Cold-injured primary blackberry bud (left) and uninjured secondary bud (right). Photo credit: Fumi Takeda, USDA-ARS-AFRS
The net result of these types of weather events is cold injury to many perennial crops. During the fall of 2015, after a brief cool-down, temperatures fluctuated considerably. Unusually warm temperatures occurred in December in PA mainly because a bend in the jet stream allowed warm air from the South to spill into the Northeast.
Winter injury in brambles (raspberries and black berries) is often expressed as dieback of the terminal ends of the canes or dead buds in spring. There is little we can do to influence temperature, other than select good planting sites or grow plants in protected environments such as high tunnels. If we understand how plants are influenced by temperature, we may be able to develop cultural practices that maximize their cold resistance, and also adjust pruning practices to retain the maximum number of buds that will be fruitful.
Winter Injury Terminology
Before going any further it is important that we understand the terminology concerning winter injury. The terms "winter hardy" and "cold hardy" refer to the ability of a plant to tolerate cold temperatures. However, it is important to realize that the cold hardiness of plants will differ during the winter and the relative cold hardiness of different varieties may depend on environmental conditions. The best example I can think of to illustrate this is to consider the peach rootstock 'Siberian C'. This rootstock survives winter temperatures of -20⁰F in Ontario, New York and Michigan, but it is killed by temperatures above zero in South Carolina and Georgia.
So a variety that is winter hardy in one region or during a particular winter may not be very hardy in a different situation. This is why it is so difficult to evaluate cold hardiness of plants. Many years are needed to evaluate plants under varying field conditions. Therefore, pomologists have relied on controlled freezing experiments to evaluate hardiness by exposing plants or parts of plants to a range of temperatures in freezers and then recording the amount of injury sustained by the plant either by rating tissue browning or evaluating growth in the greenhouse.
This is not to be confused with cold-injury related to the chilling requirement being met too early in the season, as is the case when cultivars that are better-adapted for southern conditions (certain low-chill varieties of blackberries or blueberries) begin to come out of dormancy too early in late winter or early spring and then are subsequently injured by cold temperatures.
Acclimation and De-acclimation
"Cold acclimation" is the process by which plants develop cold hardiness. Raspberries cease growing in late summer and blackberries continue to grow into the fall. In mid-summer raspberries shoots are killed at about 18°F. During the fall, the leaves sense the shortening days and this induces the first stage of cold acclimation and by Mid-October the plants can withstand about 10°F. The second stage of acclimation is induced by temperatures just above or below freezing. As a result of a mild frost, the cold hardiness of woody plants can increase by 6 to 10 degrees within 24 hours. By early November raspberries can withstand about 1°F and by early December they can survive -10°F to -35°F depending on the variety.
Early in the acclimation process the plants enter a period of dormancy called "rest". There is quite a bit of confusion and different terms concerning dormancy and different stages of dormancy, but for this discussion I will assume that there are two stages of dormancy. When plants are in rest they will not grow even if they are place in environmental conditions that are ideal for growth (long days and warm temperatures). Plants remain in rest until they have been exposed to adequate chilling.
The ideal chilling temperature is about 47°F; temperatures above 65°F and below 32°F do not contribute to chilling. The chilling requirement for most brambles is usually between 800 and 1,000 hours. A study in New Jersey in cooperation with Cornell University showed that holding potted summer-bearing red raspberries at 40°F for 40 days was adequate to satisfy chilling for greenhouse production. In Pennsylvania, the chilling requirement for brambles is usually satisfied by early January. After the chilling requirement is satisfied the plants will de-acclimate in response to warm temperatures and they will also lose some cold hardiness. If exposed to lower temperatures gradually, the plants will partially re-acclimate and regain most, but not all of their cold hardiness. This is why fluctuating winter temperatures can be so harmful.
