Start with the place you can grow

The owner of the west balcony, an instructional example this course returns to, is walking through a garden center in May. A trident maple stops them: a thick little trunk and fresh three-lobed leaves, still tinged red. It would look lovely by the balcony door.

Before the tree, think about what the balcony would ask of it. On July afternoons, sun beats onto its concrete floor from early afternoon until sunset, the air reaches 30 to 33 °C (86 to 91 °F), and wind blows in at the open end. On the coldest night of a typical winter, the pot would sit four floors up in air at about −15 to −17 °C (0 to 5 °F), with no ground around its roots. There is no tap outside, and every August the owner is away for two weeks.

None of that is printed on the maple's label, and none of it would change if the owner fell for a different tree instead. The maple might do well there, with some planning. A ficus could not survive a January night out there at all. Which trees could thrive on this balcony depends first on the balcony.

Why the place comes first

You can choose among hundreds of kinds of trees, but you have only a few places to grow them, and you can change those only a little, with a bench, a screen, a drip tray, or a grow light. You cannot turn a west balcony to face east, move your city into a milder climate, or add a tap where the building has none. It is far easier to find a tree that suits a place than to make a place suit a tree.

The place matters this much because a bonsai's needs are a full-sized tree's needs (Course 01, Lesson 1): strong light for its leaves (Course 01, Lesson 6), water and air for its roots in a small pot that dries quickly in sun and wind (Course 01, Lesson 8), and, for a temperate tree adapted to warm summers and cold winters, its seasons, including a cold winter rest (Course 01, Lesson 13). A tree in the wrong place often declines slowly, and it can look fine for weeks or months before it shows.

What to look at in a growing place

A growing place is any spot where a tree could live: a balcony, a yard, a roof, a doorstep, or a window. This course follows three, all instructional examples in one temperate Northern Hemisphere city:

• The west balcony. A fourth-floor apartment balcony facing west, 4 m (13 ft) wide and 1.8 m (6 ft) deep, with a concrete floor and a glass railing. The balcony above roofs the back half.
• The bright window. An apartment with no outdoor space, but a large south-facing living-room window with a sill 25 cm (10 in) deep and a radiator below it.
• The small backyard. A fenced yard on the south side of a house, with a paved patio along the warm house wall, a lawn, a neighbor's large maple beyond the west fence, and an unheated garage.

Look at the same things in each: light, shelter, water, access, time, and the local climate. The west balcony is the worked example.

Light through the day and the year

Light is the first thing a tree needs from a place, and the easiest to misjudge. What matters is direct sun at the height where the tree would stand: how many hours, at what time of day, and in which seasons. In the Northern Hemisphere, where these examples are, south-facing places get the most sun, and the sun is higher and up longer in summer than in winter; in the Southern Hemisphere, swap north and south.

The west balcony gets direct sun from early afternoon until sunset: about six hours in midsummer. That is good light for an outdoor tree, but it comes in the hottest part of the day, so it heats pots and dries leaves more than the same hours of gentle morning sun would. In winter the low sun reaches farther under the overhang but sets early, giving about three hours.

At the bright window, direct sun reaches the sill on clear winter days from late morning to mid-afternoon; in summer the high sun barely gets inside. Even so, a window that looks bright passes far less light than open sky, and the light falls off steeply a few steps back into the room (Course 01, Lesson 6). Lesson 4 shows how to map sun and measure light; for now, note when direct sun reaches the spot, and for how long.

Shelter, heat, and cold air

Walls, overhangs, fences, and corners make differences a tree can feel, even a few steps apart. Wind and heat raise the evaporative demand on leaves, the pull that draws water vapor out of them (Course 01, Lesson 8), and strong wind can tip a pot over.

On the west balcony, the open south end is windy, while a solid privacy screen shelters the north end. An air-conditioner condenser on the floor in the north corner blows hot air whenever it runs in summer. In the small backyard, the house's south wall and the patio along it soak up sun and give it back as heat on summer afternoons, and the southeast corner, the lowest part of the yard, collects cold air on still, clear nights. Mark spots like these in your own place; Lesson 5 explores them.

Water and where it goes

A tree in a pot depends on you for water much of the year. Every thorough watering should wet the whole root zone and end with water running from the drainage holes: roots need air as well as water, and a pot that cannot drain stays waterlogged (Course 01, Lesson 7). So ask two questions: how far away is the water, and where will the runoff go?

