How much of a temp increase can stonework provide fruit trees?

Hi, first post.

I'm in Missouri, in zone 6A, and I've planted two figs a few weeks ago, which I'm sure will be doing an annual dieback + regrow, but will otherwise be fine (unless we get a particularly bad winter - then they might die entirely). It'd be nice if I can keep their temperature such that they don't need to dieback at all, though.

Further, I also see that there are species of pomegranate (Wonderful Pomegranate, and Red Russian) that can kinda-sorta-almost grow in Zone 7 (again, I'm in zone 6A).

I was wondering how I might create a microclimate around those trees (my existing two figs, and the two imaginary pomegranates that I don't have but would like), to give them a boost and help them survive and thrive. My "tree area" isn't anywhere near a building or stone wall to plant against.

I was thinking, what if I poured some flat-ish concrete bricks (as thermal mass), with black coloration (to absorb more sunlight), and lay it down around the trees? They'd be sloped to catch and run water towards the trunk.

This was what I was thinking:


How many degrees (Fahrenheit) do you think that might raise the temperature during winter?
I understand it doesn't actually generate heat, but absorbs heat during day and and releases it during night, "averaging" out the heat. I'm just trying to get a ballpark estimate to know what I can and cannot plant.

But what about when they get covered by snow? Yes, they'll melt the snow most times, but what if it snows enough that it remains covered for the next day or two, or on particularly cloudy days, so the bricks aren't able to absorb sunlight, creating a single extra-cold night or two? Is two hyper cold nights enough to a kill a tree, or does it require a longer period of extreme cold for a tree to die?

It'd be great if I was even able to plant some lemons too (and oranges and limes and kittens and jelly beans...), which are more like zone 9. For citrus trees, what if I also wrapped their 2-4 t-posts surrounding them a foot away in a plastic drop cloth, to create a temporary "greenhouse" over them, just for the first two or three years?
(Note: Meyer's lemon won't cut it. I need a more sour lemon species)

If you were in my Zone 6A, how would you experiment with growing citrus fruit outdoors? Year-round, outdoors, I mean. Planting in a pot and bringing it in every year is not what I'm after.

Jamin Grey wrote:Hi, first post.

I'm in Missouri, in zone 6A, and I've planted two figs a few weeks ago, which I'm sure will be doing an annual dieback + regrow, but will otherwise be fine (unless we get a particularly bad winter - then they might die entirely). It'd be nice if I can keep their temperature such that they don't need to dieback at all, though.

http://www.lowtechmagazine.com/2015/12/fruit-walls-urban-farming.html

Google search for UK Fruit Walls

I hope some experts come along with good answers for you.  Does your ground normally freeze in winter?  Would you expect these raised pots to freeze (barring any stonework)?  

I wish there was a resource to say what mechanism of coldness kills each type of plant.  Is it the roots freezing?  Branch tips under a certain diameter freeze and die?  Is it excessive temperature changes while dormant?

I think the more mass you have the better because it can hold the heat longer into the night.  I think the mass shouldn't be too thick or the heat won't fully soak in during the day.  I think a back stop of cement (facing south) would give you the best "surface area of mass" to "surface area of sunlight".  I think if you can use the heat of the earth around the plant it will help.  Tenting a greenhouse over the plant or frost blanket would definitely help, especially if it has a wide foot print to expose more of the warmer earth to the air around the plant.  Blocking the wind should help.  

Side story, I've heard of people trying to insulate their figs in Chicago by wrapping them with fiberglass insulation and then plastic.  But they wrap them up like a corn dog (tight at the trunk) so the only thermal mass inside of the insulation is the 6 ounces of fig wood.  I think if they tented them at the base to include several square feet of soil it would help greatly.

@J Anders: Thanks for the link to the Fruit Walls. I've heard a little about those before, but that link went into much more detail and was very interesting! The serpentine walls in particular caught my eye.

Building a full wall won't work in that particular location; I guess I'm looking for something easier and cheaper that can help the trees that *almost* make it by not quite - the figs (which merely dieback), and maybe pomegranates.

At some point, I'll want to build a full greenhouse, and I strongly suspect lemons and other citrus will have to wait for that. =(...

