From seed to seedling

Here is an instructional example: a bonsai starter kit arrives as a gift, with a small pot, a bag of seed mix, and a packet of seeds with a tree's name on the front. Its new owner soaks the seeds, sows them, and keeps the mix moist on a warm windowsill, exactly as the directions say. Weeks pass, and nothing comes up.

Were the seeds dead? Possibly. But a healthy seed can also sit in moist mix and refuse to grow. A seed is alive and waiting: for water, for air, for the right temperature, and sometimes for a signal, such as a long spell of winter cold.

What is packed inside a seed

A seed is a young plant in a protective package, with enough food to get it started. Every tree seed packs three things:

• The seed coat, a tough outer layer that shields the inside from drying, injury, and decay.
• The embryo, a tiny, partly formed plant. It already has a young root called the radicle, a short stem called the hypocotyl, a shoot tip from which the young stem and its leaves will grow, and one or more seed leaves, called cotyledons.
• A store of food: starch, oils, and proteins the embryo will live on until its leaves can make sugar in the light.

Where that food is kept depends on the plant. In a maple, the two long, folded cotyledons hold it themselves. In many other flowering plants, such as wheat and corn, it sits beside the embryo in a separate tissue called endosperm. A pine seed wraps its embryo in a pale nutritive tissue full of stored food, and the embryo has a ring of several seed leaves, often five or more.

Germination, step by step

Germination is the process in which the embryo resumes growth and breaks out of the seed. Germination and the growth that follows run in a consistent order:

1. The seed takes up water. A dry seed holds so little water that its cells are nearly inactive, which is how it can wait for months. Now it soaks up water and swells, a step called imbibition.
2. Its cells restart. Respiration resumes, at first using small amounts of sugar already in the seed; once growth begins, the main store is broken down into sugars and other building blocks. Respiration needs oxygen (Lesson 7), so a waking seed needs air as well as water.
3. The root comes out first. The radicle pushes through the seed coat and grows down, anchoring the young plant and taking up water. Botanists usually count germination as complete here; what follows is the growth of a seedling.
4. The stem pushes up. In many trees, the hypocotyl grows upward bent into a hook, so the bend, not the delicate shoot tip, leads the way through the soil.
5. The seed leaves open. Above ground, the hook straightens, and the cotyledons spread in the light and begin photosynthesis (Lesson 6).
6. True leaves follow. The shoot tip between the seed leaves makes true leaves, the first leaves with the form of the plant's later foliage.

Until its first green leaves reach light (in maples and pines, the seed leaves), every new cell of root and stem is paid for from the seed's stored food, its packed lunch. Watch that handover in the interactive: on the Sugar supply chart, when does each line rise or fall, and where do they cross? Compare the Pine-like seed, then try Order it without looking back at the list.

In both seeds the root led, and the photosynthesis line stayed at none until the seed leaves opened in the light. The lines crossed around the time the first true leaves appeared, but not because of them: young leaves are sinks that import sugar while they are small (Lesson 9). The green seed leaves, working in the light, had begun supplying more sugar than the dwindling store; the timing varies by species. That crossover is the turning point from living on a packed lunch to living on light. Notice, too, that a maple's two strap-shaped seed leaves look nothing like maple leaves.

What germination needs

Water starts germination, but it is only one requirement. Germination needs the first three of these together, and some seeds also need the fourth:

• Water. Enough to soak in and to keep the seed moist while it grows; a seed that dries out after it has started can die.
• Oxygen, for respiration. Oxygen moves roughly ten thousand times more slowly through water than through air, so in waterlogged mix a seed can take up plenty of water and still fail or rot.
• A suitable temperature. Too cold, and the seed's chemistry runs too slowly for growth; it may wait until conditions warm. Too hot, and many seeds will not germinate, and some are damaged.
• Light or darkness, for some seeds. Some seeds, often small ones, germinate only after sensing light, which keeps them from starting too deep to reach the surface; a few are held back by light. This is light as a signal, separate from the light a seedling later needs to make sugar.

Dormant is not dead

Sometimes a seed has water, air, and warmth and still does nothing for months, like the seeds in the starter kit. It may be dead: a seed's viability, its ability to germinate given the right conditions, falls with age and poor storage, and the acorns of many oaks die if they dry out.

Or it may be dormant. Seed dormancy is a state in which a living seed will not germinate, even in good conditions, until a particular requirement has been met. A seed that fails only because conditions are wrong will go ahead once they are fixed; a dormant seed keeps waiting.

Dormancy protects a tree's offspring. Many temperate trees shed seed in autumn, and a seed that sprouted in the next warm spell would leave a tender seedling to face winter. Two kinds of dormancy are common in tree seeds:

• Dormancy in the embryo. The embryo is held back, often until it has spent a long period cold and moist, as it would through a winter outdoors. Growers imitate that winter with stratification: holding seeds moist and cool, usually a little above freezing, for weeks or months. The chill readies the embryo; germination usually follows once conditions warm.
• A hard, waterproof seed coat. Water cannot get in, so imbibition never starts. Growers use scarification: nicking, sanding, or otherwise weakening the coat (or, for some seeds, a hot-water soak) so water can reach the embryo. Hard coats are common in the pea family, which includes black locust.

Some seeds have both kinds and need scarification followed by stratification; some have neither.

Test this in the simulation, predicting each result before you sow. Start with Seed A, which has no dormancy, and change one condition at a time: try Waterlogged, Dry, and Too cold. Then give Seeds B and C the conditions that worked, and try each pretreatment on each seed.

Every seed needed the same basics: water, oxygen, and a suitable temperature. Waterlogged mix stopped even Seed A, though it offered the most water. Seed B took up water and respired but stayed dormant until it had its cold, moist period; Seed C stayed hard until its coat was scarified. Each treatment removes a different block, so one cannot stand in for the other.

Every seed has its own instructions

Dormancy requirements vary between species, and even between batches of one species collected in different places. Some seeds need weeks or months of cold, some a warm period and then a cold one, and some take more than one season to come up. No single recipe fits every seed, so find out what yours needs from a reliable source, such as the seed supplier or Iowa State University's guidance on germinating tree seed.

From seedling to tree

Once a seedling lives on its own leaves and roots, it is established, but far from safe. Its few shallow roots sit where the mix dries first, and it has little stored food to fall back on. In dim light it stretches, pales, and makes too little sugar to build roots; in wet mix, crowded in still, humid air, it is prone to damping-off, rot caused by several soil fungi and water molds that can kill seeds before they emerge or collapse young stems at the soil line.

Even a healthy seedling is a long way from a bonsai. After its first season, many are barely thicker than a matchstick. Building a trunk worth styling takes years of largely unrestricted growth, and how many depends on the tree, the climate, and what you want to make. Lesson 12 explains why a tree has to grow long to grow thick.

Growing from seed has real rewards: it is inexpensive, you see a tree's whole life from its first root, and each seedling differs a little from its siblings, giving you variety to choose from. Course 13, Propagation and Creating New Material, will teach it in depth. But it is an option, not a requirement. A young nursery tree, like this course's nursery juniper or trident maple, has already done years of that growing, so you can start learning to care for and shape a tree this season.