The west balcony has no outdoor tap, so every can is carried from the kitchen, and drips land on the neighbors' balcony below; pots against the back wall, under the overhang, get little rain even in wet weather. At the bright window, the kitchen tap is a few steps away, but runoff from the sill would reach the radiator and the floor. The small backyard is the easiest: a hose tap on the house wall, and runoff that soaks into the lawn. Carrying every drop also limits how many pots one person can keep (Lesson 7 covers drainage and a workable setup).

Access, rules, and safety

A place is useful only if you can reach it in every season. A roof hatch locked in winter, a garden plot you visit only on weekends, or a front step buried in snow all fail just when a tree may need checking, and a resting tree still needs its moisture checked through the winter (Course 01, Lesson 13).

Buildings and landlords often have rules too: nothing on or hanging from railings, limits on weight, no dripping onto balconies below, fire escapes kept clear. Ask before you plan around a spot. Safety matters as well: a pot on a fourth-floor railing can be knocked off by wind or an elbow onto whoever is below, so on the west balcony, pots stay on the floor or on stable stands inside the railing.

Your time, and who can help

A healthy bonsai does not need hours of work, but it does need someone to look at it often. In the growing season, that usually means a few minutes of checking on most days: is the root zone starting to dry, and how do the leaves look? In summer heat a small pot can dry out in a single day, so checks become daily, sometimes twice daily. Winter asks for less, but not nothing. Course 03, Watering and Daily Observation, will turn this into a routine. So record your time honestly, including your travel and who could water while you are away, once shown how:

• The west balcony's owner travels for work about one week a month and is away two weeks every August, in the hottest part of the year. A friend can water if shown how.
• The bright window's owner works from home and rarely travels.
• The small backyard's owners are away two weeks every July, usually the hottest month. A neighbor can water.

Read the local climate

Every place sits inside a climate, which sets limits no bench or screen can change. For a tree in a pot, these facts matter most:

• Winter lows: how cold the coldest nights get, and for how long. They decide which trees can stay outdoors and how much protection their pots need.
• Summer highs: how hot afternoons get, and how often heat waves come. Heat speeds water loss and can overheat roots in a small pot.
• Frost dates: when frost usually ends in spring and starts again in autumn.
• Rain: how much falls, and when. Rain helps, but it rarely waters a pot reliably.
• Wind: where strong winds usually come from, and in which seasons.

The example city has warm to hot, humid summers, with July afternoons often 30 to 33 °C (86 to 91 °F) and occasional heat waves near 36 °C (97 °F). Its winters are cold, with January nights often −5 to −10 °C (14 to 23 °F). Rain falls in every season.

A frost date is the average date of the last frost in spring or the first frost in autumn, worked out from many years of weather records. Here the last spring frost usually comes in late April, and the first autumn frost in mid-October. The frost-free months between them are the growing season, about five and a half months in this city: the part of the year when trees do most of their growing, and the longest a frost-tender tree could spend outdoors. A ficus, which many growers keep above about 10 °C (50 °F), gets less. Frost dates are averages, not promises. Some springs bring a frost a week or two after the usual date, and a low corner can frost on a night when the rest of the yard stays clear.

What a hardiness zone tells you

Plant labels and catalogs often give a hardiness zone. In the United States, the Department of Agriculture (USDA) maps the country by the average annual extreme minimum temperature: the coldest temperature reached each winter, averaged over many years (1991 to 2020 for the current map). Each zone covers a band of 10 °F (about 5.6 °C), split into halves marked a and b. The example city's coldest night in a typical winter is about −15 to −17 °C (0 to 5 °F), which places it in zone 7a.

A label that says a tree is hardy to zone 5 means it usually survives the winter lows of zone 5, which are colder than zone 7a's, when it is planted in the ground. In the ground, the nursery juniper from Course 01 is hardy in roughly zones 4 to 9, and the trident maple to roughly zone 5. The ficus, treated as a tropical tree for placement, is damaged or killed by frost, so in this climate it can be outdoors only in the warm months.

Other systems are similar. In the United Kingdom, the Royal Horticultural Society rates plants rather than places, from H1, for plants that need a heated greenhouse, to H7, for plants hardy below about −20 °C (−4 °F). Many countries publish their own zone maps, and some, such as Canada's, combine several climate measures. Either way, you compare the tree's cold tolerance with your place's winter lows.

A zone tells you one thing well: how cold a typical winter's coldest night gets. It tells you nothing about summer heat, humidity, rain, or wind, about how long cold spells last, or about a sudden hard freeze that arrives before trees have built their cold acclimation in autumn. The warm end of a range such as “zones 4 to 9” often hints at where summers get too hot or winters too mild for that tree, but the map itself measures only winter cold. And it describes plants growing in the ground.

