Bloom on apple trees was much later than usual this spring. This was just as much due to below normal temperatures we experienced during the winter months as well as during the spring. For the buds of apple trees and other deciduous plants to emerge in the spring, the dormant buds must be exposed to so many hours of chilling to break dormancy. For apples this is typically around 1,000 hours of exposure to temperatures between 32 to 45 degrees F, but can vary from about 800 hours to over 1,200 hours. When the temperature drops below 32 degrees the plants do not accumulate additional hours of chilling, and during this past winter, our temperatures stayed below 32 degrees for much of the time. As a result, we did not accumulate 1,000 hours of chilling until about mid-April. In comparison, the chilling requirement is normally satisfied in about mid- to late- February, but with the mild winters we had been receiving, it was achieved as early as mid-January during the previous three winters. With this year's delay in satisfying the chilling requirement, the plants could not respond to exposure to warmer temperatures that may have occurred earlier. The ISU Iowa Ag Climate Network now tracks chilling hours during the winter as well as growing degree days during the growing season.
With the late bloom that occurred this spring, we can anticipate a later harvest season for apples, but much will depend on the conditions we experience for the remainder of the growing season. If conditions are normal, we can anticipate a one-day delay in harvest for every three-day delay in bloom for cultivars that normally mature in mid- to late-September or later. For earlier maturing cultivars the delay will probably be longer, with the earliest maturing cultivars experiencing the longest delays. If we have a summer that is cooler than normal, we can anticipate a longer delay for all cultivars. With this uncertainty in harvest time, and because cool temperatures enhance red color development but not fruit maturity, it will be important to test fruit maturity before harvesting to assure customers a quality product. Such testing can be done either by taste or by the starch-iodine test. For cultivars that have some storage life, the starch-iodine test is more reliable and allows you to maximize the storage potential of the fruit.