@Mike Jay: I actually don't know if my *ground* freezes , but we certainly get below-freezing temperatures, averaging around 10° in January, and sometimes dipping quite a bit lower. The lowest I've experienced here after multiple years is -5° or so, but the historic record was something like -15°.

I would suspect the raised wooden beds (with bottoms open to the ground) are likely a degree or two colder than the surrounding ground, just because of potential wind-flow around them. Haven't ever measured it though.

What do you mean by "a backstop of cement"? Also, how could I use the heat of the earth?

My fig trees - like most my trees - have multiple t-posts about a foot away from them, so they can be supported using the "stanford tie" method. Most then have chicken wire wrapped around the tree (about a foot radius away) from t-post to t-post to keep deer off.
My "solution" for the figs this first year, just to ensure the plants survive since they were just planted, was to wrap the chicken wire, and up over the tree, with a plastic drop cloth, and to fill the chicken wire and up over the young 3'-4' tree with leaves. However, part of my issue is low maintenance.

I don't want to do this every year - it's only to protect it for it's first winter. I'll let it just die back in future winters. But.... if there is some "permanent" solution that doesn't add yet another annual task to my tasklist, I want to put in that labor - if financially cheap. I rather spend sweat than money, but prefer spending neither, and enjoy putting in up-front labor that provides long-term no-maintenance benefits.
Even if I can just raise the temp by 5°, that seems a worthwhile improvement, and if a particularly horrendous winter comes by, may mean the difference between life and death of some trees.

(I already mulch the trees, and have spiral white trunk-guards on them - which I check every year to make sure they aren't constricting them or accidentally holding in moisture)

Jamin Grey wrote:@J Anders: Thanks for the link to the Fruit Walls. I've heard a little about those before, but that link went into much more detail and was very interesting! The serpentine walls in particular caught my eye.

Building a full wall won't work in that particular location; I guess I'm looking for something easier and cheaper that can help the trees that *almost* make it by not quite - the figs (which merely dieback), and maybe pomegranates.

At some point, I'll want to build a full greenhouse, and I strongly suspect lemons and other citrus will have to wait for that. =(...

@Mike Jay: I actually don't know if my *ground* freezes , but we certainly get below-freezing temperatures, averaging around 10° in January, and sometimes dipping quite a bit lower. The lowest I've experienced here after multiple years is -5° or so, but the historic record was something like -15°.

I would suspect the raised wooden beds (with bottoms open to the ground) are likely a degree or two colder than the surrounding ground, just because of potential wind-flow around them. Haven't ever measured it though.

What do you mean by "a backstop of cement"? Also, how could I use the heat of the earth?

My fig trees - like most my trees - have multiple t-posts about a foot away from them, so they can be supported using the "stanford tie" method. Most then have chicken wire wrapped around the tree (about a foot radius away) from t-post to t-post to keep deer off.
My "solution" for the figs this first year, just to ensure the plants survive since they were just planted, was to wrap the chicken wire, and up over the tree, with a plastic drop cloth, and to fill the chicken wire and up over the young 3'-4' tree with leaves. However, part of my issue is low maintenance.

I don't want to do this every year - it's only to protect it for it's first winter. I'll let it just die back in future winters. But.... if there is some "permanent" solution that doesn't add yet another annual task to my tasklist, I want to put in that labor - if financially cheap. I rather spend sweat than money, but prefer spending neither, and enjoy putting in up-front labor that provides long-term no-maintenance benefits.
Even if I can just raise the temp by 5°, that seems a worthwhile improvement, and if a particularly horrendous winter comes by, may mean the difference between life and death of some trees.

(I already mulch the trees, and have spiral white trunk-guards on them - which I check every year to make sure they aren't constricting them or accidentally holding in moisture)

Why won't a full wall work at that particular location? Would a small greenhouse work? Clear plastic pipe over the trunk might make the difference? Anything you can do to make a micro climate would help.

Jamin Grey wrote:What do you mean by "a backstop of cement"? Also, how could I use the heat of the earth?

I meant kind of a cement slab version of the backstop for a baseball field.  So maybe a foot to the north of the plant you prop up some cement slabs as a wall.  It may reflect light/heat to the plant during the day and will hopefully direct thermal radiation at it during the night.  If the wall of cement is curved it could protect the plant from more wind and catch early and late sun as well.  Much like the serpentine walls in J's link.