Roots in a pot need a margin

In the ground, a tree's roots sit in a great mass of soil that changes temperature slowly. On the coldest night, the soil around the roots stays much milder than the air, especially under snow. In a pot standing above ground, cold reaches the roots from every side, including from below, and over a long cold spell the whole root ball can get nearly as cold as the air.

The roots are also the tenderest part. Roots are generally less cold-hardy than the buds and twigs above them (Course 01, Lesson 13), and many are killed by temperatures the branches survive easily. So a zone rating overstates how hardy a tree in a pot is.

A common rule of thumb for plants that spend the winter outdoors in containers is to choose ones rated at least one or two zones colder than your own. Even then, bonsai growers protect their pots in hard freezes, often in an unheated shelter that stays cold, so the tree still gets its winter rest (Course 06, Seasonal Care and Timing). In zone 7a, the juniper has about three zones to spare and the trident maple about two, and both still need their roots protected on the coldest nights.

Where to find local climate data

You do not need to guess any of this.

• A national weather service publishes climate normals for its stations: average monthly highs and lows, extremes, and often frost dates.
• Extension services and botanic gardens publish frost dates, zone maps, and plants that do well locally. In the United States, each state's cooperative extension service does this, and the USDA Plant Hardiness Zone Map can be searched by ZIP code; elsewhere, agricultural or horticultural advisory services do the same job.
• Local bonsai clubs and nurseries know which trees come through the winter in pots where you live and how growers protect them. Their experience comes closest to your conditions.

A weather station measures air in an open, standard spot. A balcony four floors up, a sunny wall, or a low corner can be warmer or colder, so your own observations belong in the record too; Lesson 5 shows how to check with a thermometer at pot height.

Turn each constraint into a criterion

Each fact about a place is a constraint: something any tree there will have to live with. A selection criterion turns that fact into a requirement for the tree and its setup. “A front step with direct sun only in the morning, about four hours” becomes “trees that grow well with a few hours of morning sun.” “The building allows nothing on the railings” becomes “pots on the floor or on stands inside the railing.”

Some criteria describe the tree: how much light it needs, how cold-hardy it is, whether it needs a winter rest. Others describe the setup: a bench, a tray, a shelter for the coldest nights, a helper for the holidays. Both come before any tree is named, because while you are still choosing, a criterion a tree fails is a reason to choose a different tree; once the tree is home, it becomes a problem to solve.

In From a place to a shopping list, start with West balcony. Read each card under Observations and choose what it leads to under It leads to. Every choice explains itself, and you can change an answer at any time. Each right match adds a line to This place asks for… under the picture. When the list is complete, press Next: Bright window, and then do the Small backyard. Your answers for each place are kept while you switch.

One climate gave three different lists. On the balcony, the tightest limits were the hot afternoon sun, roots exposed to winter cold, and watering while the owner travels. At the window, they were light and the missing winter cold, while time was plentiful. The backyard offered the most: sun and shade, an easy tap, and a garage for winter shelter, with the hot paving, the low corner, and the July trip as its limits.

The tempting wrong answers failed for a few recurring reasons. A zone label is not enough for roots in a pot. “Tropical” and “indoor” trees answer neither summer heat nor time away; they only move the problem. No tree in a pot does without water, and a pot with no drainage hole traps water instead of solving the runoff. A south window is still far dimmer than outdoors, and an “indoor bonsai” label does not make a temperate tree an indoor one.

And none of the finished lists named a tree. That comes next. Lesson 2 covers what to find out about a tree before you choose it, and Lesson 3 whether it belongs indoors or outdoors, so that a list like this can become a short list of trees.

A site profile for the west balcony

A site profile is a short written record of a growing place and the criteria it leads to, on a page or two. It is worth writing even for a single place, because it keeps the facts in front of you when a tree in a shop is tugging at you.

Here is the west balcony's, written by its owner in early summer after a week of observations:

The profile also notes what is still unknown: how cold the balcony floor really gets in January, where the pots could shelter in a hard freeze (a question for the local bonsai club), and whether the friend can come every couple of days in August. It sets a date to look again in midwinter, when the low sun reaches farther in.

It already works as a filter. A ficus fails the winter line unless the owner can also offer a bright indoor place for most of the year. The nursery juniper and the trident maple both pass, with plans for heat, wind, and the August trip; the maple, whose leaves can scorch in strong afternoon sun and wind, needs those plans most. Lessons 2 and 3 do this sorting properly.

Profile the place before you shop