milehighgirl:

Chill hrs for apples are a rather nebulous factor. Quite a few will grow and fruit with much less chilling than traditionally thought. And if you are talking apples for zone 5b, you can grow any apple with no concern about chilling. I assume you are in the Denver area. That area has twice the chilling needed for any apple.

The Fruitnut

Maybe I'm mistaken then, I was looking for apples with a very high chill hour rate so that they don't break into bloom too early. In Denver we often have warm days followed by a blizzard. So will higher chill hours help to prevent this?

No, higher chill won't help. You get way too much chilling. Any apple in your climate will be primed and ready to go by Jan or Febr at latest.

But there are apples that bloom later than others. My latest in Amarillo was Rome. But it's not much of an apple. Quite a few nurseries give tables listing relative bloom dates, quite often for pollination purposes. Raintree has such a table in their catalog.

A new apple I'm going to try is supposed to bloom very late. It's called Suncrisp. You can buy and read about it at Adams County Nursery.

The Fruitnut

Wow, I can't believe I was so mistaken. Does this apply also to peaches? I have selected a few very high chill hours peaches for the same reason. I just assumed that high chill also meant late bloom!

milehighgirl:

With stonefruit there is some reason to think that higher chill may help. Certainly you wouldn't want the very low chill cultivars. But the highest chilling peaches and nectarines I've seen are about 1200 hrs. That should help in some areas of Texas where chilling ranges from 100 to 1200+ hrs. But will it help in Denver where you probably have 2000-2400?? Probably not much.

With peaches the approach to avoiding spring frost has mostly been to chose cultivars that are more resistant to low temp at bloom. Two I've grown that fit that bill are Surecrop and Contender. Once made a few peaches with Surecrop after two night in a row of 13F at full bloom.

I'm in an area with ~700 chill hrs. Chill hrs or whatever does have quite an influence on bloom date here. 200 chill hr peaches bloom in Jan or Febr. High chillers in late Febr or early March. But the freezes that get us are usually late March or the first two weeks of April.

Another factor that is involved is the heat requirement a cultivar has after chilling has been satisfied. Grapes have a very low chilling requirement, about 100hrs, but a very high heat requirement to initiate spring growth. As a result they bloom late.

The Fruitnut

I wouldn't worry about it. You have enough hours to fill any apple and the tree won't instantly burst into flower once it wakes up. It will still look for proper conditions.

Chill hours are a concern for those who don't get enough.