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The Impact of Microclimates...block walls in Phoenix and the last Peach orchards in Paris

Over the last 21 years that I've lived in Phoenix, I've realized we do some strange things here in the desert, mostly because we don't understand our climate.  And we don't understand climate because we have modern conveniences, like air-conditioning and as much water as we want right out of the tap.  This mitigates the effect of some of the design "Don'ts" that are so common here.

Like block walls surrounding our yards...

Like MY block wall.  The block wall that I spent thousands of dollars on.  Sure, it's pretty, but what I didn't realize is that I was that I was basically increasing the heat in my backyard by building it.

Why?

Block walls:

--block breezes and airflow

--trap warm air in the yard

--store heat in their thermal mass and release it during the night when temperatures drop, adding to the Urban Heat Island effect

--If the wall is a light color, it can bounce reflected light around in your yard and burn nearby plants.

Am I anti-fence or anti-wall?  Nope.  But if I had it to do all over again, knowing what I know now, I'd do things differently.

Like what?

I'd install an open fence - something in wood, or metal - even chain-link.  Then I'd plant thorny plants on the outside in the alley (natural burglar protection) and vining plants ON the fence.  Inside the fence, I'd plant some screening shrubs and some overstory trees that some of the taller vines could grow up into.  This would give me some natural AC as the plants would transpire and the breezes would flow through.

So a lot of people don't think that microclimates make all that much difference.  How much hotter could it be near a wall with a lot of thermal mass?  Well... it can get hot enough so they can grow peaches in Paris - not an area known for peach growing! 

Check out this beautiful blog post at Messy Nessy Chic.  She chronicles the history of peach growing within the city of Paris - a booming business back in the 17th century.  The peaches produced were coveted by royalty as far away as Russia.

How'd they do it?  They manipulated their microclimate.

All those things that make block walls a problem here in the hot desert, make them ideal for heating up a cool climate.  Here are a couple of quotes from the blog:

"The peculiar architecture, known as “Murs à pêches”, wall for peaches, served to protect peach trees planted near the walls and adapt them to a much colder environment than the fruit is typically used to."

"The 3 meter high walls were more than half a meter thick and coated in locally sourced limestone plaster, giving them a high thermal inertia and the ability to store heat. Their intentional north-south orientation allowed solar energy to be stored in the walls during the day and transmitted to the trees during the night, preventing them from freezing and accelerating the ripening process. Within these walled orchards, temperatures were typically 8 to 12 °C higher than outside." 

Let's revisit that last part "temperatures were typically 8 to 12 °C higher than outside".

That's 14 to 22° F higher!  And Paris is in a "gentle" climate zone.  They don't get up into the 100's for months on end like we do.  So I can only imagine what the temperatures are like on my south and west facing walls when it's 112° F. 

The upshot of all this?  Microclimates matter.  A lot. 

Visit the blog for more lovely shots of the Peach orchards of Paris.

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Good points. I'm with you. My neighbor replaced his fence with a solid 8 ft block wall and we share it... It is so hot, I've had challenges growing anything near it except citrus and pomegranates. In the summer, I can actually feel the temp change of like 20 degrees within a few feet of the wall. It's brutal. I am planting sugar cane along the wall this year to buffer the heat a little.  We'll see how that works- If only my Thrice Mulberry would hurry up and get shady...

The other walls in my yard, I put a reed fence in front of. I think the reed fence helps. If I could ever talk the neighbors that I share fences with into chain link, I think I'd follow your lead.

Now let's talk about driveways... I'd love to get rid of the cement. What are some alternatives?

Yep - I've used citrus to mitigate the heat coming off of my south-facing and west-facing walls.  They can take the heat plus it's a good micro-climate for them in the winter.  Let me know how the sugarcane works - that's an interesting solution.

Reed fences - yep - that would help mitigate some of the reflected light which could be a problem in the summer.  It would also stop some of the heat absorption too as sunlight would not directly fall on the thermal mass.

Driveways.  Yeah...  So I know that Nick Irvine (water harvester extraordinaire) took out his cement driveway, dug it down a bit and backfilled with woodchips from a local arborist.  Essentially his driveway became an infiltration basin for his front yard.  When the chips started to decompose, he'd just top them off with another woodchip delivery.  Added bonus, the carbon locks up some of the pollutants that may drip from a car.  I believe he did get pushback from some neighbors who wanted to know what the heck he was up to.

