Mugo pine bonsai

Placement

Pinus mugo is a pine from the mountains of central and southern Europe that often forms low, sprawling shrubs on bedrock near the tree line. As a bonsai it lives outdoors year-round and needs full sun: in shade, the needles lengthen and the tree loses its compact look. It dislikes strong wind but tolerates a short dry spell.

Like all pines, it must spend the winter cold and dormant, for several weeks at roughly 32–45 °F, so it cannot live indoors for long. In the ground it ranks among the hardiest conifers, but a root ball in a pot freezes far sooner than garden soil. Where nights often drop below 14 °F, keep it in a cold frame or unheated greenhouse, ventilated on sunny days so the tree stays dormant. Elsewhere, wrap the pot in insulation or bury it up to the roots, in a spot sheltered from wind.

Start with an ordinary nursery mugo pine rather than a dwarf rock-garden cultivar such as ‘Mops’, which grows weakly and responds poorly.

Soil mix

Mugo pine needs a very free-draining mix that water passes through at once. A classic pine mix combines akadama, pumice and lava rock in equal parts. Sift the akadama to remove dust, which clogs the pot; in a rainy climate, add more lava rock, and if you cannot check on watering often, add more akadama.

Its roots live in partnership with fungi called mycorrhizae, which are essential to its health. When repotting, do not wash the roots, and always leave at least half of the root mass untouched to preserve these fungi.

Watering

Water when the surface of the mix starts to dry, soaking the whole root ball until water runs from the drainage holes. In midsummer, a tree in full sun can need water every day or even more often; in winter, while dormant, it needs little, but the root ball must not dry out. Too much water, such as prolonged rain, lengthens the needles and invites root rot, so shelter the pot from long rainy spells.

Fertilizer matters, because a vigorous pine is easier to work. Feed generously with an organic or slow-release fertilizer from spring to late summer.

Pruning

Mugo pine produces just one flush a year. You should therefore never remove all of its candles, as is done with Japanese black pine: it would not form new shoots until the following year.

In spring, when a candle reaches 1–2 in and clearly outgrows the others, break off its tip with your fingers to redirect energy toward weaker areas. Let candles grow freely, on the other hand, on a branch that needs to thicken or lengthen. At each branching point, reduce the buds to two so that the branch forks, in early spring and again in summer.

In early summer, once the year’s growth has hardened, cut it back hard: to one or two needle pairs in vigorous areas, and to three or four pairs inside and low on the tree. Always leave at least one needle pair per shoot. This pine has a valuable trait: it back-buds where old needles remain. Keep them where you want new buds, and pluck last year’s needles from the tops and undersides of branches where you do not. The young buds that result are fragile, so wait at least two years before pruning or wiring them.

Structural pruning is best done in late winter. Never remove more than half the foliage of a nursery pine in one growing season, and reduce trunk height in stages. A removed branch can become a jin, deadwood stripped of bark and bleached with lime sulfur; carve it in early spring or late summer, and only on a vigorous tree.

Wiring

Its wood is fibrous and resinous, but young trees accept styling well. Wiring and styling are best done in late summer or early fall, and remain possible until early spring; a young, vigorous tree also tolerates wiring in summer, right after its growth has been cut back. Use copper wire, which holds better, about one third the thickness of the branch, applied at 45°. Watch the wire during the growing season and cut it off at every turn before it marks the bark. On heavy branches, protect the bark with damp raffia or use guy wires.

Repotting

Repotting as the buds swell is possible, but mugo pine often reacts badly to it. The safest window is summer, after the first flush has been cut back and before the season ends; early fall also works, leaving fine roots time to regrow before winter. On a healthy tree, remove the old, compacted mix by hand without washing the roots; if its vigor is uncertain, remove only half and finish at the next repotting. Do not reduce the foliage to balance the root pruning: it helps the tree rebuild its roots.

A young, vigorous pine can be repotted, pruned and wired in a single session after the first flush, then left to recover for two to three years. A mature tree should receive only one major job per growing season, with a year’s wait before the next: a pine styled in summer is not repotted the following spring.

It is propagated from seed or by grafting. The tree pictured, donated to a public bonsai collection in 1972, is trained in semi-cascade style, a form that recalls its low, sprawling habit in the mountains. Watch for aphids, pine needle scale, spider mites, caterpillars and sawfly larvae, as well as tip blight, rusts and rot in soggy soil. Its needs in the garden are covered in the mugo pine plant profile.