Overwintering Tropicals, outside

So here is my plan for overwintering tropicals such as banana and pineapple. Keep in mind I am in zone 6, from time to time we can get quite a bit of snow and it is generally windy where we live. They will be planted outside always.

Dig a large are out of the ground that is at least 3 feet deep which should get me close to the frost line, the dug out area will slope from North to South so it will receive max sun exposure in the winter and allow for cooler air to run down to the bottom of the slope.

I plan to put large rocks on the north, east and west sides that will act as thermal mass and wind breaks.

Once the cool weather comes around I will heavily mulch up to a foot around the base of the plants and even cover them.

I also will build a simple triangle shaped cover that will have a glass face that will point towards the south. The back interior of it will be reflective to bounce sun rays back against the glass and intensify the solar energy.

I am hoping that these things alone will keep the plants warm, but I realize that the temperatures may drop below ideal at night or on cloudy days. I don’t know how far it will drop though because I have to test all of this out next winter.

Some ideas I had for additional heat.

-Throw food scraps/compost into the covered area for slow release of heat
-Use a mini wind turbine to power a heat source
-Add more thermal mass, although this will take up more room than plantings so might not be worth it.

Any criticisms or further ideas?

Hi.

I've done quite a bit of out of zone planting here in SE Michigan.  Quite a few years back, I bought a copy of Dr. David Francko's seminal work, "Palms Won't Grow Here, and Other Myths" about his research in growing subtropicals on and around the campus of Miami University in Oxford, Ohio, which is in SW Ohio near Dayton and Cincinnati.  That inspired me to experiment with a lot of things.  Some worked, some didn't.

OK, first and foremost, there is a major difference between true tropicals and subtropicals.  True tropical plants not only can't take a freeze, many of them are damaged or killed by temperatures below about 50 degrees Fahrenheit (the exact temp varies by species from somewhere around 40 to about 55 for the most delicate species).  Subtropicals and plants from moderate temperate zones, on the other hand, can take periods of cold and even freezing, and the lowest temperatures they will tolerate will once again vary by species.  I guess a good example of this would be the difference between the cold tolerance of the coconut palm, Cocos nucifer, which can be severly damaged or even killed by exposure to temperatures in the low 40s for more than a day or two at most, and the cold tolerance of the windmill palm, Trachycarpus fortuneii, which is hardy to zone 7, will routinely tolerate lows in the low teens without damage, and can tolerate brief dips to around zero.  Windmill palms are routinely grown in places that receive snowfall every year, whereas Coconut palms are hard to grow anywhere in the continental US with the exception of extreme S. Florida.

So, pineapples -- well, you'll probably find that it will die from chilling injury and rot after a few weeks at temps below about 55.  My potted pineapple spent some extra time outside here last October because I had emergency surgery October 3rd and wasn't able to bring it indoors in time.  It never saw any actual frost, but it was a very cold October and most days were in the 50s with nights in the 40s and a lot of cold rain.  This poor plant turned yellow and lost most of the leaf tips and some of the lower foliage by the time I was finally able to rescue it.  It has regreened and started to grow a bit again finally, but I could really see the effects of chilling on this plant.

Now, bananas will work better.  People in zone 7 can routinely fruit some varieties of bananas, albeit mostly ornamental ones without useful fruits, such as Musa velutina and Musa basjoo, but some people have had luck with some of the fruiting bananas, such as Musa 'Raja Puri'.  The key with bananas is NOT to let the pseudostem freeze if you want to have them fruit.  If you just want the foliage, it's sufficient just to overwinter the tuber, which is easier. 

I like your concept of a sunpit.  I've used pits filled with dry mulch, covered, tarped, and mulched heavily on top to overwinter subtropicals as tender as Zone 9a species with a lot of success against the weather.  However, the problem I've had here is with voles getting into the pit and destroying the plants.  My particular region seems to be overrun with voles and I've been fighting them for years, so that may not  be a problem for you. 

However, as you point out, the problem with a sunpit is that, like any greenhouse, it will cool off very quickly on nights and during cold snaps.  I actually would like to propose either a modification or a change of plans for you, which is to either augment or replace the solar gain from a clear covering with the heat from some Compact Florescent lightbulbs.  Even though they don't take much electricity and they don't put out a lot of heat, since they are low wattage and very energy efficient, if you are VERY good in designing the structure with a very high R value (30 or over, the more the better) you can trap the limited heat put out by the bulbs with great success.

I found out about this method by reading posts on the Gardenweb.com Palms and Cycads board, in particular those by a man from Zone 4 in the mountains of northern Utah who routinely overwinters subtropical palms like Windmills, European Fans, California Fans, Sabal minor, Pindo palms, etc., outdoors inside of heavy foamboard boxes with about 6 to 8 CF lightbulbs in each one.  These are LARGE palms, too, one of them is about 12 feet tall.  I'd recommend that you go to that forum and do a search for his posts, his username is arctictropical.  He has some awesome photos of his plants and his protective structures.

