Chill Out!

Chill Out!
How many times have you heard that expression in your life?!
It was coined about 1980 or so and means to relax, calm down or go easy. The dictionary doesn't give the meaning of the phrase the way I am using it today. I am going to tie it into horticulture.
Chill, relates to cold. In the world of plants, cold can be a death nail. To some plants and trees however, it is essential for propagation and the future. I am, of course, referring to Chill Hours or Chill Units. These are an approximation of how many hours of weather between 32 degrees and 45 degrees a plant requires to properly go dormant so it can wake up and blossom and/or set fruit. If you really want to get fancy and be ready for an appearance on Jeopardy, this is called vernalization (which comes from the Latin vernus, meaning Spring). As a side note, surprisingly, there is no additional benefit from lower than 32 degree temperatures.
Deciduous fruit trees, bulbs and some other plants that go dormant during the Winter need a minimum number of these hours. However, the problem is, it is very difficult to measure chilling hours precisely. If you are looking at a catalog of fruit trees, the listed chilling requirements are, as I said above, approximations or estimates. Of course, every location has a different amount of Chill Hours. Here in Charleston, SC our average is 400-600 hours. Central Florida receives between 100 and 300 hours. Up in New Jersey they have about 800-1000 chill hours per year. So what does this all mean to the average home fruit grower? PLENTY!

Let's use Apples as an example. They have a pretty wide range of chill hours, (200-1,800) but many of them need about 800 hours. I should also add here, nursery catalogs often will contradict each other. One may say that a certain tree needs 700 hours and another says the same plant needs 900 hours. In such cases, it is better to use the higher number to reduce the potential of not meeting chill requirements. So, using the information above, let's say I wanted to grow Winesap Apples. They have a Chilling Hours requirement of 800-900 hours. In New Jersey, they would do great. Here in Charleston and down in Florida, not so much.  Why you ask?
If the flower buds do not receive sufficient chilling temperatures during Winter to completely release dormancy, trees may develop problems such as delayed bloom, delayed leafing out, reduced fruit set, or no fruit set and reduced fruit quality. When I tell people this, they then usually ask: Well, if I plant a low chill apple, say Anna (200-300)in South Carolina won't that work? The best answer I can give them is "maybe". Remember I said this is all averages. If a tree only needs 300 hours and it is growing in a 400-600 hour area, what happens when the 300 hours are reached and there is a warm spell. The tree breaks dormancy thinking it is Spring and begins to flower. Then old man Winter returns with a icy cold snap, killing the flower buds...result, no fruit. Now, if it stays relatively warm, the tree will be fine and you will get fruit. As a general rule of thumb, when looking at a range of chill hours, stay within about 100 hours either side of your area.
Confused yet?
The sad part is, this kind of information is rarely printed on plant tags, so home gardeners are left in the dark about the proper fruit tree selection. This information also goes for Plums, Cherries, Peaches, Apricots and many other fruit trees and plants.
I am not going to knock any of the big box stores, but before you buy any of their "great" Spring fruiting trees, make sure that you do some research and see if it will do okay where you are. I have a story, somewhat related, about Blueberries. There was a store in Charleston selling Northern Highbush Blueberries. As you might guess, NORTHern Highbush will not do well here in the SOUTH!
This type of thing is, hopefully, where my blog and I can help. If you are unsure of something, please feel free to ask. My e-mail is TheCitrusGuy@netzero.com
I hope I have cleared this often confusing topic up a little. If you have any questions at all, please use the e-mail address above. When it comes to growing fruit, one of the biggest things you need to remember is...Just Chill Out!
Happy Growing!
Darren