High Chill Apples?

[PlanterTomato.com]

A few years ago, I purchased some apple varieties like Cox's Orange Pippin and Ashmead's Kernel etc. I later discovered that they are considered "high chill" varieties and was disappointed, thinking that they would not grow in my area (northern California, USA - Hardiness Zone 9).

As it turns out, my trees did blossom and fruit. I compared average monthly temperatures in my area to England, the place where these "high chill" apples originate. I'm not seeing a big difference between England and Northern California (see below). So what am I missing here? What makes these varieties high chill? Most places in the US would qualify as high chill if these varieties can grow in zone 8 or 9.

Northern California (Palo Alto)
January Avg Low: 41° Avg High: 47°
February Avg Low: 41° Avg High: 49°
March Avg Low: 43° Avg High: 53°
April Avg Low: 46° Avg High: 58°
May Avg Low: 50° Avg High: 64°
June Avg Low: 56° Avg High: 70°
July Avg Low: 59° Avg High: 73°
August Avg Low: 60° Avg High: 74°
September Avg Low: 56° Avg High: 67°
October Avg Low: 51° Avg High: 60°
November Avg Low: 46° Avg High: 53°
December Avg Low: 42° Avg High: 47°

England (Manchester)
January Avg Low: 36° Avg High: 44°
February Avg Low: 36° Avg High: 45°
March Avg Low: 39° Avg High: 49°
April Avg Low: 41° Avg High: 54°
May Avg Low: 46° Avg High: 61°
June Avg Low: 51° Avg High: 65°
July Avg Low: 54° Avg High: 69°
August Avg Low: 54° Avg High: 68°
September Avg Low: 51° Avg High: 63°
October Avg Low: 46° Avg High: 56°
November Avg Low: 41° Avg High: 49°
December Avg Low: 37° Avg High: 45°

[macmanmatty]

Apples have been found to require much less chill than originally thought to leaf out bloom and fruit. Also They can't even agree on what constitutes a chilling hour because very little is understood about them. ( I can think of at least 4 methods of calculating them of the top of my head.) So if you don't know how many degrees constitutes a chilling hours how can you accurately measure how much chill trees require. So they don't A lot of these numbers are just guesses and they are all the time changing. many apples have been found to leaf and bloom with less than 200 hours and they are working on growing apples in africa with a program to help the native people there.

[OrangePippin-Richard]

Chilling hours are hours spent at temperatures *just above freezing*. Most of the colder US climate zones are therefore, in a sense, too cold - because winter temperatures are well below zero for long periods.

In England winter temperatures are usually around or just above freezing for most of the period December - February, which makes England a perfect high chill climate. On that basis the cooler areas of California are quite close to England in terms of winter temperatures (but certainly not summer temperatures).