So I found this article about how the Russians grew Citrus in simple trenches before 1950! For zone 6 I think the 6 ft deep 6 ft wide trench method would work!
And you have their creeping and horizontal training methods to practice in the trench which would protect more wood!
As far as citrus, I am thinking that it would work better than ever with some of the newer colder hardy Satsumas that can take down into the upper or mid teens and things like kumquat that can take 15F. And also the more hardy hybrids like Citrangequat or Citrandarin etc that can take 5 or 10F…
Then on other fruit…
Here in zone 6 a simple 6 ft deep and wide open top trench with horizontal limb training may be the simplest way to protect things like figs, pomegranite, etc… You may not even need to worry about a top cover for things like that that can take single digits… If so, fig and pomegranite grown horizontally in open top trench plantings may have impressive results throughout zone 6 with no top even needed, and even in zone 5 if a top cover is used for winter.
Any thoughts or experiences?
This reminds me of the use in South America of terracing and raised-fields to mitigate temperature fluctuations, particularly at high altitudes.
With raised fields they would alternate raised beds and canals, having the effect (besides irrigation) of the colder air being attracted to the water rather than the soil.
The stone walls of the terraces would absorb heat during the day and then release it into the soil at night.
So many of these older techniques are effective as well as fascinating. Perhaps a combination of different techniques could prove useful with citrus.
Thanks for posting this!
In Florida I have seen trenches dug between rows of citrus. The idea was to collect the cold air in the trenches, but knowing the water table in Florida, it probably also worked like the canals mentioned by @TangTang.
Such trenches will collect heavy gas CO2 (instead of oxygen)? Why not to build greenhouse instead.
But main issue is that this area in Russia has bacteria, fungi, and viruses which tropical plants (or any non-native) mostly don’t have “immune” from.
They still do it in Russia, some people even for tomatoes and potatoes (for earlier harvest)
I liked this article, very full, and “creeping citrus” (low to the ground) is interesting
https://www.lowtechmagazine.com/2020/04/fruit-trenches-cultivating-subtropical-plants-in-freezing-temperatures.html
I personally planted many roses 10" deeper than soil surface, in a hole, in order for them to survive first Winters, and now they are doing great, I covered with sand and leveled
All this happened in Communist times before they started to integrate with other countries; much easier. Export oil, vodka, import lemons, banana.
When we first moved to our little farm, we wanted to build a walipini (like the one pictured below) for year round growing. We dug down about 2 feet and the ditch begun to fill with water. We soon learned that our water table is too high for something like that. It was a bummer, but at least we don’t have issues with not having enough water (looking on the bright side of things).
Ok my Salavatski pomegranite died back even well protected, I thought it was dead but it has recently sprouted from the root. I think I am going to dig it up and pot it and bring it in this fall and grow it in a pot til I get time to dig a trench to plant it in! Then grow it in a russian trench like above pictures!
Hey Paul, did you ever get around to trying out growing subtropical fruit in trenches yet? A couple days ago I stumbled across a recent observation on iNaturalist of a Russian man growing Feijoa (subtropical fruit in Guava family) at 48 degrees north latitude in a trench: Feijoa from Городищенский р-н, Волгоградская обл., Россия on April 10, 2018 at 09:18 PM by Воротилов Антон. После снятия зимнего укрытия!!!!! · iNaturalist that made me think of your old post on the subject. Please update us on any progress.
The other thing to remember is any trench deep enough to hold a tree can be a pretty dangerous undertaking. Without proper support, a trench will eventually collapse, sometimes suddenly and potentially with you in it. Proper support is not a DIY project.
A greenhouse would be more cost effective than a safe trench.
I really don’t see an uncovered trench providing much protection. But it probably won’t take much of a cover to greatly improve upon uncovered.
A south facing slope would allow for a greenhouse that was partially underground. If all of the north wall and part of the east and west walls were underground that would add protection.
If you look at the old photos of the Soviet trenches posted above, you can see the wooden covers set to the side. They only uncovered them during milder, sunny weather, I believe.
I like the idea of a partially underground greenhouse on a south-facing slope, but it likely would have similar problems with collapsing without proper support. Soil wants to flow downslope!
It’s a cool idea, but probably one of those things that only makes sense in very particular circumstances.
Greenhouses at the time were probably much more expensive since they had to be made from glass, whereas these trenches were made from wood, land, and lots of labor, things the Soviets had plenty of.
Add to that the unique diplomatic situation making importing more difficult, and how important mandarins were, and still are, as a holiday food in that region, and you get a pretty big gap between supply and demand.
But for us folks, it’s hard to imagine a similar situation, especially with plastic, treated wood, fiberglass, etc, making greenhouses extremely affordable and things like incandescent Christmas lights making gentle, targeted, low cost heating easy as well. That, and the historically insanely low price of most tropical and subtropical fruit.
The biggest problem I see with the trench or walpini is that in winter when you would want sun from the low angle (greenhouse walls) you wouldn’t get it.
I think @a_Vivaldi got it right that these make sense in very particular circumstances.
I see that close to the equator the walpini is a good choice. A farm near me in VA has two large ones and I have been wondering every time I drive by them what they are using them for…. Maybe I’ll ask next time I go by!
For the trench design, you could line the inside of the covers with reflective material (mylar? doubles as insulation), and rig them to open to the north side with the ability to fix it open at an angle that matches the sun angle.