Feedback on structure for growing Citrus in Zone 6A

I use alot of citrus. I want to grow citrus.

SPECIFICALLY, I want to grow Lemons (both standard, and Meyers), Limes, and maybe Oranges of some kind.

I was thinking:
1 Meyers lemon
1 "real" lemon
1 lime
1 mandarin orange
1 tangerine
1 pomegranate

(NOTE: I am aware of this gentlemen, and will also keep his offerings in mind)

I live in northern Missouri, in zone 6A, where it can dip down to -10 (or even -13), but usually averages around +15 at night in January (our coldest month). On an average winter, I think it dips to -5 once or twice.

Even so, this is killer weather to Citrus. So, I have a plan I'd like feedback on.

(this whole structure is not near any building - I have no suitable building against which I can place it. Also, no electricity will reach this structure)

First, I intend to dig at least two feet down.
Then, I'll raise up a cinderblock wall (no mortar - just placed dry) and filled the wall cavities with dirt.
(The citrus trees will be espaliar'd against the cinderblocks - maybe two inches away from the wall)
During winter, I'll put a plastic covering over the whole structure.
Finally, during the coldest part of winter, I'll put several cubic feet of cow manure mixed with straw under the covering.

I have a MS Paint image attached demonstrating what I'm thinking.



Does this seem like it'd raise the temperature enough to prevent freezing and killing the trees?

Do I need *fresh* cow manure to compost as a heat-source, or is several month old stuff fine to compost? It'd have to last about 30 days to get me through the worst part of winter, without me turning the compost heap.
I'm also concerned that composting manure in such a tight space may choke the trees with noxious gasses?
And I don't want to burn the roots what with the compost pile being practically on top of the trees (though I could put down cardboard or something first).

Two other thoughts I had was to:
A) place 5 gallon buckets of water between each tree, as solar mass.
B) dig a post-hole an additional two feet down, and wishfully hope somehow warmer air will circulate up, despite knowing it won't.
C) I could also specifically try to find citrus trees on extra cold-hardy root stock (trifloriate or flying dragon or w/e), rather than from StarkBros where I usually get my trees.

Part of the goal is making so I don't have to keep tweaking it every day during the extreme cold weather month - i.e. no daily turning compost piles, no opening/closing extra layers of covering, no starting/maintaining fires.

Any thoughts?

One thing about thermal mass/solar mass: when it gets really cold, that mass will suck the heat out of the room just as readily as it will radiate heat into the room.  If it's -10 outside, and it's 40 inside, those block walls will become a heat sink rather than a radiator.  Now you're not just trying to heat the air around your trees, but you're trying to heat block walls that are -10 on the outside and sucking away what little heat you have around your trees.

That's pretty cold.  If it were me, I'd want some sort of way to keep the temp above freezing in the event of a brutal cold snap.  The strategies you are suggesting might work in the event of normal cold, but if you got a 3 or 4 day cold snap, I think these systems would be inadequate.  Thus, you need a heater of some sort.  Passive solar, compost, and thermal mass aren't enough.  Even if it's something like an old pot-belly stove that you could feed with a couple of big hunks of wood before you go to bed at night, and again when you get up first thing in the morning, that might be the difference between losing all those trees or saving them for another growing season.

Further, citrus trees bear fruit in the cold winter months, so they do not go dormant.  They are still producing sugars to make the fruit taste good.  You need to keep them warmer than just survival temp if you want the food to taste good.

It's easier to heat the air in the room than it is to heat the walls and water.  Yes, air temp rises and falls quickly, but the BTU's needed to keep those block walls and water tanks above freezing is considerably more.  All batteries go from a state of fully charged to uncharged, and then more energy is needed to recharge them.  So just having buckets of water sitting around really doesn't bring much energy (heat) into the room unless you find a way to recharge (reheat) them.  The short days and low angle of the sunlight in Dec. and January will not be enough to make any meaningful difference.  You'll need an additional energy source, as you are constantly fighting against the battery draining power of the cold outside air that is sucking energy from the walls.

