Hardiness is the ability of the plant to withstand cold temperatures. In the fall, plants develop cold hardiness in response to cold temperatures (and maybe to day length changes). Because it was so warm this fall, there was concern that bud hardiness was not developing normally. On December 8-9, we exposed cut uprights with flower buds to temperatures of 10F and 0F for 5 hours. The uprights were then left in the lab at room temperature for 3 days to allow any damage symptoms to develop. Buds were then cut and examined under a dissecting scope for damage. For the cultivars Early Black, Howes, Stevens, Ben Lear, and Crimson Queen, we found no significant damage after exposure to either temperature. This indicates that by December 8, the buds had developed hardiness to at least 0F (-18C).
Temperate fruit crops have a chilling requirement - the need for exposure to some number of hours of cold conditions - in order to properly develop flower buds and fruit. This chilling exposure also contributes to the development of winter hardiness (see above). Chilling requirement for cranberry appears to be ~1700-2000 hours below 45F in MA field conditions. We are at about 750-800 currently, about 2 weeks behind average. In average years, the requirement is met by about mid-February to early-March. If temperatures remain average from this point on, chilling would be satisfied in another 45-50 days or by the last week of Feb - to first week of March.
There is a body of research that indicates that chilling may be lost in warm temperatures (generally above 55-60F). For this reason, it is prudent to guard against winter warm spells by having the bog under flood. The cold water then buffers against loss of chilling. Not having the bog under flood can also increase the risk of certain insect infestations, particularly yellow-headed fireworm.
Spring development and loss of hardiness
Once the chilling requirement is satisfied (late Feb-Early Mar), the plant is capable of beginning growth once it is exposed to enough heat units. Generally, cranberry growers remove the winter flood by mid-March. With the water removed, the plants can accumulate heat units and begin to lose hardiness. Once again, keeping the flood in place longer can be a buffer against unseasonably warm temperatures. This may be desirable since the warmer it is, the faster dormancy and hardiness are lost.
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