From: pharvey@quack.sac.ca.us (Paul Harvey)
Subject: Hardy Citrus was List of cold climate gums
Organization: The Duck Pond public unix: +1 408 249 9630, log in as 'guest'.
	<1eepk0INNao9@uniwa.uwa.edu.au> 
	<1992Nov19.070101.25938@sserve.cc.adfa.oz.au>
Date: 21 Nov 1992 02:32:46 UTC

In article <1992Nov19.070101.25938@sserve.cc.adfa.oz.au> 
rim@csadfa.cs.adfa.oz.au (Bob McKay) writes:
>Where I thought the guide was over optimistic, Dave's argument strengthens
>the point: if a species can't take that temperature as an overnight minimum,
>it's not going to like it for weeks on end. So it really comes down to the
>two species - pauciflora and stellulata - where I thought the guide was
>underestimating their cold tolerance. In both cases, I'm talking about their
>behaviour in their wild habitat: high (for Oz) mountain areas, with snow
>cover for some months of the year, and with little more Winter diurnal 
>temperature variation than you get in the continental States (probably more
>Winter sun, but that's an issue I've already covered).

The more I think about it, I bet the snow cover is the difference.

>On the other hand, to take 
>Dave's point, Canberra is right on the limit of the ability to grow citrus: 
>there is one particularly cold-tolerant variety of lemon (Meyer) that is 
>reasonably reliable (if you cover it for the first few Winters). You can't 
>grow other citrus in normal circumstances. But then again, citrus are 
>fairly thin barked, and so are perhaps more susceptible to short cold events 
>than are gums - it's all too complicated!!!!!

Ref: California Rare Fruit Growers, Fullerton Arboretum, CSU Fullerton,
Fullerton, CA, 92623, ISSN:1049-4545, Feb, Apr 1992, Vol. 24, #1,2, "The
Hardy Citrus of Texas" as reported by C.T. Kennedy from the notes of
John R. Brown:
Cold-hardy citrus is a tantalizing mirage, so close yet ever so far
away! The genetic material to produce hardy citrus exists in trifoliate
orange, and near-hardy citrus species abound. For many years, the
promise of new and hardy citrus has motivated a circle of
horticulturists - notable William Chapman, John Brown, Stewart Nagle,
and Major C. Collins - in east Texas and Georgia, to produce many
hundreds of experimental crosses in search of a dessert fruit that can
be left to grow in the open ground, far to the north of commercial
citrus regions. During the 1980's, the Indoor Citrus & Rare Fruit
Society conducted a round-robin newsletter among the hardy-citrus
enthusiasts. Since the demise of that society, results of hardy-citrus
breeding in Texas has had no proper journal for the record. C.T. Kennedy
of CRFG has transcribed the recollections of Dr. John Brown, formerly of
Houston and now of Franklin, from notes, correspondence and
converstations of past year's accomplishments.
	The trifoliate orange, Poncirus trifoliata, may be perfectly
hardy as far north as St. Louis, but it is deciduous and produces small,
dry fruits with a repellent, resinous flavor within the fruit. Its
supreme hardiness makes it the ultimate standard of hardiness in
hardy-citrus breeding, and it figures in the parentage of nearly all the
hybrids I mention below. We wish we could impose the hardiness of
trifoliata on dessert-quality fruit, but it is strongly polyembryonic,
and so its hybrids are few and far between, except by using its pollen
on 100% monoembryonic (ie gametic) females. ...

P. trifoliata is hardy to USDA zone 7, average lows of 0oF.

Ref: All About Citrus & Subtropical Fruits, Ortho Books,
ISBN:0-89721-065-4:

Hardiness differs according to species, and sometimes variety, of
citrus. Trees can usually withstand temps 3-4oF lower than those that
will damage the fruit. Also, ripe fruit can withstand lower temperatures
than can immature or green fruit. For this reason gardeners in marginal
citrus areas select early-maturing varieties, which usually ripen before
the first frost. Fully dormant "Satsuma" mandarin trees can withstand
temps as low as 18oF without defoliation. Kumquats may tolerate temps a
degree or two colder. Most other mandarins can take 22-23oF; grapefruit
and oranges 23-24oF; lemons 26-27oF; and limes 28oF. The average
freezing point for mature fruits of "Satsuma" mandarins and "Temple"
tangors is 28oF. Ripe navel oranges freeze at 27-28oF ...
Duration of cold is also very important in determining whether fruit
will be damaged. It usually takes 3-4 hours as 27-28oF to injure navel
oranges, but it may take only 30-60 minutes at 29oF to injure small
lemons.