I'd say that your strategy might work for November and February, but you need to find an additional energy source in December and January.  Just my opinion.  It would be easy enough to test without trees in the building: just install a thermometer that takes hourly readings and sends them to your computer.  Looking at a spread sheet, is it possible to keep the air above 30 degrees or so without an additional heat source?  If not, how much additional energy is needed?  Would a wood stove give you enough?  Do you have a source of inexpensive wood to fuel the stove?

Those are just my initial thoughts.

Best of luck.
m

Thank you, Marco; I appreciate the shared wisdom!

I have plenty of wood for a stove, though I'd have to buy an ax (and a stove) and cut wood if I wanted anything substantial in size - seems like way more work than I'd want to put in (I prefer upfront labor rather than reoccurring labor, whenever possible). Plenty of half-inch fallen branches though.

I hear what you're saying about the heatsinks actually working against me in some situations - it's unfortunate I won't be able to passively make it work, but such is the nature of trying to fight uphill against the local climate. I can still grow plenty of other awesome things here - figs, apples, peaches, berries, etc...

One of these days, I intend to build a proper greenhouse with a rocket mass stove in it. It seems like my citrus will have to wait until then. =(...

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I also wasn't aware citrus produces through winter - could I not just discard the winter fruit as inedible, and accept mere survival temp for them?
And what if I solely used trees from McKenzie Farms - hybrids that are "cold hardy" down to about 15°F. Would my objective of passive heating be achievable if I """merely""" had to keep the temp above 15°F?

There is plenty of wisdom on this site about building a rocket mass heater with a big thermal mass attached to the exhaust to capture all that energy and then dissipate it into the room over time.  I've never built one myself, but the design is proven (as many here at Permies can attest).  That might be the kind of system that would allow you to bank the heat you need to keep the greenhouse warm throughout those cold winter days and nights.

Most stoves send the majority of their energy right up through the exhaust pipe and out into the cold.  But a Rocket Mass Heater catches and stores that energy in the battery (the mass) and slowly gives it up.  

I realize I've hijacked your thread -- please forgive me.  But if you're looking for an honest opinion, if the only source of energy you're bringing into your system is the heat from decomposing compost, I just don't think that would be enough.

Is it worth it?  Well, nothing like fresh citrus all winter!  I just cut down my lemon tree by 2/3rds and threw at least 500 lemons into the compost, so I wish I could just say "Come on by and pick as much as you need".  

Best of luck.
m

Marco has brought up several ways to make sure your greenhouse has enough heat for citrus. It has been my own experience that using rolls of plastic for a greenhouse holding citrus doesn't work nearly as well as a frame that has either polycarbonate or plexiglass as the glazing.
Sunlight for citrus needs to be more direct than what can get through the milky white of visquene type materials, they do let light in but it is very diffused and that actually leads to less photosynthesis by the tree leaves, which means less energy for the trees.

I love the idea, I don't, however, think it will be a good fit for growing citrus in Missouri any more than it would work in Arkansas.
With the current weather changing, We (Wolf and I) are preparing to build a wood framed, double glazed green house that will be the home for avocado, lemon, lime and mango trees. I am hoping to build it so it looks more like a Victorian conservatory than a modern greenhouse.
I will be building a RMH inside at the center of the structure so I will be able to keep the interior at an acceptable temperature level and it will need to have at least 10 venting windows both at the ridge line and at the footer of the structure for summer ventilation. (think along the lines of the needs of orchids)

Good luck and let me know what design you end up with, even though we are in different USDA zones we aren't that different overall.

Redhawk

I suggest you look into fruit walls. The concept is that they can provide extra warmth and protection.

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

Jamin,

I share a lot of the concerns raised by Marco and RedHawk.  I have two thoughts.

First, this seems like an ideal application for a good RMH built into the structure.  I think that you could use water as a thermal mass, but it had better be hot water.  Water has one of the best specific heat capacities of any common substance.  Perhaps you could integrate water into a RMH to capture and slowly release heat to maintain a survivable temperature.

The second thought is entirely different.  Possibly you could grow dwarf citrus trees in half whiskey barrels, keep them outside in warm weather and then move them in during the cold months.  You would still need a building heated with something, but the actual growing might be easier.

These are just a couple of my thoughts, please let me know what you think.  And if you don’t like, that’s ok too.

Eric