Don't Panic!

Some of you may be wondering if your fruit trees are at risk for breaking bud and growing with all this warm weather. Certainly, if it were later in the winter, this might be the case. And, reports of (ornamental) cherry bloom in the mid-Atlantic area (Washington D.C., and Newark, NJ), don't help. (Don't Panic!) We're not too worried about apples. Stone fruit chilling and grapes, however, are further along and with continued warm temperatures (with additional heat accumulation) there is potential for injury, as winter is just beginning. A review of chilling, dormancy, and rest might help explain what is going on.

First, the concept of dormancy and chilling hours/units. Deciduous trees, such as apple and peach, gradually acquire cold hardiness and dormancy beginning in the fall with cooler temperatures and decreased day length. They have adapted such that they don't wake and start growing in early or mid-winter, because obviously this would be a problem. If they broke (or swell) buds, started growing, and then the temperature dropped below freezing, they would be toast. In fact, this concept of endodormancy (aka “rest”) has been well studied and quantified.

These chilling hours (or units as they are called when somewhat modified), in their most simple form, accumulate when the temperature is between 32 and 45 degrees F (it is generally agreed). Some studies have gone further and adjusted this accumulation based on other temperature factors, such as when the temperature is above 45 but chilling units are still accumulated (but at a slower or faster rate). For apples, it is generally accepted that approximately 1,200 chilling hours between 32-45 are necessary before they might start growing again. It is, however, somewhat variety dependent, with a range of app. 800 (low chill varieties like ‘Anna’) to 1800 chilling hours/units. Peach, however, has significantly lower chilling hours/unit requirements: 500-1,200, again highly dependent on variety. All this is important too in warmer climates where sometimes not enough chilling hours/units are accumulated to properly break bud and flower (and fruit).

Buds are most hardy (resistant to minimum winter cold temperatures) when the chilling has not been close to met and temperatures have gradually declined. This allows buds to increase in hardiness. Once the chilling requirement has been met, it is the amount of heat units that accrue that move buds forward to bloom. (But only after the chilling requirement has been met!) After chilling is met, temperatures above 32 degrees F. allow for internal development in the buds.

So, where do we stand in chilling hours/units right now? A pretty neat website, http://getchill.net, calculates them for you for any location that has a Weather Underground weather station (http://www.wunderground.com/). For the UMass Cold Spring Orchard (KMABELCH4) since November 1, 2015 (through December 22), the calculation results:

• Below 45 Model: 633 chill hours
• Between 45 and 32 Model: 522 chill hours
• Utah Model: 679 chill units

Zestar! flower bud, low risk for injury
as chilling requirement has not been met (yet)

The important numbers are Between 45 and 32 Model (633 chill hours) and Utah Model (679 chill units). Both well below 1,000. Apple buds aren't going anywhere (yet). Peach buds, well, their chilling hour/unit requirements are less (as low as 450). There could be an issue with stone fruit if the warm weather persists (especially with some chilling still). Even though we might not have enough warm temperatures to actually break bud, they will be more sensitive to mid-winter cold once rest has been broken. I would not fret too much about it yet, however, as most peach varieties we grow have fairly high chilling hour/unit requirements.

Peach buds at higher risk because
chilling requirement is close to met;
in fact, these buds look swollen on 12/23/15

Second, and this is more problematic. Any time very cold temperatures -- particularly a rapid drop in temperatures -- following very warm temperatures during the dormant season, can result in plant tissue damage. This is a cold hardiness issue. Stone fruit are more sensitive to this because of their genetic disposition to just stay “greener” all winter. Keep in mind that fruit tree buds (and the cambium layer just under the bark) do not fully stop metabolizing during the winter and are still quite “alive” despite their dormant appearance. Very warm early or mid-winter temperatures followed by a rapid drop to extreme cold is definitely not good news, although certainly it is not always a disaster either. (Just plain bad news.)

So that you might be able to sleep tonight, consider apple trees are pretty well adapted to survive and grow despite the vagaries of weather. Peaches (and other stone fruit) are less suited for surviving wild temperature fluctuations, i.e., they are not as well adapted as apples to our wildly fluctuating weather here in central New England. Now, what the long-term effect of climate change (warming) on our fruit trees is another question? (Weather is short-term, climate is long-term. See Northeast and Northern Forests RegionalClimate Hub Assessment of ClimateChange Vulnerability and Adaptation andMitigation Strategies.) Ask me again in April how today’s “weather” will affect our 2016 tree fruit crop. (Hint: we have a long way to go until apples and peaches are harvested and in the cooler in 2016. And I might just tell you the answer is 42.)

Many thanks to my colleague Win Cowgill at Rutgers University for helping me track down and understand the concept of chilling hours/units and dormancy of deciduous fruit trees, as well as supplying some of this text. You might also want to check out About Chilling Units and Hours from University of California Davis (although the emphasis is on low chill varieties suitable for normally warmer climates) and Cold Injury to Fruit Trees (a presentation I gave at the Ontario Fruit & Vegetable Growers's Conference in February, 2014.) And, if this is all too mind boggling, well there is always The Answer to the Ultimate Question of Life, the Universe, and Everything, which of course is...

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