I moved my entire potted subtropical palm collection into my lexan and brick greenhouse this past summer and decided to use his method as well.  I previously had tried to use the greenhouse over the winter for potted plants and growing, but the heat bills were just way too high to justify it -- cost twice as much as it would have to just buy the plants new in the spring, and I couldn't afford it.

So, this November, I bought a bunch of foamboard and R30 fiberglass insulation, packed the palms in, and put a total of 12 CF lightbulbs in there.  So far, it's worked beautifully, the plants look fantastic, and we're past the halfway point of winter.  I also ran an electric space heater overnight on those nights where the temp was to go below about 10 degrees, which has been about 8 nights so far.  I don't really think I needed the space heater, I was just too chicken to go "cold turkey" without any additional heat on those few extra cold nights.  So far, total additional electric costs over my normal bills for the period from November through now has been about $40, which I certainly can afford.

So, my suggestion for your solar pit is to add some CF lightbulbs and a controller unit (you can get these for about $40) that will turn them on when temps fall below a desired temperature, and to either forget the clear cover entirely and use a heavily insulated cover (the lights will provide all of the light the plants need all winter), or, if you are willing and able to watch it closely, go with the clear cover but also have some kind of backup insulated foam and fiberglass outer cover to put over the clear cover during cold snaps and remove when the weather moderates.

Also, two other things -- when plants are stressed by cold, it's really important to keep them on the dry side, since moisture will promote rot, and moderately water-stressed plants also have more concentrated fluids in their cells, which further depresses the freezing point of the cells, making them less likely to be damaged.  Finally, be sure to use as much rodent control as possible.  In my particular situation, even using a LOT (like 5 or 6 packages) of poison bate in my pit wasn't enough to stop the buggers, but I have a really high vole population here, have had for almost 20 years, after they bulldozed the vacant orchard across from me for development, had a tremendous influx of rodents whose children took up permanent residence.

I hope that this helps you.  Good luck with your project.

What can I say but Wow, very well thought out response and very thorough. I really appreciate that kind of information and suggestions. In response to your CFL suggestion, I was actually looking to build a system that would use wind to power a small generator that would produce 12 volts of electricity. Since I am not good with this kind of thing I am currently looking on how to do it properly. I have undercabinet lighting in my kitchen and the lights themselves can run on a 9volt battery, and with a transformer they run on 12v from 120AC. These suckers get really hot as well, so my thought is wind powered lights since I have a pretty steady wind, they could also be plugged into AC like your CFL’s but the transformer is super expensive. I think the shorter fix is definitely to go with you CFL idea though.

Voles,….yuck! Now I am angry, because of all the times I have seen posts on voles I have ignored them because I have never heard of voles and figured I didn’t have them….well after a little research apparently voles can be considered a field mouse, which we do have! Now I gotta go back and look at some anti vole strategies, so I am glad you said that as well!


Thanks again, this is very helpful!


Edit: Do you have a srouce for the thermostat CU?

Blitz:

I bought the temperature controller that I use about 10 or 12 years ago from a local store of a place called Grainger's that sells parts for various auto, mechanical, farm, and industrial things.  It was about $40 at the time, and is "old" analog type technology, with a mechanical dial to set the temperature, and a metal coil type thermocouple to read the temperature.

I did a quick online search, and found a lot of different places selling various models, ranging in price from about $20 to hundreds of dollars (for professional applications in refrigeration, agriculture, science, etc).  It looks like you can get a pretty good one still in the $30 to $50 range -- now, they are all digital, programmable, and so forth.  Some of the little bit pricier ones ($60 and up) also have wireless remote sensors, which would be good for some uses, but probably not necessary in a small sunpit greenhouse.

I'd check out amazon.com first.  Or google it, try using the search terms "temperature controller" and "thermostor" or "thermocouple" (themostor and thermocouple are two different types of remote temperature sensor probes -- either one is fine).

I don't know if local stores would sell too much of this sort of thing or not -- you might try a farm supply, hardwares, or perhaps nurseries and garden centers that sell greenhouse supplies if you have such a thing in your area.  Odds are, you'll probably have to mailorder for it.

Good luck.
Dennis
SE Michigan

Cheers Dennis

Yes, good info.  (would help if everyone put their location in their profile so we can tell where the advice is coming from.)

