Lee Poulsen
Wed, 21 Jul 2004 12:58:00 PDT
I don't know about *all* the Mexican taxa, or that they can all 
reacclimate to Minnesota's climate. But I do know that the Mexican race 
of avocados withstands more cold than the Guatemalan race, and that 
some trees growing wild have been found at fairly high elevations in 
northeast Mexico. Apparently there is no real desire to breed 
cold-hardiness into commercial cultivars, but I did track down a 
variety where the mother tree had been growing in Uvalde, Texas for at 
least 25 years. The original seed was said to have come from an avocado 
growing in the lower mountains of northeast Mexico. Uvalde is in USDA 
Zone 8. Even here in southern California, there are some avocado 
varieties that are iffy if not grown in a fully Zone 10 location. My 
clone growing in the ground at my parents' home in Austin, Texas (also 
Zone 8) with no winter protection survived the two super-freezes that 
hit Texas in the 1980s. During one of those two freezes, the coldest 
night dropped down to 5 deg. F. and did not go above 32 deg. F during 
the next day either. The tree is still growing and fruiting and is 
about 25 or 30 feet tall these days. I believe it may be the furthest 
north that a mature avocado tree is growing outside without protection 
in Texas.

--Lee Poulsen
Pasadena area, California, USDA Zone 9-10

On Jul 21, 2004, at 6:49 AM, Boyce Tankersley wrote:
> Just a quick comment. During the last ice age, the ice sheet stretched 
> across much of eastern North America. This had the effect of shifting 
> the temperate zones well into Mexico. At least some of the higher 
> elevation plants collected in Mexico retain those cold hardiness 
> adaptations and some grow successfully as far north as Minnesota 
> (Quercus polymorpha).
> Perhaps with more experimentation we can learn if all of these Mexican 
> taxa can reaclimate to cold temperatures or if we've just discovered 
> some of the exceptions to the rule.

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