Fruit trees must first flower and attract pollinating insects in order for any chances of a fruit crop on their branches later in the year. One of the greatest threats to a good flowering is temperature; frosts and freezes can damage or kill flowers, and trees will not grow new blossoms that year. The effects of frosty temperatures vary by fruit tree, but if too much of flower petal blossom is exposed, the flower is greatly damaged or fully aborted when the central sex organs are killed.
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Fruit Tree Types
Citrus trees usually bloom late enough to avoid frosts and freezes in the subtropical parts of the United States. Farther north, untimely frosts and freezes are expected across spring, often alternating with streaks of warm weather that coaxes the dormant flower buds to swell and open prematurely. The earliest flowering fruit trees are pears, almonds, apricots, peaches and sweet cherries. Apples and sour cherries bloom a bit later, often avoiding seasonal threats of a late frost.
When fruit trees are dormant, and their buds are tightly closed on the branches, subfreezing temperatures cause no harm. Once buds swell after sap flows in the tree, the buds become increasingly vulnerable to damage from frost and freezes. When a bud opens to reveal green tips, it is killed when temperatures reach the -9.44 to -6.67 degrees Celsius range. Once the bud reveals petals but hasn't opened, it may survive temperatures down to about -5.56C without detriment. Open flowers on fruit trees begin to die at -2.22C, with most killed all across a tree if the temperature drops down to -3.89C. These temperature thresholds are quite consistent among all species of fruit trees.
Frost and freeze damage on fruit tree blossoms appears as crinkled or blackened tissues. Depending on the temperature and duration below freezing, the damage on flowers or buds can manifest with 24 hours or take as long as 72 hours. The least harmful damage is the loss of flower petals. Only when the sex organs -- the female pistil and male stamens -- are killed by cold, does annual productivity of the fruit tree end. Without sex organs, no transfer of pollen to fertilise the ovules in the pistil occurs, preventing any fruit development.
Within the same tree, the harmful effects of frost can be variable. Open flowers are killed before closed buds nearby. Outermost branch tips may sustain more thorough damage compared to blossoms nestled deeper in the canopy. Lower branches may endure colder temperatures near the ground since cold air sinks. Flower clusters may also reveal varied damage. Inner flowers may be unscathed while outer flowers are hit hard by penetrating cold. Even across the garden, the effects of frost damage on fruit tree flowers can vary. Tree elevation, exposure to wind and early morning sun rays can affect the extent of blossom damage that occurs on trees in an orchard.
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