Keep in mind that not even Poncirus trifoliata could survive the December 1989 or January 1982 freeze in Chicago. The wind chill during the December 1989 freeze in Minnesota was more than 90 F below zero!

OK - this is an interesting post to read. I think the big difference between northern Europe (incl. my area around Vancouver) and the southern United States, are the freezes and their duration. My understanding of the USDA zone system is to predict the ultimate low temperature a given area will experience. Of course there are exceptions to this from time to time. I think I am somewhere in between but have similar issues in the nearly 5 years of experimentation. The occasional freeze events we experience in Vancouver can be - maybe for a week duration and where the temperature does not get above freezing for an entire week. This is very difficult for my citrus to take - except for the poncirus trifoliata.

In the southern united states the freezes are typically a dip overnight then back up above freezing the next day. sometimes the temperature will drop to its low for maybe 1 to 2 hours or even less. We too have frosty nights where the temperature dips to -5 or -6c (21F) then up to +5 or +6c during the day this does not seem to harm my citrus so long as they are protected from the wind.

My big headaches occur when we get covered in a arctic air mass where temperatures stay below freezing and sometimes well below freezing for a long duration. I live in the only corner of Canada that is mild enough to grow about 5 varieties of Palms.

On another note, we were short on snow at one of the Olympic sites, the ski runs were bare snow had to be trucked in . Since the athletes left town nearly 3 metres (9 feet) of snow has fallen on the local ski hills
Go Figure

Greg

I is now going to get obvious what survived last winter. One of my sample of about 15 C. ichangensis seedlings (4 years old) survived with only little damage. All others show severe damage.
PoncirusXSunki-Mandarin had a bit protection from a evergreen plant and has survived quite well. Only sun-exposed parts are dead.
All Citrumelos are dead. Most Citsumas are dead some survived.
One of two Trifeolas may survive.
One positive surprise is, that the scion of Citsuma "Prague" which I had grafted to Poncirus in Autumn seems to live. This Citsuma produces sweet Satsuma-like fruit.
I will have to wait and see.
/Robert

You will never need to worry about citrus psyllids or Huanglongbing disease in zone 7B or colder. The asian citrus psyllid is only hardy down to about 28F. It is a tropical insect that cannot tolerate freezing termperatures for long.

there is something positive in nearly everything
/Roberto

Skeeter, Your sisters satsuma - was it a seedling or a cutting? I have heard satsumas are difficult to root (cuttings). I have tried a few different methods - all without success. We didn't get much frost this winter and NO snow ! - only a few frosty nights in December. My (in ground) satsuma, Changsha and for the first time my Juanita are in bloom now - meyer lemons have finished blooming

Greg

There are 2 big problems in case of hardy citrus varieties.

1. Lowest temperature to survive is only one part, but most important is kind of winter - how many frosty days or hours of permanent frost etc. etc. - for example plant can survive 1 hour of -17 Celsius, but is easily killed by -10 celsius if exposed for 2 days.

2. Problem of taste - I think that ability of poncirus to survive cames from that well known oil, which makes poncirus inedible. So, if we improve taste by crossing it always means, that we reduce presence of that poncirus oil.

btw: the goal is to get fruit. So, because of short season in countries with frosty winters - acceptable plant have to start flowering in spring, and fruit have to by mature in fall - max. end of november.

I would add to the point 1 of Radoslav my observation that some "hardy" citruses are coming out of winter dormancy too quick. Here in Northern part of France we can have very big fluctuations in winter temperatures.
I lost my C.ichangensis during one of the winters when after near zero centigrade temperatures we had for 10 days +15C followed by frosts of -8C. Ichangensis started a new growth and even show the flowering buds but was completely destroyed by this rather mild freeze. Hybrids adapted to this sort of unstable winters should have a profound winter dormancy that is a feature of poncirus and some kumquats.

Fruits of "hardy" varieties are not freeze hardy, probably because they do not contain much of the factors that determine hardiness of leaves and stems. They are mostly protected by the degree of dissolved solids (sugars and acids) that are measured by Brix index. Poncirus fruits start to freeze at -8C.
Oil in the fruits could be one of the antifreezing factors, but clearly it is not playing a major role, poncirus overall hardiness being a multifactorial, polygenic trait.
That means that future zone 7-8 fully hardy varieties should ripen early in the autumn.

I will dare another try this winter. Citrumelo (scion from Ilya -thank you!) on top of 1m Poncirus. Protected by cloth and straw. Please keep your fingers crossed!
/Robert

"Frost tolerant" citrus breeder Don has new web page - looks like he ends most of his breeding projects.
http://hardycitrus.com/

Thanks, I have been wondering what he's been up to. He keeps changing the url of his site.