High Elevation Gardening Web Log "Blog"

Many parts of Northern Arizona experience killer spring frosts that limit fruit production. Though our average daytime temperatures are similar to other parts of the country, our nighttime lows often freeze flower buds. Before flower buds begin to open, they are pretty hardy and can handle our cold winter temperatures. But once the buds break dormancy and start to open, they are far more susceptible to cold. For apples, 90% of open blossoms will be killed if the temperature drops below 25°F for just one-half hour. Other types of fruit blossoms are slightly more susceptible than apples to cold temperatures. However, most other fruits including apricot, peach, pear, cherry and plum bloom many weeks before apples so their flowers have a greater chance of getting nipped by frost.

Why was 2006 an outstanding year for fruit production in Flagstaff?

To get some insight to my question, I went to the local weather data that’s available on the internet and looked at temperature and moisture data for 2004, 2005 and 2006. Weather data collected by the Arizona Meteorological Network (AZMET) are available for the Flagstaff area for the last three years. The station is located in the Continental Country Club area and the data can be found at cals.arizona.edu/azmet/. You can also get weather data from the National Weather Service at www.weather.gov/.

Why did the killer frost that occurred on March 14, 2006 do so little damage to fruit tree flower buds? Temperatures hovered around 0°F in the wee hours of that morning. It’s likely that the unusually dry winter conditions delayed bud opening. The mid-March snow storm eventually provided much needed moisture to allow for buds to break dormancy and start growing.

A look at the temperature data confirms what most of us know – we received few killer spring frosts after mid-March. A couple of nights in March dropped down to the teens but for less than an hour so minimal damage occurred.

Remember the fruit crop of 2005? There wasn’t one. On March 16, 2005 temperatures dropped into the single digits likely killing all opened blossoms and damaging developing buds. Perhaps the wetter than normal December and warmer than normal January and February encouraged early bud development increasing susceptibility to killer frosts.

2004 was another really good apple year though not so for other earlier flowering fruit. On several mid-March days the temperature dropped below 20°F for several hours likely damaging the just opening blossoms of apricot, peach, and cherry. Later blooming apples had yet to open their blossoms and thus retained their winter hardiness. Subsequent April temperatures seldom dropped below freezing.

Temperature has a lot to do with why fruit trees bear a crop or not but why is this year’s crop so heavy? For an explanation, we have to look at what happened the previous year when flower blossoms started to develop. Remember that in 2005 there was little if any fruit crop. This allowed for the carbohydrates produced that season to go towards new bud initiation and storage instead of fruit growth.

Back to 2006, this fruit season also benefited from a milder than normal winter that limited winter damage to flower buds. Nighttime temperatures rarely dipped below 0°F.

Once bloom occurred successfully, conditions were excellent for pollination. There were many days in April were temperatures reached 65°F which is perfect for bee flight to ensure good pollination. In addition, the slightly warmer April temperatures likely helped with pollen viability.

If you are curious about how fruit production will fair on your property, get a maximum/minimum thermometer and compare your local temperatures to those posted in the paper or on the internet. This will give you an idea about the microclimate of your property. The warmer your gardening site, the greater your chances of success.

Knowing that fruit production in Northern Arizona can be frustrating and highly unpredictable, I have yet to give it a try. My thanks go out to all the friends and neighbors who dropped many bags of cherries, peaches, plums, pears and apples at my doorstep. Receiving such gifts is almost as rewarding as growing it myself.

By Hattie Braun
The author is the Master Gardener Program Coordinator for Coconino Cooperative Extension. If you have a gardening question, call the Master Gardener hotline at (928)774-1868 ext. 19. To learn about local gardening events, visit the Master Gardener blog at highelevationgardening.arizona.edu.