Fruit Trees and Their Chill Hours

Red Baron Peach Tree(1 of 2 in the garden)

There are posts and then there are posts.  I consider this following write an important post for those who don't know the rules about "chill hours" and are considering adding fruit trees into their gardens.  This topic is especially important for those who live in warm or tropical areas around the world.  It was an education for me last year as I began to plan my fruit tree garden in several sections of El Presidio.  I hope this information is helpful and answers some questions about the needs of fruit trees.....

If you were in the desert southwest during this extreme freeze last week, you may have had several ways of looking at this cold snap.  1.  Oh boy, cover the plants!  or 2.  Yes!!  My fruit trees are getting their chill hours! Or 3. It was a mixture of both feelings which I suspect was the case for many gardeners.  Chill units (or CU) are important for the development of fruit on trees.  Temperatures, anywhere from 32-45 degrees, are important for the development of buds,etc on our fruit trees.  If we have a mild winter, it is likely that we won't have a lot of fruit developing on our trees....if any!!  Chill hours are the amount of hours needed for a fruit tree to produce a crop of fruit and it is essential we get those cold nights for a good harvest:)

Yummy Plums...don't forget to have a cross pollinator...check your varieties to make sure you have the best match!  Plums grow in Tucson, but you many times need another variety of plum tree(or the same) to create that delicious fruit!
Fruit trees are incredible because many times they create beautiful flowers and/or scents in spring.  In summer, they provide shade and in several cases fruit!  In fall, the leaves will drop  and in winter, they create artful structures with their branches in the garden.  So let's go back to chill hours.....here is a technical definition for this fascinating event! "Dormancy is triggered by the lengthening nights and dropping temperatures during fall and early winter. Physiological changes take place that allow the cells in some plants to survive without damage even when exposed to very low temperatures. We can think of this as nature’s anti-freeze. A period of time at temperatures between 32 deg F and 45 deg F is required to negate the effect of these physiological changes. The length of time required varies plant to plant depending on the climate to which they have become adapted through evolution, selection, or breeding. Once this length of time has passed, the plant is technically released from dormancy and can return to normal growth once cultural conditions have become favorable. If temperatures remain low, the plant may remain suspended in a dormant state. However, once the chilling requirement is met, the soil temperature need only rise above 45 deg F for 3 – 4 days to initiate growth. Most stone (peaches, plums, etc) and pome (apples, pears, etc) fruits have a chill requirement. Even some bush fruits, such as blueberries, have specific chill requirements. To be successful, a fruit grower must know the average number of CU’s(or nights below 45 degrees!) they can expect in a normal winter in order to select varieties that will successfully bloom and set fruit in their location. If a variety requires significantly more CU’s than they can expect, it may never bloom or set fruit. However, if a variety requires significantly fewer chill hours than they can expect, it may break dormancy and bloom too early. A hard freeze during bloom will damage or kill the flowers and no fruit will be set. Damage may occur to leaves and tender tissue as well. It is very important to know your Chill Hours Average and to select only those varieties that will grow and produce fruit in that range. Keep in mind that these are averages and that yearly extremes may temporarily affect production. You may find that you can locate micro-climates within your yard that vary by as much as 7 -10 degrees. This may permit you to push a chill zone one way or the other in these areas. You may wish to keep frost cloth on hand when growing fruits in the lower chill hours ranges. If killing frosts or freezing weather threaten a tree that has broken dormancy, the frost cloth may protect the blooms sufficiently to set fruit."

Source: http://www.thegardenacademy.com/fruit-gardening-classroom/understanding-chill-hours

Here are some personal observations and experiences that I have with my apricots, plums, peaches, apples, nectarines, and pears on my property.  One.  Before you buy, research at your local garden centers what fruit trees do well in your area.  Don't be lured into buying that cool looking fruit tree on the stands at Lowe's or Home Depot without researching....you'll waste money.  Many times they will have plants for our area, but sometimes they slip in some enticing things that just won't grow in our areas.....like oh I don't know...those infamous lilacs!  Research first....then shop:) 2. Once you've found your trees or bushes, look at the chill requirements for these plants....some will say 500 or 450 hours.  For some areas in Arizona, like Phoenix, they don't always get that many cool hours at night.  A fruit tree there might have something like 250 chill hours while Tucson can and usually does range between 300-500 chill hours. When purchasing the fruit tree, check the chill hours needed. Different varieties have different CU requirements. Do not purchase anything over 500 chill hours unless you just want a green plant in the garden....which is possible:) An area like Prescott would have a fruit tree that allowed for 500 or more chill hours like say, for example, 800 hours.  Higher altitudes=higher hours of chill=different varieties of fruit trees 3.  Once you've put your fruit tree in the garden, make sure you don't plant them in extreme shade.  A lot of desert gardeners think that fruit trees won't grow in our desert and that simply isn't true.....peaches and apricots thrive here!! So plant in full morning sun with afternoon protection from the summer sun.  Again it depends on the variety.  Sun is needed to ripen the fruit...the lesser the sun; the lesser the sweetness of the fruit.  I grow Red Baron peach and Blenheim Apricots.

And wait....I'm not done:)  4.  When your trees start to blossom, and they will, it is very important to protect the trees from our fierce wind storms in the spring.  If a blossom is blown off, you will have lost a potential fruit.  5.  Finally, when fruit starts forming, you can do several things to prevent the birds from eating them.  Take a small paper bag and cover the fruit or put a net around the tree.  Some gardeners believe that leaving some fruit on the tree for birds is important as they believe it is a give back to nature kind of thing.  Plus they are probably bird lovers:)  Some key notes on fruit trees.  Some self fruit while others do not.  Self fruit means that they will produce fruit on their own while others need a "cross pollinator" from a same type plant.  The plum is an example of this.  Check your varieties for best cross pollinations.  I've put a Satsuma Plum next to a Santa Rosa Plum.  There are also things to consider when purchasing peach or nectarine trees...are they freestone, clingstone, or semi-freestone?  Freestone just means what it means....is the pit of the peach attached to the fruit or is it easily removed from the fruit?  If the pit can be pulled out easily from the center, it is called a freestone variety.

As you can see, a lot of planning goes into a fruit tree, but it is also worth the time and effort.  So while you are shopping at the garden places, educate yourself on what you want in your garden and what will actually grow in your own backyard. I will share a quick story with you before I leave my writing for today.  I won't be able to write on Saturday as we will be in Sierra Vista and later to see a perfomance of Wicked!!!   Here's my story....

Last year, I planted A LOT of fruit trees around the garden.  PS. The plum trees were the last to leaf out and the last to drop their leaves.  However, all the fruit trees produced several large fruits on their branches.  I watched as a neighbor walked past the tree and saw a peach.  The smile on her face as she reached and took a bite from that peach made everything I do in the gardens worthwhile.  It was so sweet and juicy that it dribbled down her chin.  There was an excitement amongst the neighbors as they all started to discover various parts of the garden with different fruits....limes, peaches, lemons, etc.  Someone recently found my secret hiding kumquat tree!!:(  I personally hide my favorites in places where some people can't get to them as easily.....like the plums.  What the cold recently did to my fruit trees was create large and healthy buds on the branches which will open in March.  You MUST protect your trees once they start budding and leafing out from last minute freezes....which can happen during this last month of potential frost. March here is the sign that it's time to green up!  Until next time, happy gardening!