Spring Frost
Most blackcurrant varieties flower soon after mid-April, when they are still at risk from damaging frosts and cold winds. A few varieties flower later, notably Ben Alder & Ben Tirran, which begin flowering at the beginning of May.
Blackcurrant flower tissue is susceptible to frost damage as soon as the “grape” clusters can be detected within the expanding bud, normally at any time after mid-March, and when the temperature falls to -1.9°C (28.5°F) or below. During frosty spells, the temperature on a still night may drop slowly to give only a short time below the critical temperature. Provided the flowers remain dry, it is unlikely that damage will ensue from a period of up to one hour at a temperature of -1.9°C. Protection will be gained from the “canopy” effect of expanding leaves as well as from miniscule pockets of still air acting as an insulator within the flowers.
The symptoms of frost damage will show up within 24hr - 48hr following a severe frost. The entire flower may be blackened or only the stigma and anthers change colour. Alternatively, the ovary (the shiny green “receptacle”) might appear wrinkled or blanched. When the flowers have suffered chilling or the effects of a succession of minor frosts, the symptoms take longer to appear. Either the flower withers, becomes papery or then drops off, or the sepals and ovaries gradually become deep red in colour, before they eventually drop off.
Wind frosts are usually a more serious threat to the grower than radiation frosts because heat will be removed from the tissues more rapidly. With a wind frost, the external cold air movement rapidly cancels the insulating effect of air space inside the flower buds. Some protection may be gained from shelter belts and areas of nearby woodland, where they have been positioned so as to deflect any cold air currents away from the plantation.
Frost Protection
The technique of protecting plant tissue from frost damage by continuous water sprinkling was devised at East Malling Research Station by Rodgers and Modibowska. Water is sprayed onto the planations using hundreds of irrigation sprinklers, and by ensuring that the sprinkler nozzles rotate at least once per minute, the temperature of the ice that forms around the flower buds is prevented from falling below freezing point even though the surrounding air may be at a lower temperature. Frost protection is mostly used in the east of England, but with present day later flowering varieties growers are not generally investing in this type of system.