Anyway, I am in Zone 9 A I think but it's been hard to tell this winter.  (What!!! Snow in central Florida!!! well it didn't stick to anything but metal.)  A "greenhouse" of single ply without additional heating is not gonna keep true tropicals happy through winter anywhere that snow is likely to cover the ground.  I don't think many of my pineapples are doing well after the cold we have had.  The bananas, papayas, moringa, ginger will all come back from the roots but even stuff in a cold frame got burned where it wasn't well sealed.  I have an aquaponics system under single layer greenhouse film.  tomato plants survived just fine but some of the papayas in there lost more leaves than I would have expected under protection.  And the Aquaponics system is a huge thermal mass with all that gravel and water.  A single layer of plastic just does not insulate and getting enough heat in during the day to warm the thermal mass often causes too extreme temperature fluctuations for the plants to do well.  The recommendation of insulated boxes is definitely a good one depending on what you are trying to do.

Throwing a few food scraps on the mulch under the cover is not going to provide enough heat.  You need a true Hot composting pile to provide any appreciable amount of heat.

I hear ya about the zone thing We have over 4 feet of snow which is rare in our area.

It looks like I need to experiment with temperatures in an enclosed area first to see what kind of temp ranges I get. I imagine that since the roots are below the freeze line they will be ok, but could probably use soil warming.

Ok so I am following the logic of not having a glass top, however I think I can take advantage of the solar heat and not have a glass top. What if my south facing cover was the Styrofoam insulation with a rather large solar heat collector built into it so the excess heat gets pumped into the chamber where the plants are held? Of course the lights would be used as auxiliary. Question, would rocks in the pit absorb the hot air and save it for later as thermal mass?

Look up solar greenhouse because there are ways to capture heat.  It might require some extra stuff to say pump the warm air down into the rocks to help warm them up on sunny days but I'm sure something is possible.

I pump water from an old chest freezer up to the roof though solar pool heating panels and this provides some extra heat but it costs a fair bit in electricity.

If you are short on rocks, you can use containers of water to act as thermal mass.  You might even be able to arrange things so that your solar collector might be filled with salt water and hooked to some sealed water containers inside the box and set it up so a thermosiphon will bring the heated water into the container in the box to warm things up when the sun is shining on the collector.  Look up thermosiphon solar hot water heating.  It can work in a closed loop set up like you would need.  I mention using salt water so that the plumbing outside the box will be less likely to freeze in the panel on cold nights, I don't know if this would be sufficient in your climate though.

I have toyed with that idea as well, and I have plenty of rocks in fact that is the one thing Im guaranteed to have! Saltwater would probably freeze for me so I could use some kind of antifreeze, though I doubt it has the same kind of thermal mass.

Do you guys think any coconuts could live in Zone 10?  It gets down to 32 probably twice a year for a couple hours.  Coconut has to be one of the best foods. Island people can live off of it.

Maybe if the boulders like mentioned above aren't enough, the north side of a pond would be?

I don't know about coconuts since I live in zone 6. But if you did have enough thermal mass and sun exposure you should be ok for those short times, if temperature is the only factor. As for the pond, I believe that the reflection of the sun onto the north part of the dry area is what you are talking about? If so, the sun rays will not hit the ground, but will warm the upper part of the vegetation. I dont think you coud rely on that alone though. If you look at how Sepp does his setup, the sun reflects onto a mass of rock and vegetation. Someone who has been there can correct me if Im wrong please

Rob S. aka Blitz wrote:


...Dig a large are out of the ground that is at least 3 feet deep which should get me close to the frost line, the dug out area will slope from North to South so it will receive max sun exposure in the winter and allow for cooler air to run down to the bottom of the slope.

I plan to put large rocks on the north, east and west sides that will act as thermal mass and wind breaks.

Once the cool weather comes around I will heavily mulch up to a foot around the base of the plants and even cover them.

I also will build a simple triangle shaped cover that will have a glass face that will point towards the south. The back interior of it will be reflective to bounce sun rays back against the glass and intensify the solar energy.

I know I am responding to an old post, but I was surprised to see someone else doing almost the exact same thing as I am here in the North Georgia mountains (zone 7B-8A)
I am using a very similar setup to protect two Satsuma Mandarin trees planted a few years ago. I do have backup heat in the form of two 40watt bulbs, one per tree. These are switched on automatically by a greenhouse thermostat when the temp falls below 30F / 0C inside.

The differences from the setup described above to my setup are that I do not mulch at all (Bare dirt absorbs more heat during the day and releases it at night) and also I have automatic vents that open at 70F / 21C to allow the little greenhouse cool off on sunny winter days, this is to prevent the citrus trees from coming out of dormancy in the winter.
Also I have a plastic sheet front instead of glass. The Back is made of metal foil bubble insulation.
I also put bales of hay, concrete blocks, jugs of water inside to store heat.


I am also over wintering cold hardy banana plants outside (Musa Basjoo / Musa Sikkimensis) with "tee pees" made of plastic sheet and bamboo poles.
These are filled with leaves, pine needles,etc.

I also have a bunch of cold hardy palms (windmill palms, needle palms, fan palm, pindo/jelly palms) the less hardy or younger ones get the same teepees as the bananas.