My neighborhood is old enough to have the two strips of cement as a driveway.  I took those out a long time ago and now that side of the house is where my deciduous orchard is (shady in winter).  I reserve a small graveled parking pad in front.  If I had it to do over again, I'd do woodchips like Nick did.

4' chain link. Blackberries for the alley.  Grapes and kumquats for the neighbors.

As for block: Paint it white helps a lot.  Reflects more light into yard to help with partially shaded areas.  I have grape vines growing great up against it (cooling it further) --- too early for grape production yet.  Going to have to figure out a wire trellising system for it this year.  And this is a 8 foot high western facing wall to boot!

Painting walls white (or other light color) does help reflect sunlight away from the thermal mass, alleviating heat gain and is particularly useful for houses.  Your AC bill will drop in a light-colored home.

For walls surrounding gardens, white paint can reflect that light back into the garden which is OK for winter, but problematic for summer (the "solar oven effect").  Some fruit trees (and fruit) can be damaged from reflected light if they are planted to close to the reflective source.  It's like that practice of painting the bark of fruit trees white to reflect light away from the wood to stop sunburn.

Grapevines - do you have them on southern and western exposures?  I've grown them on those exposures but not right up against a wall.  I'd be interested in how they do.  Do they leaf out early enough that they mitigate the reflection/heat coming from the wall?

"Some fruit trees (and fruit) can be damaged from reflected light if they are planted to close to the reflective source. "

I read that too.  Doesn't seem to be a problem for me, thankfully. Mine are a mix of cherry, pluot, plum and apricot trees about 7 feet from the wall some with grape vines in between a foot or so from the west facing block wall.  Though my best growing grapevine ("Blueberry") in the yard sits next to it completely exposed.

I think the fact that your trees are 7 ft away helps a lot.  And the grapevines in between - even moreso.

I'm curious to know if your grapes leaf out and cover the majority of the wall before the real heat of summer sets in.  I think grapes can withstand more reflected light than say, a peach.  Which wall is it? (what exposure). 

And I would love to see some pics of your blueberry grapevine!

Ok. Mine is an 8 ft West Facing wall that is in full sun. I think it's about 40 feet long or so. I think I might try painting it white. That part of my yard really suffers because of that wall.

Is it a house wall or a garden wall?  I ask because, at least from what I've experienced, you'll need to mitigate the reflected light off of light colored garden walls somehow - usually by planting something really hardy in front of them.  For a western wall I'd plant something that is evergreen in front of it like citrus if you have the room.  I now also recall hardenbergia doing well on a west facing wall.

I've also noticed that my light walls reflect and bounce light off my gravel pathways too.  The glare gets pretty intense even with some tree canopy.  Gravel pathways are something I would NOT choose to do again if I was to do things over.

I use these "Lady Banks Roses" around my fence where I grow out trees for our property. They are virtually care free except for the chopping back. I use them for mulch too. The thorns help protect my young trees from varmints digging & chewing them up.

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Nice, Wayne!  How much water do you give them?  I have had trouble getting mine to establish - lost 2 out of 3 on the west side of a north-south wall even though they were shaded with shade cloth all summer.

I don't have to water them, but I give them a drink every few weeks in the summer, and once a month or so in the winter. They do like to be fed & mulched pretty heavy. I have them on my N.,E., & W. fences. They also drape over the other side almost to the ground.

I took that pic this morning. I wish they bloomed more than once a year, but than I wouldn't want to chop them up for mulch. I get quite a bit of biomass out of these beasts.

Don't shade them. For me, they grow best in direct sun & temps over 90*. They grow faster the hotter it gets. 

Wayne, I agree with you.  Rose banksia thrive on neglect it seems.  A lot of desert or desert-adapted plants do.  I don't know how many desert-adapted plants I've killed because I paid too much attention to them (over-watered, over-shaded, etc).

I finally had to remove a rose banksia because it got shaded out.  But in a sunny spot, WOW - they are awesome. 

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