How Important Are Chilling Hours for Fruit Trees?

Q. This is really great information that you put on your blog!! Thank you so much for doing this, and it gives me a bit more hope about expanding my home orchard in Phoenix. 

http://xtremehorticulture.blogspot.com/2015/11/special-fruit-tree-orders-no-longer.html

http://xtremehorticulture.blogspot.com/2011/04/recommended-fruit-tree-varieties-for.html

How many chill hours are you getting where you are located? Specifically related to Pluots, you are recommending some with some pretty high chill hours, such as the Flavor Supreme which is listed on DWN as 700-800 chill hours. But as you mentioned above that may have a lot to do with location and micro climate.

Also for Apples, Pink Lady is a pretty late ripener, and I’ve heard that late harvest don’t do as well in the extreme heat of Phoenix, something similar to where you live. But again, I'm hoping the placing them is certain cooler places within my backyard and trying to create a micro climate that is a bit more forgiving to these fruits could have a large impact as well.

Would love to hear you take on the points point. Thanks again for putting some much time into this.

A. Thank you for appreciating the hundreds of hours that it took over a period of one and a half decades to put this list together. Many kudos to the Master Gardener volunteers who supported it.  I will try to address each of your questions separately.

            Most of the trees used were donations by Dave a Wilson Nursery, Tom Spellman in particular, who worked with us since 1996. This orchard is located in North Las Vegas Nevada at right about 2000 foot elevation.

            The orchard is exposed to the North West by cold winter winds which helps explain why citrus was not included on this list even though it was tried. Las Vegas is not citrus country but many people further south in the Valley have had success with some citrus and even limes and blood orange in warm microclimates.

            Our chilling hours are somewhere between 300 to 400 hours per year. You are exactly right. If you take chilling hours literally many of the recommended trees should not produce in this climate yet they have for over 15, now going on 20, years. Some of these so-called high chilling hours tree fruits have shown no sign of a lack in chilling hours.

            Tom Spellman was the first person to bring this idea to light for me. It challenges many preconceived ideas about chilling hours and there has been much speculation and even disbelief in this information.

            Personally, I believe that chilling hours are more important in some types of fruit and even some varieties than others. Is chilling hours important? Yes, definitely. Have we followed chilling hour recommendations to literally? Yes, definitely.

            Too many, Phoenix and Las Vegas have similar climates if you don't live in either of these locations. You and I both know that as far from the truth! Phoenix is a totally different animal from Las Vegas but I believe there is a wide variation in how plants view this difference. I believe most apples are more prone to chilling hours than peaches, nectarines, apricots and plums and their crosses.

            I don't know all pink lady apple performs in Phoenix but it is an outstanding Apple in the Las Vegas area. Like pomegranates, this particular Apple seems to have better flavor quality as temperatures drop. In our harsh desert climate it seems to develop a thicker, tougher skin but the flavor quality of the "meat" is superior. It is worth giving a try in Phoenix, Yuma, Parker, Bullhead or Lake Havasu.

            I think planting apples in areas of the landscape that avoid the late afternoon sun is a good idea. Contrary to some information out there, if your soil has lower amounts of organics in it, amend it with compost at the time of planting. I also believe you will see huge benefits if the soil is covered with wood chip mulch under its canopy to a depth of at least 4 inches.

            Keep the mulch away from the trunk for the first five years of the trees life to avoid collar rot. If rabbits are problems, protect the tree from rabbit damage with 1 inch hexagon with chicken wire for the first five years as well.