Bonsai winter care: dormancy, protecting roots from frost and spotting common pests

TUREFF• •0

Winter is when well-meaning beginners lose outdoor bonsai, often by bringing them inside to keep them warm. Most temperate trees need the cold, and the real risk is to their roots, not their branches. Tropical indoor trees are the opposite. Here is how to tell the two apart and look after each, plus the pests to watch for.

Dormancy: why outdoor trees need winter

Temperate trees, such as maples, elms, junipers and pines, go dormant in winter. Growth stops and the tree rests. That rest is part of their annual cycle, and skipping it by keeping them in a warm room weakens them over time. So outdoor trees should stay outdoors as long as possible, with protection only from the worst of it.

Protect the roots, not the tree

In the ground, roots are insulated by the earth around them. In a small pot, they are exposed on every side, and roots are the most cold-sensitive part of a bonsai. The aim is to stop the pot from freezing hard and repeatedly, not to keep the tree warm. Common methods:

Grouping pots together and sheltering them by a wall or under a bench.
Burying the pot in the ground or a box of mulch so only the tree shows.
A cold frame, an unheated, covered box that shields trees from hard frost and winter wet. Open it on mild, sunny days so the trees do not warm up and start growing too early.
An unheated garage or shed for colder spells, if it stays cool and gets some light.

How much protection a tree needs depends on the species and your local winter. Hardy native species need less; less hardy ones need more. A local bonsai club knows what works in your area.

Watering in winter

Dormant trees use little water, but they are not maintenance-free. Trees under shelter may not get rain, and wind can dry them. Check the soil regularly and water on mild days when the soil is not frozen.

Tropical trees indoors

Ficus and other tropical species do not go dormant and must not freeze. Bring them in well before frost; many growers move them inside as nights start getting cool. Indoors in winter:

give them the brightest spot available, as light levels drop
keep them away from radiators and cold draughts
check watering more often if heating dries the air
expect slower growth

Common pests

Most bonsai pests are the same as on garden and house plants. Inspect your trees regularly, including the undersides of leaves, and treat early.

Aphids: small soft insects clustered on new shoots. They suck sap and leave sticky honeydew, which can grow a black sooty mould.
Spider mites: tiny and hard to see; look for fine speckling on leaves and fine webbing. They tend to like hot, dry conditions, so indoor trees in heated rooms are at risk.
Scale insects: brown or white bumps on stems and leaves that do not look like insects at first. A cotton bud dipped in rubbing alcohol can remove small numbers.
Mealybugs: white, cottony clumps in leaf joints and crevices.

Gentler treatments such as insecticidal soap or horticultural oil are a common first step. Follow the label, test on a small area first, and check whether the product is suitable for your species.

Not every problem is a pest

Yellowing leaves, dropping leaves and weak growth are more often caused by watering, light or temperature than by insects. Indoor trees often drop some leaves after a move or as light falls in autumn. Confirm you are looking at a pest before reaching for a spray.

A short winter checklist

Know whether each tree is temperate or tropical.
Leave temperate trees outside and protect their pots.
Bring tropical trees in before frost.
Keep checking the water.
Inspect leaves and bark for pests.

How do you overwinter your trees where you live? Share your set-up, or ask about a pest you have spotted, in the comments.