Friday, July 15, 2011

Chilling Requirement for Plants

Apple Tree in the Snow

As we are entering the hottest part of the year (in the northern hemisphere), I thought I would discuss a much cooler topic... Chilling Requirements for Plants.

There are certain plants that require cold temperatures to produce fruit.  This is why, for example, apples are not grown much in Florida.  Apple trees require a minimum number of hours below a certain temperature, and Florida just doesn't get cold enough for long enough.  Now there are always exceptions to the rule, and certain apple varieties have been developed to grow in warmer temperatures, so if you live in southern California, you are not without hope for growing apples.  However, we need to be aware of what our chilling hours are and what our plants need to fruit reliably.

Chilling Requirement is a term used to describe the idea that a plant needs a period of cold to blossom.  The requirement is usually expressed with the interchangeable terms Chilling Units or Chilling Hours.  One unit or hour is equal to one hour at or below the chilling temperature.  Some plants have a Chilling Temperature that is below freezing, some others may need to be under 45 F (7 C), and others only need to be under 60 F (16 C) for example.  Every plant has a certain chilling temperature.  If a plant does not obtain its required chilling hours it either may not flower at all or will flower much less and therefore produce a lot less fruit.

How Ice Wine (or Eiswein in German) is made... seriously!

Now, there are two stages of chilling.  I always consider the first stage of chilling to be like a baseball pitcher's windup.  The first stage is reversible.  As the season starts to cool down, the plant is getting ready for its period of dormancy.  If the temperature warms up for a few days or a week or so, then the plant gears down from its dormancy preparation.  It's like the pitcher stepping back off the mound.  It's a do-over.  No damage is done.

The second stage of chilling is like the pitch.  It is the point of no return.  It is irreversible.  At some point (a certain temperature or a certain temperature for a specific amount of time), and this is very difficult to tell, a plant has committed itself to dormancy.  Even if things warm up, the plant will remain dormant until other triggers cause it to break dormancy.  If the temperatures are low enough for long enough throughout the winter, then the plant will be able to blossom well in the spring.  If it is not a very cold winter, or if we have a plant that should be growing in a colder climate, then we may get little or even no flowering... and then our fruiting is poor.

It is important to know that the Chilling Hours do not need to be consecutive.  Typically, the plant just needs cumulative Chilling Hours.  We may have nightly temperatures that drop below the required threshold for our plant, but our days warm up above that temperature.  If we have enough nights doing that, then that may be enough for most plants.

On the flip side, some plants have hair triggers to break out of dormancy.  It the temperatures rise too high for too long, let's say in an uncommon warm spell in the late winter, then a plant will wake from dormancy and may start production of blossoms.  When temperatures drop again, the new growth may be damaged or killed.  This is what causes concerns about late frosts or early blooming plants in a garden.

Chilling Unit Map for the U.S.

So how does this affect us and our plants on a day to day basis?  All it means is that we need to select plants that are suited to our climate.  That's it.

We need to find out what our Chilling Hours/Units are for our area and then select plants that fall within or under our cut-off.  I could only find one map that provided general chilling hours for the U.S.  I did find a number of local state maps, but you will have to do a little looking on your own to find your specific local information.  Most plant sellers will have this information for you if it is important for the species you are looking to purchase.

Plants that have a Chilling Requirement to produce blossom well

  • Pretty much all fruit that grows in a Temperate Climate (Apples, Blueberries, Cherries, Grapes, Peach, Pears, Plums, Strawberries, and many, many more)
  • Oranges and other citrus (although they don't really go completely dormant, the chill produces more flowers and better tasting fruit)
  • Many vegetables need some chill to produce seeds (Cabbage, Carrots, Celery, Sugar Beet, and many more)
  • Almost all Bulb Plants
  • Many seeds need a period of cold to sprout.