootstock. You can’t change that now, but it’s helpful to know. Overwatering can make the leaves a lighter green too.
If the tree is in lots of sun, it should flower well. But Fuertes can be slower than some other varieties to start fruiting. Still, don’t give it much more time. Some Fuertes are drones and they’ll never produce well. Don’t hesitate to cut your losses and replace the tree if it doesn’t start producing soon.
Greg, I remove all mulch from under a frost tender fruit tree, and before a predicted frost I irrigate the ground under the tree.This enhances the movement of thermal energy(conduction& radiation) from the earth into the tree’s canopy;Thetemperature of the soil is around 52 degrees Fahrenheit .
If you’re dealing with a small frost tender fruit tree (mango) try enclosing the tree during the freeze with a temporary tipi(inverted cone) made from a Dacron comforter.Use clothpinsto keep it closed and bricks to weight down the base.I witnessed a young mango go through a 3 day freeze with absolutely no damage here in the San Fernando Valley !
Hey Greg, I LOVE the calendar – what a wealth of information! I featured you and your gardening calendar in our community published newsletter for January 2022. You may get some orders for your calendars from our neighborhood.
Thanks, Chris. Great to hear!
I love the calendar as well! Happy solstice to all and to all a good night, unless your trees are fighting frost.
Good call on how rocks absorb and re-emit heat. I’ve noticed in the summer that grass at the base of stones is burned away, not just in my yard but elsewhere. What does this mean? Stack rocks around a tree in the winter; remove them in the summer? You are just the sort of researcher to test this idea. I may be, too. Thank goodness for the rain in So Cal!
Hi Greg, what about the water in the pipes? Doesn’t it freeze and blow the pipes? Have you drained your lines?
Questions…
Luckily, we don’t get any frost yet here in southern Portugal.
Thanks, Vasco
Hi Vasco,
My location doesn’t usually get cold enough to need to drain the pipes, luckily.
Hi Greg, same experience here.
We had a couple of frosty nights (-2ºC) in the first January week.
One of the valves (facing north) at main line coming from the well leaks a few drops from time to time and an ice cone formed in the early mornings.
The water in the lines was still flowing.
Thanks, Vasco