Winter chill, bud break, and frost risk, late February, 2017

By Terence Bradshaw

February 26, 2017

I am following up on a notice I received from Mary Conklin at UCONN regarding recent warm temperatures in Connecticut and thoughts on bud break in fruit crops. I started to look at this on Friday, Feb 24, but went away to Montreal for the weekend where cooler temperatures were the norm. Prior to leaving, the forecast for Vermont showed a gradual cooling trend which I assumed would prevent any loss of cold hardiness in apple trees and grapevines. However, temperatures hit 72°F in Burlington on February 25, and although temperatures are cooling down from there, it is worth assessing the situation.

Temperate fruit crops undergo several phases of cold hardiness development. In fall, trees reduce shoot growth and export water from cells into intercellular spaces in response to shorter day length and cooling temperatures. During this period, known as acclimation, the cold hardiness of plant tissues increases until maximum dormancy is reached , usually sometime in mid-December. This state is called endodormancy, and requires a period of cold below 45° and above 32°F for the tree to ‘reset’ and initiate hormonal processes that will allow it to bud out in spring. Without this process, plants would bud out easily during winter warm spells, and subsequent cold could kill deacclimated bud s and other tissues. The chill hour requirement ranges from a high of about 1200 for apples to as low as 200 for some grapes. As of today, Shoreham, VT has accumulated 1173 hours since November 11, and East Dorset 926. So, warmer regions may have accumulated chill hours for apples or are near doing so, while cooler upland orchards still have a little ways to go. We should assume that all orchards will have met this requirement in the next few weeks. After chill hour needs have been met, the plants are in a state known as ecodormancy, where environmental conditions are the only thing preventing them from resuming growth. However, that doesn’t mean that buds will immediately start popping. Trees will then need to undergo deacclimation which is driven by accumulated heat units.

Unfortunately, we do not have a good handle on how much accumulated heat is needed to push apple (or cold-hardy grape) buds. I looked at the last seven years’ ‘McIntosh’ budbreak date from the UVM Hort Farm and calculated accumulated growing degree days (GDD), base 39°F (or about 4°C) since January 1 for each of those years from NEWA. This is far from comprehensive, as a true analysis would need to consider bud health going into the winter, acclimation conditions, date when chill hours were reached, soil moisture, and soil temperature conditions. But this is what I pulled together quickly on a Sunday night anyway. Bud break occurred after an average of 134.7 accumulated GDD base 39°F (range 132-174) from January 1 in South Burlington, VT. Today, we are calculating 54.1 GDD at this same site. In order to accumulate any GDD at this base, we need to see high temperatures in the high 40s and above. In the near-term outlook, I only see a couple of days (2/28 and 3/1) that might accumulate a few GDD in the Champlain Valley, and in cooler upland regions I don’t know if those days will accumulate GDD as far as apple phenology is concerned. Plus, it is likely that upland orchards still need some chill hours to accumulate before dormancy is broken, so they are even better off. In my opinion, apples are fine as far as cold hardiness to the temperatures expected in the near future and early (pre-April) bud break are concerned.

As for grapes, the news is a little worse, and yet better. There is no question that grapes have met their chill hour requirements in all of Vermont, although we really don’t know what those requirements are for the cultivars we grow. However, grapes need a bit more heat accumulation after entering ecodormancy to break bud than apples (although exposed tissue is more vulnerable to cold once it has emerged). Again, I don’t have a good handle on how much heat it will take to make grapes push bud, nor at what deacclimation stage they are in. However, the few (and not entirely reliable) long-range forecasts available are not suggesting more extreme (-0° or +50°F) weather after Wednesday. That means that even if buds have lost some hardiness (and some preliminary analysis by Tim Martinson at Cornell suggests we have), we are not likely to see the deep cold needed to cause damage, while we are not likely to accumulate the heat required to push budbreak until well into March. We’ll keep an eye on things, but I do not see, at this time, no cause for worry.

Here’s a good rundown on how things are developing in the southern U.S.:

It’s good to remember that the chilling hour requirement and generally cooler temperatures overall help keep Vermont and other northern country growers a bit more protected than those who are living in areas with warmer winters. It’s why we had fruit in 2010 and 2012 (albeit a smaller crop) when Tennessee and Michigan did not.