Deciduous

Most people know that ‘deciduous’ refers to trees that lose their leaves each year, but there is more to the word and the process than meets the eye.

When a plant no longer needs a flower’s petals, those petals are allowed to fall away. When fruit becomes ripe, it is also allowed to drop. This act of ‘allowing to fall away’ is at the heart of deciduousness. Did you know that a deer’s antlers and your own baby teeth are also considered deciduous?

Summer almond

Winter almond

Spring almond

Pros and cons of deciduousness

​Unlike evergreens, deciduous trees and shrubs, and some herbaceous perennials, lose their leaves each year. This is called abscission. In the Northern and Southern hemispheres, leaf drop normally occurs in autumn or early winter. In tropical regions, deciduous plants lose their leaves during the dry season. In each case, leaf loss occurs at a time when having leaves is not in the plant’s best interest. For example, broad-leafed hardwood trees might collect too much snow or ice in winter, causing limbs to break off, leaving open wounds, while tropical plants are unable to maintain heavy leaf cover without rainfall. This annual leaf drop is believed to be a mechanism by which some plants interrupt pests and disease triangles. It also means a plant must have enough food stored to last through the winter and to begin growing again in spring.

Another benefit of abscission is related to something called cavitation. Cavitation refers to times when water tension within a plant becomes so great [think rainy season] that the sap vaporizes within the tree and the oxygen held in that water expands rapidly enough to cause a loud ‘crack’ - you may have heard this, if you spend any time in forests. The problem with cavitation is that it damages the xylem. Plants can usually repair this damage, but not aways. One way deciduous plants protect themselves against cavitation is by dropping their leaves, which, in turn, allows them to have larger xylem vessels. These larger xylems allow deciduous plants to take up more water than evergreens in the summer months.

The chemistry of deciduousness

During spring and summer months, deciduous plants are busy producing chlorophyll, a green pigment. Shorter days (or drought stress) trigger plant hormones (auxins) to reduce chlorophyll production and to start drying the connection between the stem and petiole of each leaf. In some cases, the plant also withdraws the nitrogen and carbon held in those leaves for use in spring. Lower levels of the green pigment are what allow us to see other colors. Some of these colors, the yellow, brown, and orange carotenoids are always present, while red and purple anthocyanins are produced in autumn, as sugars become trapped in the leaves. These changes are triggered by shorter days and cooler nights. In areas without those conditions, the leaves simply dry up and fall off.

Deciduous trees and shrubs

Common deciduous trees include almond, pomegranate, quince, apricot, nectarine, peach, olive, persimmons, plum, fig, pear, walnut, and apple. Your grapes, kiwifruit, blackberries, raspberries, strawberries, and blueberries are also deciduous. In fact, nearly all fruit and nut crops occur on deciduous plants, citrus being a notable exception.

Winter is the best time to prune deciduous trees. [Except apricot and cherry, due to eutypa dieback.] The absence of leaves makes it easier to see the true structure of deciduous trees and shrubs, allowing you to see and remove dead, diseased, crossed, and poorly placed limbs. Winter is also the best time to apply dormant oil, to control many pests, such as scale.