Why Leaves Change Color

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Oh, my aching muscles...

By Marty Wingate

Winter is a time when much of the garden closes up shop.  Apart from late (and early) perennials and bulbs, such as hellebores and crocus, and a few winter-blooming shrubs, the garden is quiet.  We call this winter dormancy, but just what does that mean?  Is there something going on inside stems and underground, while we dream of spring?

For the most part, the plants have slowed their metabolism, almost to a standstill.  They have hunkered down to wait out the months where there is little light for photosynthesis (and therefore, growing), little chance that any flowers would be pollinated (and so, reducing the chance to further the species) and a big possibility that tender plant parts would be damaged by the weather.

Deciduous trees and shrubs go dormant because the temperature and the day length trigger certain physiological responses.  The first thing we notice is that they drop their leaves.

The departure of the hormone auxin from the leaf abscission zone - the place where leaf joins stem - causes a breakdown in the vascular pathways that were open all spring and summer.  These pathways had transferred the products from photosynthesis down to the roots (by way of the phloem) and water from the roots to the leaves (by way of the xylem).  The cells break down at the precise point, the base of the petiole (the leaf stem); the leaf receives no nutrients or water, and so they die and drop.

Winter is a good time to observe the twigs, stems, branches and trunks of deciduous trees and shrubs.  We can check out the health of the plant, look for broken stems that need to be pruned or inspect stems for those grayish, Styrofoam-like egg casings of tent caterpillars.

Apart from giving trees and shrubs a checkup, we can also learn a thing or two about their anatomy.  In winter, it's easy to see how much growth a plant has put on, because the newest growth, at the end of the stem, will have a different color from the older bark.  Coral bark maple (Acer palmatum 'Sangu-kaku') is an easy one to spot, as its newest growth is also the brightest color.

Other observations may require closer examination.  A hand lens is a useful tool to carry about the garden or your walks in the woods, because it will give you the opportunity to see small structures more clearly.  A 10x lens will give you plenty of magnification to study such parts as a leaf scar.

The leaf scar shows you the point of abscission; often, you'll be able to see dots that were the phloem and xylem pathways before leaf fall.  You also can take a good look at leaf buds, those structures that, like us, are waiting for spring.  The terminal leaf bud is the apical meristem, the point at which grow begins in spring.  During dormancy, the apical bud is well protected from the elements by layers of leaf scales.

The apical meristem holds the key to a plant's bursting forth in spring.  When the time is right, this terminal bud breaks first, because it produces hormones that inhibit buds further down the stem from breaking.  Once the apical bud begins to grow, the hormone production weakens, and successive buds also break.

Spring growth depends on a healthy plant, and occasionally winter weather can cause damage that will weaken a tree or shrub.  It all begins with the hardening off process.  As those leaves fall, the plant slowly settles in for winter.  But we have all seen what happens when our mild, often warm, autumn turns suddenly cold.  An early freeze or a rare dump of snow in November can harm deciduous plants that not have quite shut down for the season.

Even during winter, when the plants have slowed extensively, certain events can create problems.  In the midst of typically cold weather, when morning sun hits the trunk of a tree, the warmth can cause the sap to begin to flow while the trunk is still in its slightly constricted winter phase.  This causes vertical breaks called frost cracks (sometimes called sun scald) on the trunk.

An early warming can interrupt dormancy; a bout of warm weather can cause the star magnolia, for example, to start blooming.  Then, when a late freeze hits, early flowers can be damaged.  This occurrence may destroy one year's flower show, but a healthy plant can withstand that.  It's only if the plant is declining for other reasons or if there are repeated blows that it can do lasting damage.  A weakened plant is a susceptible plant, and disease pathogens can enter the frost cracks or the mushy remains of previously frozen buds.

But we hope for a fairly normal winter, when buds stay put until they are well and good out of the woods.  Many plants begin their leafing out and bloom cycle on predictable cues.  Apples, for example, need from 1,000 to 1,400 hours at 45 degree (temperatures in Fahrenheit) before they break.  Although days of 50-degree air temperature might not make a difference, when the bud temperature goes up to 50 degrees the resting period is overcome.  Other plants break dormancy with a combination of a period of cold followed by a period of lengthening daylight.

Before bud break, before spring, while all is fairly quiet above, below ground the root system continues - although in a much reduced fashion - to take up water and nutrients.  When a plant is dormant, it is a good time to apply fertilizers.  The roots take the nutrients up and hold them until spring, when everything goes into high gear.

Winter conditions, with colder temperatures and lack of sun, make it necessary for plants to hunker down and wait out the season.  Dormancy is a way to conserve resources until conditions are right to spend precious assets in order to grow leaves, and to flower - in order to continue the species.  It's also a good time for us to slow down, and take pleasure in branches and stems and the occasional winter flower.

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