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

See my comments in the fruit and orchard posts.