Growth cycles, dormancy, and reproduction

Instructional example: the nursery juniper from earlier lessons goes home in spring with a tag that says “indoor bonsai.” It sits on a sunny windowsill above a radiator, watered with care, and through summer, autumn, and winter it stays green.

The next spring, new growth starts late and comes in short and pale. Over the following months the foliage thins and the inner tufts turn brown. The tree was not neglected; it was in the wrong place for what it is. A juniper is a temperate tree, adapted to warm summers and cold winters, and it spent its year short of light, waiting for a winter that never came.

A tree's year, read in stages

Trees do not keep a calendar, but they do keep time. Phenology is the timing of a tree's recurring, visible events. Follow the trident maple, an instructional example, through a year outdoors in a temperate climate:

1. Bud break. As spring warms, buds swell, their scales part, and new leaves unfold, often tinged red, drawing at first on sugars stored the year before (Lesson 9).
2. Shoot growth. Shoots extend in a spring flush (Lesson 12), and a vigorous tree may flush again later.
3. Bud set and hardening. In late summer, shoots stop extending and form the buds that will carry next year's growth. The soft new shoots then firm up and turn woody, which growers call hardening.
4. Autumn color and leaf fall. As days shorten and nights cool, the green pigment chlorophyll breaks down. Yellow and orange pigments that were in the leaf all along show through. Many maples, the trident among them, also make new red pigments. Then the leaves drop.
5. Winter rest. The tree stays alive but stops growing until spring.

These stages arrive on different dates in different places and shift with each year's weather; south of the equator, July is midwinter. So this course times care by the stage you can see on the tree, not by the month.

Day length and temperature set the clock

A tree starts preparing for winter well before the first frost, by reading two signals.

The first is photoperiod, the length of daylight in each day. In any one place it follows the same pattern every year, whatever the weather. In many temperate trees, shortening days in late summer are the cue to stop extending shoots and set buds, while it is still warm.

The second is temperature, which varies more from year to year and fine-tunes the timing; in some trees it matters more than day length. Together, shortening days and then cool nights and early frosts drive cold acclimation, also called cold hardening: over several weeks, the tree's living cells change so that they can survive freezing. A fully acclimated twig survives cold that would kill the same twig in summer.

Acclimation fades in warmth, and a tree that spent the autumn in a warm room built little of it. Moved suddenly into frost, such a tree can be badly damaged, even if it would have been hardy had it stayed outdoors.

Dormancy: resting, not dead

Dormancy is a period when a tree, or a bud, is alive but not growing. Buds can also be held back by a growing shoot tip above them (Lesson 12), a different reason for staying inactive. In temperate trees, winter dormancy has two phases, and the difference between them answers this lesson's question.

Deep rest. By the time the leaves fall, the buds are in endodormancy: they will not grow even if the weather turns warm, because signals inside the buds hold them back. This protects the tree. Otherwise a warm week in late autumn could coax out tender shoots for the next frost to kill.

Time in the cold ends this rest. Each tree has a chilling requirement, an amount of time its buds must spend in cool weather before deep rest is released. Cool weather a few degrees above freezing counts most, hard cold counts for less, and warmth does not count. Freezing is not required. The amount varies widely among species, and even among varieties.

Waiting for warmth. Once chilling is met, the buds can grow, but cold still holds them back. This phase is ecodormancy. As warmth builds in spring, the buds break.

Without enough chilling, deep rest is never fully released. The next spring, bud break comes late, uneven, and weak: some buds open long after their usual time, many never open, and the new shoots are short. Less foliage makes less sugar, so the tree grows and stores less, and over several winters it declines.

Evergreens rest too

The nursery juniper keeps its foliage all winter but follows the same cycle: as days shorten, its shoots stop growing and harden, its foliage acclimates to cold, and its buds enter deep rest. Many junipers turn bronze or purplish in cold weather, a normal winter color that fades in spring. Like many temperate conifers, junipers generally need winter chilling too.

Indoors, an evergreen meets a second problem. Its foliage keeps respiring, faster when warm (Lesson 7), while even a sunny window gives far less light than open sky (Lesson 6), so it struggles to make the sugar it uses. The windowsill juniper missed its winter and ran short of light at the same time, and because it stayed green, both problems stayed hidden.

A missed winter shows up a year later

A missed winter does no visible harm at first, so the season wheel runs three years. Try these in order:

1. With Trident maple and Outdoors all year, press Play or step with Next through Year 1. When does Bud set and hardening begin, and what are the day length and temperature doing then? When does Winter chilling reach Met?
2. Switch to Warm room all year and compare Year 1, Year 2, and Year 3, watching Winter chilling and Condition.
3. Try Outdoors, warm room in winter, the move many beginners make to protect a tree from frost.
4. Choose Juniper in a warm room and switch Indoor light between Strong and Dim.

Outdoors, the maple set its buds while it was still warm, cued by shortening days, and its chilling was met in late winter, leaving the buds waiting only for warmth.

In the warm room, Year 1 looked normal, because the tree had spent the winter before at a nursery. No chilling built indoors, so Year 2 brought late, weak bud break, and by Year 3 the tree was declining. Bringing it in before the first frost did the same harm, stopping its chilling almost as soon as it began. The juniper declined too: in the model, strong light did not make up for its missing winter, and dim light made things worse. A warm room shelters a temperate tree from the very cold it needs; a sheltered place that stays cool is a different matter.

Trees from warm climates keep a different clock

Not every tree evolved with winter. Tropical trees come from warm regions near the equator with no cold season, and subtropical trees from the regions bordering them, where winters are mild and frost is rare and light. The ficus, the course's third example tree, comes from tropical and subtropical regions; for placement, treat it as a tropical tree.

A ficus has no chilling requirement and no winter rest to complete. Light and warmth set its pace: it grows strongly in long, warm, bright days and more slowly when either drops. But it has little ability to acclimate to frost. Frost damages it, a hard frost can kill it, and many tropical plants suffer from cold even above freezing. Some tropical trees do rest, but through a long dry season rather than a cold one.

Return to the season wheel, choose Ficus, and compare the three placements with Indoor light set to Strong and then Dim. Which placement harmed it, and what decided its condition indoors?

Left outdoors, the ficus was damaged by the first frosts and severely damaged by hard frost. Indoors, light decided its condition: it stayed healthy in Strong light, weakened in Dim light, and declined if kept dim all year. So a warm room can suit a ficus, but only where the light is strong; indoors it was short of light, not warmth.

Flowers, cones, and seeds in the cycle

Reproduction runs on the same seasonal clock, once a tree is mature enough. Young trees pass through juvenility, a phase when they cannot flower however well they grow, which can last from a few years to decades.

Many spring-flowering trees and shrubs, such as flowering apricot, crabapples, and azaleas, form next spring's flowers inside buds during the growing season before, a stage called flower bud initiation. The trident maple does the same: once mature, it opens small, yellow-green flowers with its new leaves, from buds formed the summer before, and later sets pairs of winged seeds.

Flowers and fruit are strong sinks, drawing sugars and mineral nutrients away from shoots, roots, and reserves (Lesson 9), so a heavy crop can set back a weak tree.

Conifers such as junipers make cones instead of flowers, and a ficus hides its tiny flowers inside its figs. The seeds of many temperate trees have a chilling requirement of their own: like the seeds that needed cold stratification in Lesson 11, they wait out a winter before they germinate.

Choosing where a tree spends its year