Now that I have your attention. Here's the real deal---I have a 16 by 32
greenhouse with a Subterranean Heating and Cooling System (http://www.roperld.com/hycg/SGHwSHCSBuild.htm) in
which I've planted citrus, figs, palms, camellias, tea roses and tons of other
stuff. Some items are in pots but most are planted directly in the ground.
Because I am in such a cold climate (-53F is the all-time low here in Lone
Rock, WI) I've tended to select the very hardiest cultivar/species of each type
of plant. For example the palms are Rhapidophyllum, Sabal minor McCurtain Cty. OK, Sabal
xbrazoriensis and Trachycarpus sp. The
reason that I've done this is because I don't yet know how cold the greenhouse
might get if it's 40 below outside and the electricity or LP furnace
malfunctions.

This winter is the second season of operation and I've easily
kept the temperature just above freezing (my winter goal) with a mere low of
-20 outside. But during mid-winter the
greenhouse day temperature might not rise much above freezing if the weather is
cold and cloudy. I do have to have some
midwinter cold because certain plants I have need a proper winter cooling
period. Because the SHCS stores heat in
the greenhouse soil mass, a full winter of operation might cool the soil
significantly by spring. Sunny winter
and spring days rise into the 60's and 70's regularly. I'm not sure how near-freezing temps for
several days might affect different avocado cultivars or how low soil temps
might affect different rootstocks.

I'm thinking that I could stuff in one (or perhaps two)
avocado tree(s) if I keep them pruned heavily although naturally smaller trees
would probably be better. I want great
fruit and I'd like the/(one of the) tree(s) to fruit over the summer and fall
to coincide with our tomato season (guac rules!) After reading a lot of online
material some things come to mind….

One variety that might work would be 'Mexicola' (not 'M.
Grande'). It's relatively cold hardy and
good tasting but doesn't appear to fruit until later in the autumn. (Sources vary a lot of course.) On the other hand then I wouldn't have to
worry much about a January freeze wiping out the over-wintering crop of one of
the 'Hass' types. 'Stewart' sounds like
a better tasting Mexican type. It's
relatively compact but has problems setting fruit in many areas? (including my
greenhouse?) Either of these could make
a hardier scaffold to have other varieties grafted to them eventually for
pollination, variety, etc.

Another route would be to go with one of the 'Hass' types
and not worry too much about hardiness issues.
But the usual (somewhat) compact suspects, 'Holiday', 'Wurtz', 'Gwen'
and 'Reed' are among the least hardy trees although the latter three would
likely bear some fruit in the summer if all else goes well. But these ripen the year after
flowering. Will the fruit continue to
ripen over a chilly winter?

And what about 'Day', one of the Florida types? Generally I don't like the Florida avocados
but 'Day' is supposed to be hardy to 22F and I read that it's creamy and more
like the California types. Can anyone
tell me from experience what 'Day' is like?
Or if it ripens the same year that it blooms?

Even if my eventual selection is not 18 degrees hardy like
'Mexicola' I'd like to have something that will tolerate freezing to some
degree. I also read that 'Gwen' has a greater
need for cross pollination than most others.
Should this be a deal killer?
Other than the 'dwarves', I could
go on and on with pros and cons for various cultivars based on what I've read,
but at the end of the day I guess my questions is still, "What's a girl to
do!?" Any suggestions, thoughts about cultivar selections would be
appreciated. Steve H. (loneroc1) at Lone
Rock, WI USA USDA zone 3

My biggest concern besides freezing the top would be cold soil temperature. Your soil in winter will be in the 30s for months. My expectation is that might cause root disease issues.

I've chosen not to try many subtropicals and no tropicals in my greenhouse because of either production issues or fruit eating quality issues. In a cold winter climate you'll get better fruit quality with crops native to those temperatures. I've gotten superb eating quality on stone fruits of all kinds, grapes, blueberry, and trailing blackberry.

