Many fruit tree species grown across the temperate regions of the world must endure an extended period of winter to produce flowers in spring. Flower buds and open petals are not exempt from damage or death from an untimely late spring frost. In fact, trees like peaches, apples, cherries and plums will not regenerate flowers if the first ones are destroyed. With diminished numbers of flowers, gardeners expect little or no harvest of fruits later in summer.
Late winter and early spring typically is a battle between cold and warm temperatures, having a direct impact on the timing of plants ending winter dormancy and beginning new growth. A "late spring freeze" implies untimeliness, as a potentially harmful dip below 32 degrees Fahrenheit is expected. Late-spring freezes occur when plants in general are already unfurling tender leaf and flower tissues because of weather that has occurred to coax the end of dormancy.
Iowa State University comments that two different types of freezes occur: advection and radiation. Advection freezes are the most intense and little can be done by gardeners to offset the strong infiltration of cold. An example of advection freeze is when a polar air mass with cold winds enters a region. The depth and size of the cold air is impressive. Radiation freezes result from cold air and no wind, allowing any surface warming to radiate up and out of the atmosphere. Radiation freezes are often offset by trapping heat, preventing it from escaping away from growing plants.
Informational charts from Iowa State and Michigan State Universities reveal that species of fruit trees naturally tolerate different degrees of subfreezing temperatures once buds have swollen and flower and leaf buds appear and unfurl. Generally speaking, apple trees' buds tolerate temps down to 16 degrees Fahrenheit, while peach tree buds tolerate to only 23 F. The more growth and exposure of petals and leaves, the less resilient the tissues are to exposure to cold. Thus, an open flower is readily killed by subfreezing temps, while a tightly closed bud may endure the same temps without harm. Frost/freeze damage is seen by blackened tissues, or full drop of the damaged leaves or petals.
While wind machines, heaters and irrigation spray are all means an orchard grower chooses to proactively protect fruit trees when an untimely spring frost occurs, none are fail-safe. Iowa State University states that proper site selection and tree species/variety selection are key to reducing concerns of inevitable untimely late frosts. A common recommendation is to plant fruit trees on hillsides oriented away from the prevailing cold winter winds but angled to capture the sun's warming rays. Freeze-inducing air is heavy and congregates in low elevations. In regions where late spring frosts are common, avoid planting early flowering fruits like sweet cherries or peaches and focus on types that bloom later when there's less chance of spring frosts.
While covering fruit trees with protective cloth sheets or warming the air with heaters and mixing air with fans is an option to offset potential damage from late spring frosts, they are neither cheap nor easy. California Rare Fruit Growers Association mentions that in some fruit trees, an application of the plant hormone gibberellin can protect flowers from cold and help initiate additional production of flowers. The dosage application is critical, as is timing; significant research is needed to manage fruit trees, spring freezes and gibberellins properly and effectively.