Bonsai Plant Care: Watering, Soil and Repotting

Before anything else, find out whether your tree is tropical or hardy, because the answer decides where it lives. Missouri Botanical Garden divides bonsai into two groups: tropical and subtropical species that thrive indoors - weeping fig, dwarf pomegranate, natal plum, rosemary and Serissa - and hardy types such as maples, pines, junipers and boxwoods, which require outdoor growing conditions for best growth and should only be brought inside briefly for display.

That single distinction accounts for most failed bonsai. A juniper bought as a gift and kept on a desk is a hardy outdoor tree in the wrong place, and no amount of watering or feeding fixes it.

The rest is ordinary horticulture done at small scale: a lot of water, a fast-draining soil, light feeding, and periodic root work. Bonsai is a technique applied to a tree, not a species.

Small pots dry fast. Missouri Botanical Garden's guidance is that in spring, summer and autumn watering is often required at least daily, and on hot days twice; use a fine mist and continue until water drains through the holes.

The Arnold Arboretum's collection staff check by placing a palm flat on the soil surface: if they detect any moisture they do not water, and if it is dry to the touch they do. Their watering method is to fill the pot, let it drain, then fill it a second time so the whole root ball is wetted. They also avoid wetting the foliage on sunny days, because water droplets can concentrate sunlight enough to burn spots into leaves.

That second filling matters. A single pass over a dry bonsai soil often runs straight through without wetting the middle of the root ball.

Missouri Botanical Garden recommends a well-drained soil mixed with screened sand. The Arnold Arboretum gives proportions: for conifers, one half sand, one quarter loam and one quarter peat; for deciduous trees, one third sand, one third peat and one third loam, with sand particles between one and three millimetres.

On repotting frequency, Missouri Botanical Garden says deciduous species every two to three years and pines and junipers every four to five, done in early spring. The Arnold Arboretum's figures are similar - larger specimens every four to five years, smaller plants every two to three - and describe removing two to three centimetres of roots and soil from the sides and bottom of the root ball, and cutting any root thicker than a pencil.

Root pruning is what keeps a tree small in a shallow pot without starving it.

Missouri Botanical Garden's feeding options are a water-soluble fertiliser such as 20-20-20 during the growing season, or a time-release product where one application in early spring generally lasts the full growing season.

The Arnold Arboretum feeds a very dilute chemical fertiliser, roughly 0.01 percent NPK, every one to two weeks from spring through mid-July, then every two to three weeks until October, stopping in November when the plants go dormant.

For pruning, Missouri Botanical Garden says to remove new growth regularly while leaving enough leaves to maintain the vigour of the plant. The Arnold Arboretum times it by species: deciduous trees such as cherries and maples in early spring, pines and spruces in mid spring, junipers and hinokis in early to mid summer, removing at least half the new growth.

A hardy tree in a shallow pot has almost no soil mass to insulate its roots, so winter is a management problem rather than a hardiness problem. The Arnold Arboretum stores its collection at 33 to 36 F from roughly mid-November to mid-April, checks water weekly, and finds the plants typically need watering about once a month. Plants must be fully dormant before they go into storage.

At home the equivalents are an unheated garage, a cold frame, or burying the pot in a sheltered spot outdoors. What you are protecting against is not cold itself but the repeated freeze-and-thaw of a small volume of soil, and drying out while the roots cannot replace the water.

Missouri Botanical Garden's advice to bring hardy types indoors only briefly for display applies here too: a warm living room in January breaks dormancy at the worst possible time.

The care instructions above split cleanly along one line, so identifying the species is the first practical step, not a detail. Missouri Botanical Garden's indoor list is weeping fig, dwarf pomegranate, natal plum, rosemary and Serissa; its outdoor list is maples, pines, junipers and boxwoods.

Many bonsai are sold with no label at all, or with a generic one. Photograph the whole tree, a close-up of the foliage and, if there is one, the bark in even light, and let the app read the species first; then apply the group it belongs to.

If it turns out to be a juniper or a maple that has spent months on a windowsill, move it outdoors gradually rather than all at once, and expect it to need time. A tree weakened indoors does not recover in a week.

For a first bonsai, three things matter more than anything else: know whether it is an indoor or outdoor species, never let the soil dry out completely, and give it as much light as you can. Bonsai Empire calls watering the most important part of bonsai care and says the right frequency depends on the species, pot size, season, soil and climate - which is why no fixed schedule works for every tree.

Bonsai Baime's after-care guide for indoor bonsai adds practical rules:

• Keep the soil consistently moist and never let it dry out completely; it gives every three to five days as a starting point depending on pot size, and warns that complete drying causes permanent damage.
• Do not keep moving the tree; frequent relocation stresses it. Rotating it about twice a year for even growth is fine.
• Keep indoor trees above 60 F and away from frost.
• Do not spray pesticide unless there are pests.

Missouri Botanical Garden's figure for spring, summer and autumn is much higher - often daily, twice on hot days - so check the soil rather than the calendar.

Yes, and more than most houseplants. Bonsai Baime recommends a south- or west-facing window with the tree within about six inches of the glass, and a minimum of four to seven hours of direct sun a day. Its per-species minimums: ficus four hours, jade five to seven, Hawaiian umbrella tree three to six. It warns against keeping a bonsai in a shady spot away from windows.

If a window cannot provide that, Bonsai Baime suggests a grow light three to six inches above the tree for 10 to 12 hours a day - and not at night, which it says confuses the tree. Missouri Botanical Garden's indoor recommendation is a south or east window, and for outdoor trees morning sun with afternoon shade.

There is no single official rule of three in bonsai; the phrase is used for a few different guidelines, and two of them are well documented.

• Groups use odd numbers. Bonsai Empire says a bonsai forest or group planting requires an odd number of trees - three, five and so on when only a few are used - to create asymmetry, and that most group plantings contain 7 to 15 trees. It recommends mixing sizes, with the tallest slightly off centre, and avoiding neat rows.
• Remove no more than a third. Bonsai Empire says a healthy tree can typically handle removal of up to one third of its foliage without significant stress, and advises spacing major work apart - if you do structural pruning in spring, leave root work until the next year.

Other versions of the rule circulate online, such as proportions for branch placement, but we could not find them in the sources we checked, so we do not repeat them here.

Bonsai Empire separates two kinds of pruning. Maintenance pruning, which keeps the shape, can be done throughout the growing season from early spring to late autumn for outdoor trees, and year-round for indoor trees; most trees benefit from it two to four times a year. Structural pruning, which removes larger branches, is best done in early spring and in some cases late autumn, just before or after the growing season.

The reason to prune the top and outer growth hardest is apical dominance: Bonsai Empire explains that a tree's main central stem grows more strongly than its side stems, so trimming the top redirects growth to the inner and lower branches. Bonsai Baime's indoor schedule is a trim every three to four months depending on growth rate.

The sections on beginner basics, sunlight, the rule of three and pruning timing were added on 2026-09-25. Research and drafting were done with the help of AI, and every figure was checked line by line against the sources listed on this page.