Nice! I put my project on hold for now, but Id like to pick it up again someday. Thanks for sharing.

"-Throw food scraps/compost into the covered area for slow release of heat."

That can work - compost activity can generate heat. Because the bacteria and fungi in a compost pile are cold blooded, their response to cold is to make less heat, not more.  Large compost piles inside a greenhouse can be very effective, a thin outdoor layer is usually not so effective, a large pile outdoors might keep working through some frost.

Don't know it this will help.  It's an excerpt for Weeds: Guardians of the Soil.  An amazing book, out-of-print but available online at http://www.naturalsequencefarming.com/press/Weeds%20guardians%20of%20the%20Soil%20(3).pdf

Her words brought back days in the Indian country when I often
tramped the woods and hills with a hunter-naturalist who used to say to
me: "If you wanta scare up deer on a blizzardy day, always make for a
weed patch that ain't too close to any house. Weed patches are warm
even in coldest weather, and the deer know it."

One spring back there in those boyhood days I decided to explore the
soil in one of my favorite weed coves, with the hopes of discovering
just why it should be warmer than the surrounding land. My patch
consisted mostly of giant ragweeds, or horseweeds, bordered by annual
common ragweeds and thistles and mint. Being quite young and
unlearned in the science of geology, I at first imagined that Mother
Nature was sending the heat from the interior of the earth as a special
favor to weed coves -- or to the deer and other wildlife that sought
warmth in such coves. But the further I dug into the soil of my cove,
the more I came to suspect that the horseweeds themselves were
responsible for the warmth. How -- I hadn't the slightest idea then.

It was not until many years later that I learned why the deer could
depend on weed coves to supply them with warmth on frigid days. The
soil in such a cove is close to being an ideal organic soil, composed
mostly of plant materials in various stages of decay. And since the
bacteria that are largely responsible for transforming the weeds into
humus are very active and persistent workers when conditions are
favorable for them, as conditions are in a virgin weed cove, a great
amount of heat is being continuously generated. In such situations the
bacteria keep up their work to a degree, even in winter. This heat is the
heat of decay

I use a simple method to protect several satsuma trees here in my orchard in upstate SC.  Put 4 tee-posts around the sides of the tree.  Place several water filled gallon milk jugs around the base of the tree.  Wrap a water heater insulation blanket around the outside of the tee-posts to surround the tree, then pull a large leaf collection bag down over the top of the entire assemblage.  Leave the tree covered until after last frost to keep it from breaking dormancy too soon.  When dormant, satsumas are leaf hardy into the upper teens F, but once they have broken bud, those same leaves (and the new growth) can be killed by any below freezing temps.

basjoos wrote:
I use a simple method to protect several satsuma trees here in my orchard in upstate SC.  Put 4 tee-posts around the sides of the tree.  Place several water filled gallon milk jugs around the base of the tree.  Wrap a water heater insulation blanket around the outside of the tee-posts to surround the tree, then pull a large leaf collection bag down over the top of the entire assemblage.  Leave the tree covered until after last frost to keep it from breaking dormancy too soon.  When dormant, satsumas are leaf hardy into the upper teens F, but once they have broken bud, those same leaves (and the new growth) can be killed by any below freezing temps.

This sounds like the trees are in complete darkness through winter. I know they are dormant, but I would have thought they would need at least a little light.
This is a lot simpler than the setup I use for my Satsumas in North Georgia.

They are in complete darkness for about 3 months, but it doesn't bother them since they are dormant.  Think about an evergreen tree surviving through a winter in the British Isles.  It gets less than 8 hours of daylight with the sun spending most of that time near the horizon and behind a thick cloud bank most of the time.  Very little photosynthesis gets done during that time of year even on evergreens.

For the heat maybe a traditional muck bed would work, but not all plants may like this.
You must look at two kind of things. Do you simply want to have the tree or do you want to have the fruit?
Overwinter a tree is maybe easy,and often trees grow back after frost. But fruit? Tropical fruit require a certain amount of sun and heat what you don't get up North, especially if you put walls around, which will cut out morning and afternoon sun. And then flowers might be destroyed by frost too.
For your trials I would stick to subtropical fruit because they can take a light frost.

basjoos wrote:
They are in complete darkness for about 3 months, but it doesn't bother them since they are dormant.  Think about an evergreen tree surviving through a winter in the British Isles.  It gets less than 8 hours of daylight with the sun spending most of that time near the horizon and behind a thick cloud bank most of the time.  Very little photosynthesis gets done during that time of year even on evergreens.

That makes sense.How old are your trees/how long have they been in the ground? 

I put in the trees last summer and they went through this past winter (a severe one that killed my last big jelly palm) without cold damge in the enclosures.  They are 4 feet high.