A bonsai is a living tree

Picture two trident maples. One grows in a park, about as tall as a two-story house. The other sits in a shallow pot on a wooden bench, about as tall as your arm from elbow to fingertip. Their leaves have the same three-pointed shape, their buds sit in the same pairs along each shoot, and in spring both push out new growth from those buds. Inside both, every living cell is doing the same work.

The small one is not a different kind of plant. It is the same kind of tree, grown in a different way. So how can it stay small for decades and still be healthy?

An ordinary tree, grown in a particular way

The word bonsai is Japanese and means, roughly, “planted in a tray or container.” It names a practice, not a plant: growing a tree in a container and shaping it over many years so that it suggests a mature tree at a small scale. Bonsai developed in Japan from penjing, an older Chinese art of growing trees and miniature landscapes in containers, and is now practiced worldwide. Course 08, Bonsai Design and Styling, will cover its history and culture.

Every bonsai is a woody plant: a tree, shrub, or woody vine whose stems live above ground from year to year and thicken as they age. There is no separate group of bonsai plants, no bonsai gene, and no such thing as a bonsai seed; a packet sold as “bonsai seeds” holds the seeds of ordinary trees. The junipers, maples, elms, pines, and figs grown as bonsai are the same plants found in gardens and forests.

Two words help you say exactly what a tree is. A species is a group of closely related plants that share distinctive features and usually breed with one another. Each has a two-part scientific name: the trident maple is Acer buergerianum. A cultivar is a cultivated variety, selected for traits such as compact growth or small leaves and kept consistent through propagation. Its name follows the species name in single quotes, as in Juniperus procumbens ‘Nana’, a compact Japanese garden juniper. Even a compact cultivar grows every year.

Three instructional example trees return throughout this course, all ordinary nursery plants: the nursery juniper, a young plant of that cultivar that keeps its short, prickly needles all year; the trident maple, which drops its leaves each autumn; and the ficus (Ficus microcarpa), a broad-leaved tree from warm climates that keeps its leaves all year.

What a small tree still needs

Shrinking a tree does not shrink its biology. A bonsai's leaves, buds, and roots are made of the same kinds of living cells as a park tree's, and those cells need the same things:

• Light. Leaves use light energy to make sugars in photosynthesis (Lesson 6).
• Water. Roots take it up, and it moves to the leaves, where most of it evaporates (Lesson 2 and Lesson 8).
• Air. Leaves take in carbon dioxide to make sugars. Every living cell, roots included, uses oxygen day and night to release energy from sugars, a process called respiration (Lesson 7). Roots get that oxygen from air spaces around them.
• Mineral nutrients. Mineral nutrients are elements such as nitrogen, potassium, and iron, taken up by the roots from the water around them (Lesson 10).
• Suitable temperatures through the seasons. The trident maple and the nursery juniper need a cool winter rest outdoors; the ficus is damaged by frost (Lesson 13).

Food is missing from that list because a tree makes its own. Its leaves use light energy to combine carbon dioxide and water into sugars, which the tree also turns into starch for storage and cellulose for new cell walls. Sugars, starch, and cellulose are all carbohydrates: the tree's building material and its fuel. Mineral nutrients are essential but supply no energy. Fertilizer, which supplies them, cannot make up for too little light or too few healthy leaves, so this course does not call it “food.”

To see what depends on size and what does not, move the “Tree size” slider one step at a time from “Mature tree” to “Small bonsai.” After each step, compare the two leaves in the “One leaf” panel, then read the two cards below them. Then turn on “Show what it needs” and step through the sizes again.

Everything in the “Changes with size” card is about size and setting, including the root-zone buffer: the reserve of water and air around the roots, which falls from Very large in open ground to Very small in the smallest pot. The other card holds the biology, and with the needs switched on, the same arrows reach the leaves and the root zone at every size. The real leaf stays about the same size too; if it shrank with the tree, it would be a grain of rice on the medium bonsai and a speck on the small one.

What actually keeps a bonsai small

If a bonsai's needs do not shrink, something else must keep it small. Before reading on, sort eight things people do to keep a bonsai small. Each card explains itself after you choose, and you can change any answer.

Four things work together, and none depends on withholding light, water, air, or mineral nutrients. Two of them limit how large the tree grows; the other two make a small tree look in proportion.

A container. A tree's crown and roots grow in relation to each other. In open ground, roots often spread beyond the edge of the canopy. A pot limits how far they can go, and a limited root system tends to support a smaller canopy. The container limits size without stopping growth: a healthy bonsai still makes new roots, new shoots, and a little more wood every year.

Pruning. Each growing season, a healthy bonsai extends its shoots. Pruning removes some of that extension, so the outline stays about the same size while the tree keeps growing. It takes some leaves with it, but the tree keeps enough healthy foliage to go on making carbohydrates. Course 09, Pruning and Managing Growth, will teach how and when.

Material choice. In bonsai, “material” means the plant you start with. Species and cultivars with naturally small leaves and short stretches of stem between them stay in proportion at a small size more readily.

Design and proportion. Much of what makes a bonsai look old and large is design: a trunk that tapers from a broad base, branches arranged as they might be on an old tree, open space between them, and a suitable pot. Your eye reads those proportions and fills in the scale, even though a trident maple bonsai has, more or less, trident maple leaves. With dense, fine branching, the leaves of many trees become somewhat smaller, but nowhere near in proportion. Proportion, not miniature biology, does the work.

Each of the four practices that starve or stress the tree takes something away: light, water, mineral nutrients, or the healthy foliage that makes its carbohydrates.

Small is not the same as starved

Starvation and size control can look alike for a while, because both can keep a tree small. The difference shows over months and years. A healthy small tree carries full, well-colored foliage, makes new shoots and buds in its growing season, and responds when pruned. A deprived tree carries sparse or pale foliage, makes little new growth or only thin, stretched shoots, and gradually loses twigs.

Small does change one thing about care: how closely you need to watch. A pot holds a far smaller reserve of water and air than open ground, so on a hot or windy day it can dry out much faster than a garden bed. It can also stay too wet. After watering, water tends to linger in the lower part of a container instead of draining away into the ground below, and roots in persistently waterlogged potting mix can run short of oxygen (Lesson 7). So a bonsai typically needs closer observation than a tree in the ground, not less care. Course 03, Watering and Daily Observation, will turn that into a routine.

Health comes first

A healthy tree grows, and growth is what bonsai is built from. New shoots give you branches to choose among, buds give you places for new growth after a cut, and thickening trunks and branches add taper and age. A tree that cannot grow cannot respond to pruning, wiring, or repotting, however skillfully it is styled.

So the first job is always to keep the tree healthy, and that starts with understanding how it works. The rest of this course builds that understanding in three sections: tree structures, continuing with roots; resources and transport, beginning with photosynthesis; and growth through time, beginning with seed and seedling. The later courses these lessons name by number, such as Course 03, Watering and Daily Observation, are still being prepared.

That understanding pays off the first time you bring a tree home. Take the nursery juniper, an instructional example, sold with a tag that calls it an “indoor bonsai.” Like most junipers grown as bonsai, it is an outdoor tree that needs full outdoor light, which even a bright room cannot match, and the seasons, including a cool winter rest. Kept indoors, such junipers usually decline, often too slowly to notice at first. Nothing on the shelf warns you; only knowing what the tree is tells you the tag is wrong.