In Poland, Pocholak (1978) determined the T50 (temperature at which 50% of the canes were killed) throughout the winter for the hardy variety 'Latham' and the less hardy variety 'Malling Promise'. He reported that in October 'Malling Promise' had a T50 of about 10°F, in January the T50 was about -31°F and in February the T50 was only -11°F. For 'Latham' the T50 was also 10°F in October and -20°F in January, but -20F in February. So 'Malling Promise' and 'Latham' were equally hardy during the fall and mid-winter, but 'Malling Promise' was more susceptible to cold following a winter warm period because it was more responsive warm mid-winter temperatures and de-acclimated more than 'Latham'.
Craig and Aalders (1966) compared 6 red raspberry varieties in Nova Scotia and reported that bud survival for 'Viking' and 'Trent' during the winter of 1963-64 was 9% and 71%, respectively and they speculated that 'Viking' was affected more by the mid-winter fluctuating temperatures. Hummer et al. (1995) evaluated maximum cold hardiness of more than 120 varieties and selections of brambles at the USDA-ARS Germplasm Repository in Corvallis, Oregon.
The plant material was first preconditioned at 31°F for 4 weeks, so the tissue should have been at its maximum hardiness. Buds and sections of the canes were then frozen at a range of temperatures from 0 to -40°F to estimate the T50. Although within each group, varieties differed in their cold hardiness, summer-bearing red raspberries and purple raspberries were most hardy, followed by black raspberries, then fall-bearing red raspberries, whereas blackberries were least cold hardy. In general the T50 of canes was 2 to 16°F lower than for buds and the bud base, where it attaches to the cane, was less hardy than the buds.
Warmund and George (1990) evaluated the hardiness of primary and secondary buds of bramble varieties from the Arkansas and Minnesota breeding programs as well as plant growing in Missouri and Illinois. In Arkansas, 'Darrow' was the hardiest blackberry and 'Wells Beauty' was least hardy. In Minnesota, 'Canby' was hardier than 'Latham' and 'Titan' red raspberry. In general the more fruitful primary buds were less cold tolerant than the secondary buds, which are less fruitful.
- Brambles develop hardiness rapidly during November and buds will lose hardiness during warm periods in January and February.
- During December and until there is a warm period in January most varieties are quite hardy and will survive temperatures typical in Pennsylvania; most varieties will survive at least -25°F.
- Winter injury usually occurs during mid-winter when several warm days are followed by a cold snap.
- There is a range of cold hardiness between and within each type of bramble. Summer-bearing red raspberries are hardier than fall-bearing varieties and black berries are least hardy.
- Researchers have developed several methods for assessing winter injury that occurs in the field and growers can probably use these methods to evaluate the extent of winter injury before they start pruning in early spring. Some researchers cut canes at the base and placed them in water in warm conditions. They changed the water every other day to eliminate bacteria in the water. Before the chilling requirement is satisfied, it took 15 to 50 days for buds to start growing. By early January buds begin to grow in 10 to 15 days. Buds that begin to develop into shoots will likely produce fruit and the buds that don't grow have been killed. So a grower can estimate the percentage of buds that have been lost to winter injury. However, if growers attempt to grow the shoots too early, before their chilling requirement is satisfied, the canes will not break bud for period of weeks, and the conclusion might be erroneously reached that the canes are dead.
- Some buds can be cut lengthwise (tip to base) as they swell, and examined for blackened centers surrounded by green tissue (an indication that the flower buds were killed), or for tissue that is completely dead. Magnification, as with a 16x hand lens or greater, will be required. This is most easily done as the buds are beginning to swell.
- The warm temperatures in December would not necessarily have resulted in more winter injury, so growers should not be overly concerned about the warm conditions. Temperatures dropped gradually enough after the warm period that plants probably acclimated before the colder weather in January. Most injury is thought to occur later in the winter and early spring when the plants are less dormant, and thus cold-injured more easily (at higher low temperatures). Growers may want to delay pruning as long as possible, preferably after testing some canes as recommended in steps 5 and 6 above, and possibly leave a few more canes than usual until a judgement can be made as to how much winter injury occurred. The extra canes can be removed later.