The heat of the earth comment went along with the following sentence about tenting plastic or a frost blanket over the plant.  If you cover the plant with something like a 1' diameter tube, the tube has a lot of exposure to the cold air and a 1' diameter area exposed to the warmer earth.  If that plant was covered with a pyramid shaped covering, much more of the interior of the "greenhouse" is exposed to the warmer soil and would help keep it warm.  Without trapping the heat of the earth under a blanket/film/plastic material, I don't know how easy it is to use it to your advantage.

Nobody around here put figs in planters, they but them in the ground for extra protection. They die back to about 18inches (1.5 foot).
I throw a old chainlink gate over mines in the winter (1st snow). It bends the fig plant down to the dirt, others even add some leaf litter/stone/straw.
Come spring (April 15th-last frost), remove the weights and it will naturally correct its orientation in a month and then resume growth.

Some years I just let it winter prune to 18inches, other years I protect the Chicago hardy fig. But I always get to harvest and eat some fresh figs.

J Anders wrote:Why won't a full wall work at that particular location? Would a small greenhouse work? Clear plastic pipe over the trunk might make the difference? Anything you can do to make a micro climate would help.

Two concerns:
1) A greenhouse or wall seems like substantial cost and sweat for a single tree, based on the spacing of the trees in that area. That'd absolutely make sense in a different context, which I'd love to explore in the future when I build a greenhouse, but I'd at that point try to maximize the benefit of the wall or greenhouse. e.g. build a greenhouse and layout the plants around it, vs plant trees arbitrarily and then trying to build a wall or greenhouse.
2) There would be some aesthetic concerns - my trees are visible from the road, about 400ft away, and having a bunch of trees look normal, and then having a weird structure over one or two trees seemingly at random wouldn't really fly.

I was hoping that maybe by just improving the soil temperature around the roots (which would be cost-efficient and not noticeably "odd"), the whole tree would benefit. But I guess that's just wishful thinking on my part?

...would clear plastic pipe over the trunk help? That's a possibility. I've never seen or heard of clear plastic pipe for any size larger than 1/2". Branches would be pretty low, though - I'd imagine once the tree grows some, the lowest branch would be only 3-4 feet off the ground.
It'd have to come in two halves, so I could take it off in summer, I guess? And heat would escape through the top of it... But that'd certainly beat wrapping the trees every winter, convenience-wise. Would it actually benefit anything? What kind of pipes are you thinking of?

S Bengi wrote:They die back to about 18inches (1.5 foot).
I throw a old chainlink gate over mines in the winter (1st snow). It bends the fig plant down to the dirt, others even add some leaf litter/stone/straw.
Come spring (April 15th-last frost), remove the weights and it will naturally correct its orientation in a month and then resume growth.

Some years I just let it winter prune to 18inches, other years I protect the Chicago hardy fig. But I always get to harvest and eat some fresh figs.

By "winter prune to 18 inches", do you mean it dies back to that naturally, and self-prunes, or do you prune the dead branches off come spring?

Even after protecting the tree multiple years in a row, is it still able to bend down smoothly enough? Or do you eventually need to let it die back, to grow a new more flexible trunk?

I'm fairly confident the figs I planted (Chicago hardy, and another species nearly as cold-hardy) will do fine after the first year; I was just wondering if there's some long-term microclimate benefit I could give them without too much cost or labor, or that might permit me to plant some trees from warmer zones.

For the really warmer zones, I'll have to do as J Anders suggests, and build a fruit wall / greenhouse combo. But for trees I can *almost* plant, only a zone away...?

There is a book 'The life of a tree' or similar that describes how trees grow, share food etc with other trees and many other unbelievable issues.
It may discuss the issue of why trees freeze
from google
trees hardly ever freeze to death. ... Half of a tree's weight is just water. So once winter hits, that water turns to ice.
The trick is that trees work to prevent the water in their cells from freezing

Yes it self-prunes and die back to 18inches naturally.