A detailed writeup here: http://www.davewilson.com/home-gardens/growing-fruits-and-nuts/cultural-practices/greenhouse-fruit-growing/greenhouse-fruit-production-in-west-texas

I didn't measure soil temps this winter. I wasn't able to heat the ground over the summer so I figured it would be a good test. I shoulda measured anyway I see now. This year I'll track temperature rises and falls. The citrus on Flying Dragon and Swingle have shown none of the discoloration or leaf drop I'd expect from near freezing soil temps. I'm thinking that I'll be able to keep soil temps in the 40's. Any suggestions for a decent soil thermometer? I should probably track changes down to at least two feet.

The soil has stayed cold enough that figs are still dormant even though air temps have hit the mid 80's. I planted the figs in root control bags to control vigour. They'd out compete other things of left to their own devices.

I'm not worried about losing the avocados. Plants are cheap in the scheme off things. I just need to figure out what varieties I should try to kill first ;-) In my 50+ years as a plant hoarder (it took a while to reach a diagnosis) I long ago stopped taking plants' demise personally.

Soil temp will be near average air temp. Near freezing air day and night means near freezing soil. You could warm it slightly above that by passive means but not by much in WI winter.

Loneroc1, I have a similarly sized GH with SHCS and might be able to give you some data, although my climate is somewhat warmer and considerably sunnier than yours.

I did run heat this winter, as I have some citrus in the GH and did not want it to go below freezing. Tests last year showed that it would stay at 27F or so even when temps got to -10F. However your minimum temps will be much colder than that. Strange, but I have not thought to check soil temps; probably something I should do next winter. Winter's not over yet but it looks like I will have used ~100 gals of propane. That (plus a bit of electricity on exceptionally cold nites and when the gas heater died on me) kept the min air temps at 45F most of the time, getting down to 40F on really cold nites. At this level of heat, the citrus did fine, and I kept one tomato plant going thru the winter (still doing well). My fig tree (potted) is just breaking dormancy now.

There were a few weeks with no sun, but I think the vent fan ran some every week of the winter. Even at 20F outside, on a sunny day it would hit 90F inside very quickly. So my SHCS could so some recharging of the soil heat bank throughout the winter. Not sure if you will get much of that where you are.

With an SHCS your soil temps should start out warmer than the air. Some mulch over the soil would help keep that heat in, but that may be counter productive in terms of using that heat to keep the air warmer. You might be able to partially use mulch, around the plants which require higher soil temps, but still get radiant heating from the rest of the soil (or supply some additional heat). Do you run the SHCS fan at night in the winter to extract heat?

One thing I did was use two independent source of heat (electric heaters and ventless propane). Each being a backup for the other. At some point I would like to add a wood stove, to have a third option in case both the primary heat sources go offline.

Sorry, no input on avocados.

Thanks for the reply. I have a small area for orchids that I cover with fleece and have a little electric heater for underneath out for their protection. I'm changing over my epiphyte collection forward more frost hardy stuff a la Santa Barbara Orchid Estate. I have a ventless lp heater too. 30,000 btu's. It puts out more water vapor than I'd like but the SCHS seems to pull some humidity out of the air (and puts some in during summer). I'm thinking that I want to get me one of those Southern Burner vented greenhouse heaters. I'm not sure how much less efficient it'd be though. A ventless heater should operate at 100%. I have a pretty large area of single poly that'll be replaced by triple acrylic (the UV transmitting kind) and a lot more insulation to put in. It should help a lot with heat retention. Just add shingling, siding, sheetrock, tile, more electrical, two more large glass units, better and more bench space and an eight foot tall outswing patio door and that's it!! :-) But, the major plantings were finished two years ago and have settled in well. Except for the avocados of course. And there's that Moro Orange that I've been thinking about. And that Finger Lime................:-) You know how it goes. I figure it'll be another year to finish things up. I was injured last summer and will only have limited use of my right arm going forward. I'm going to start to have to hire some stuff to be done. It throws a wrench into the budget. But who wouldn't think that figs and peaches that cost a couple of hundred bucks a pop (or is it thousand?) are worth it? Steve H.