False spring: Same old variable weather or new territory?

February 2017 was the second warmest in history—as warm as the average March in our region.  Grass greened up, it felt like spring, and many plants started to break dormancy.  Everything was great until things went back into the deep freeze from March 20-22, when many locations in Maryland had night temperatures between 23-28 degrees for 1-3 nights.  This is what climate scientists call “false spring”—very warm weather that seems like a welcome end to winter, just to be followed by a hard freeze later in March or April.  In the past ten years, the Eastern US has experienced three devastating false springs:  2007, 2012 and 2017.

The extent of agricultural damage after the freeze during this year’s false spring depends not just on the developmental stage of the crop, but on a number of other factors—when the weather began to warm, how long it stayed warm, whether development was slowed by subsequent cooler weather, how far along crops had gotten by the time the freeze hit, air speed and humidity, how cold it got and for how many days the cold weather lasted.  Because these factors can vary widely across a region, it isn’t surprising that the damage seen after the March 2017 freeze depended on location.

Maryland farmers were lucky—although the earliest blooming fruits like apriums and apricots were heavily damaged in central Maryland, grapes, apples and blueberries were not far enough along to be affected, and strawberries were still weeks from breaking dormancy.

At Swann Farm in Calvert Co., it originally appeared that up to 85% of peach blossoms had been lost.  However, more extensive sampling of blossoms revealed that early and late blooming peach varieties sustained only 25-50% damage, while the mid-summer varieties were at their most vulnerable at the time of the freeze, and were 100% destroyed for the year.  Swann Farms published some useful pictures that show how to determine damage to a blossom by pulling off the damaged bloom to see if there is a tiny peach underneath (see white structure under the word “Peach” in this photo).

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Farmers to the South of us weren’t so lucky. In central Virginia, peaches, nectarines and cherries were damaged, and some plum orchards were lost entirely.  Fortunately, most apples bloomed after the freeze, though some crabapples used for pollination of apples were damaged.  South Carolina lost 90% of their peach crop, and Georgia peaches and blueberries were severely damaged.  Some North Carolina farmers were able to save their strawberries by pulling out the row cover—using two layers in some cases.

So, when we get a spring freeze, is it just bad luck we can chalk up to typical weather variability?  Maybe not– climate scientists have shown clearly that spring is coming earlier these days.    The 2014 National Climate Assessment showed that the Northeast (which includes Maryland in the eyes of climate scientists) now experiences an average of 10 fewer frost free days than we did between 1901-1960. Studies of plant populations show that many plants are blooming earlier in response.  Scientists from Cornell showed that apples in upstate New York are blooming an average of 8 days earlier than in the 1960s, while grapes are blooming 6 days earlier.   When Smithsonian scientists analyzed 30 years of data on bloom times of common plants in the DC area, they found that 89% are blooming earlier now than in 1970 by an average 4.5 days.

By far the most amazing demonstration of earlier blooming is shown in a study of bloom dates of cherry trees in Japan, which have been monitored since 800AD.  Despite a lot of noise due to weather variation, the bloom times remained within a fairly narrow window until about 1875, when the trend line shows a sharp decline in flowering date, leaving that historical range.  This is just what we would expect if the earlier flowering is a response to global warming, which started during the Industrial Revolution in the mid-1800s.

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How does the flowering time shift of the cherries in the DC Tidal Basin compare with that seen in Japan?  Jason Samenow of the Washington Post’s Capital Weather Gang explored this and found that although the DC trees flower a few days earlier on average than the trees in Japan, both populations have shifted at approximately the same rate.  This is good evidence that the cause of the shift is the same in both locations– warmer and earlier springs due to climate change.

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Although spring is coming earlier, temperature variability has remained the same or even increased, so those spring freezes we are used to in March and April remain highly probable.  Any way you look at it, earlier spring coupled with a continued or even increased probability of March and April frosts means that the threat of freeze damage to spring crops is likely to be a problem that farmers on the East Coast will face more and more often.

How can you protect your fruit crops from false spring and damaging late freezes?  The experience at Swann Farms illustrates one of the best ways to minimize freeze risk—plant multiple varieties that break dormancy and/or flower at different times. If varieties are at different stages when a freeze occurs, only part of the overall crop will be damaged.   Fortunately, many farmers already plant several varieties in order to market a given crop over a longer period of time.

UME Extension Specialist Joseph A. Fiola has a number of good articles on detecting and preventing freeze damage in grapes.  Links to basic freeze protection techniques that can be modified for other fruit crops can be found on his webpage Timely Viticulture.  One clever method from Michigan State that might be useful in particular situations involves improving air flow through orchards by removing vegetation in key locations; additional information about basic freeze protection is also available at that link.

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