Fig plants are super flexible, like you I was afraid to just a chainlink gate on top of it and then just stomp and jump on top of it. For some reason it doesn't kill the tree.
I grow the fig plant as a multi trunk/stem, 50% of the fruits on it doesn't get to ripen before 1st frost do the tree over. Most years I just let it winter self prune because I dont want to baby a tree.

This year I harvest my 1st bitter orange. Harvesting figs and 'orange-relative' fruit makes me feel happy.

Other fruits that winter self-prune.
I have 4 cultivars of muscadine grapes. I harvest and eat from 2, another one is super vigorous but no fruit for me and the last one basically dies back to the dirt for the past 3 winters. They all self prune back to 3ft or less so I have built them a low arbor.

Asian persimmon, the top 12 inches dies back but the plant is as tall as me now, HUGE amount of blossoms and bee, way too much fruit drop and the fruits could be more flavorful. So I built a rock mound, snake habitat.

maypop passionfruit dies back to the root too, and it sends up suckers, only 1 fruit ripens for me, but the location is shady and I could probably warm up the soil, but too much babying.

Pomegranate is my least appropriate plant. I have two different cultivar and while they survive, sometimes self-pruning back to the ground, I have never eaten a fruit from it. But I just say, that it is hard to kill.
I have given up on them and over the years I have stepped on them, broken off all branches, dug up some roots, even dug it out and just dropped it bare root on the ground and it did not die.

I once planted a hardy banana in the fall (November) it didn't make it to spring, I might try again.

Thanks S for the tips!  I assumed I couldn't grow a Chicago hardy fig up here in zone 4a.  But I do get reliable snow cover all winter.  I wonder if I flatten it like you say and let a foot of snow insulate it, maybe it would make it.  The worst temps it would see is in November before the snow accumulates where it might see a low of 15 to 20.  Do you think I'd have a reasonable chance?

Jamin, if you planted a Chicago Hardy fig, why are you worried?  I believe they are good down to zone 5 so you should be fine without protection.

@Mike Jay: You might even try bending it over and burying it in loose dirt; some people in very cold climates do that.

I'm not worried about the fig surviving, but if I could give it just enough of a boost to not need to dieback at all, that would be a sweet bonus.

Moreso I'm wondering about the trees that would survive but won't produce: e.g. if I got pomegranates.
Same with sweet cherry trees - if I can give them a few degrees improvement, they might behave better.

S Bengi wrote:This year I harvest my 1st bitter orange. Harvesting figs and 'orange-relative' fruit makes me feel happy.

How did it taste?

Did the tree have thorns on it? Were the thorns a nuisance?

Sticking my neck out here because I have little experience or scientific testing but the first year I planted my figs I surrounded the trunk with square hay bales (18" tall) leaving just a small air gap to prevent rot on the trunk.  Wrap them in black plastic to absorb sun and keep the soil much warmer.  My saplings were maybe 3 ft tall.   I'm in zone 7a-b but we do hit sub-freezing temps multiple times each winter and they did fine.

Also LOVE LOVE LOVE the fruit wall article j Anders!   I'm saving that for future reference, meanwhile it reminded me of this Youtuber beekeeper in Massachusetts who made temporary walls with T-posts and foil covered foam insulation boards.   His bees survived and he demonstrates the temperature differences.

Hi, I just wanted to ask, how it worked out. I'm thinking about citrus trees in Zone 7A...

B Georg wrote:Hi, I just wanted to ask, how it worked out. I'm thinking about citrus trees in Zone 7A...

What I ended up doing was sticking three T-posts around my fig trees, with chicken wire on the T-posts - basically a big cylinder around the fig trees. I filled the cylinder with fallen leaves, burying the fig trees in leaves. And then I covered the entire thing with some cheap clear-plastic painter's dropcloth available for like $3 bucks.

Unfortunately, despite doing all that, for the past two years, the figs still went through their annual winter die-back and regrowth in summer. This year I'm not going to bother to cover them, and hope they survive on their own, but still do the dieback and regrow.
It's possible that the area my orchard is planted, at the bottom of a small hill, might be a microclimate a few degrees colder than Zone 6A.


Eventually I hope to grow citrus myself, but I'll likely need a hoop-house with geothermal *and* a rocket-mass heater for citrus to survive. I'm in Zone 6A.