Chill Seeker: Some Fruit Trees Are Very Picky about Temperatures During Dormancy

Southwest Yard & Garden

By Dr. Marisa Thompson

Dormant peach trees in February 2019 at the NMSU Agricultural Science Center at Los Lunas. Late frosts after budbreak nipped almost all of the blooms in these trees this year. Photo credit M. Thompson.

Question: I’m trying to decide which fruit tree varieties will produce well in my yard, and many of them list a specific number of required chill hours. Where do I find out how many chill hours we get in Roswell?

-          Question submitted via Chaves County Extension Agent Troy Thompson

Answer: Many gardeners know that certain seeds need to be cold stratified before they can break dormancy and germinate. This makes sense on a survival level because seeds that drop at the end of the growing season might germinate and grow in the fall or winter and not stand a chance in the cold. Some seeds require other environmental triggers or a combination of factors to break dormancy, like a very specific moisture content within the seed or exposure to certain amounts of light. Cold stratification in the wild involves exposure to the elements, and in captivity it’s easy to just put seeds in the fridge for a few weeks or months. On a deeper level, enzymes synthesized in response to temperature and other environmental fluctuations affect production of plant hormones within the seed, like abscisic acid, and this is what really triggers release from dormancy.

OK, let’s switch this discussion from seeds to trees. Similar mechanisms in the dormant buds of certain trees dictate when budbreak occurs and trees bloom or leaves start to expand. Temperatures are known to be detected within the buds. Not all seeds require cold stratification—peas, for example—and not all trees require chill hours to break dormancy. Most plants that evolved in the tropics don’t go into dormancy at all because those seeds and flowers wouldn’t normally be harmed by frost.

Apricot flower bud just starting to open on March 10, 2018 at the NMSU Agricultural Science Center at Los Lunas. Unfortunately, several frosts nipped these blooms in the bud and there was no fruit set that year. Photo credit M. Thompson.

When I was first learning about chill hours, my predecessor Dr. Curtis Smith explained that chill hours are THE determining factor for the flowering time of many fruiting species. With our late freezes, this is really important all over the state. The number of chill hours is calculated a little differently for different species, but it tends to be defined as a total of hours between about 32°F and 45°F. Temperatures within this range are detected by the plant in its dormant buds and are “recorded,” in a sense, by the enzymatic activity and hormonal concentrations. Temperatures below freezing are not recorded. That’s why in maps depicting chill hours across the country, North Dakota and southern New Mexico are depicted as having the same number of chill hours, even though North Dakota obviously has many more hours of below-freezing temperatures.

Any map of approximate chill hours is just a model based on averages over a select period of time. The actual number of chill hours in an area varies from year to year, and depends greatly on microclimate in the area/garden and how cold/warm the overall winter is. According to several chill hour maps I found, Roswell, NM, gets approximately 1,000 chill hours each year.

https://webapps.msucares.com/chill_hours/

When selecting a fruit tree variety with a specific chill hour requirement, you want the tree to almost match the number of chill hours you’ll have in your yard. Chill hour requirements can vary dramatically within a given species. Some apple varieties, for example, only require around 400 chill hours, while others require upwards of 1,200. If a tree only needs 400 chill hours and we have a few weeks where temperatures stay in the 30s in December, the total number of chill hours may be satisfied and the tree could break dormancy and bloom in January, which would be terrible. On the other hand, if you have a variety of apple that needs too many hours and we don’t get enough, the bloom time could be delayed or really sporadic, or the tree might not bloom at all.

I saw a presentation this summer from peach researchers in South Carolina (little-known fact: South Carolina grows more peaches each year than Georgia). The researcher showed photos of peach trees in South Carolina that have been killed by lack of sufficient chill hours in recent years! That’s pretty scary, especially considering our increasing temperature averages. Experienced fruit growers in New Mexico suggest planting trees that require a few hundred fewer chill hours than are expected in your area. So in Roswell, you’ll be looking for trees that need 700–800 chill hours per year.  

Apple blossoms in April at the NMSU Agricultural Science Center made it through our last frost and many were able to set fruit. Photo credit M. Thompson.

For more gardening information, including decades of archived Southwest Yard & Garden columns, visit the NMSU Extension Horticulture page (http://desertblooms.nmsu.edu/), follow us on social media (@NMDesertBlooms), or contact your County Extension office (https://aces.nmsu.edu/county).

Marisa Thompson, PhD, is the Extension Horticulture Specialist for New Mexico State University and is based at the Agricultural Science Center at Los Lunas.

Edible New Mexico Magazine article crosslink -- GROWING YOUR FUTURE FAVORITE TREE