Scarlet firethorn bonsai

Placement

Scarlet firethorn is grown outdoors all year. During the growing season, give it full sun or partial shade: it needs good light to flower and fruit, and a spot that is too shady clearly cuts down the number of berries. In a very hot climate, shelter it during heat waves.

This species is more cold-hardy than Pyracantha angustifolia, the other firethorn commonly grown as bonsai. In a pot, though, the roots are exposed: protect the tree from hard frost and icy wind as soon as temperatures fall below 14 °F. A cold frame is ideal for winter. In spring, a late frost can damage the flowers without the damage being obvious at first glance.

The flowers need insects to set fruit: flowering during cold, wet or windy weather often yields few berries. Keep the tree in the open air while it is in bloom.

Precautions: the long, very sharp thorns injure easily; wear thick gloves for pruning and wiring. All parts of the plant contain compounds that can release hydrogen cyanide, so it is mildly toxic. The scarlet firethorn plant profile covers its toxicity and garden care in detail.

Pests and diseases: watch for aphids, woolly aphids, scale insects, spider mites, caterpillars and leaf miners, including the leaf-mining moth specific to firethorn. Scab covers leaves and berries with black spots: cut off affected shoots. Two other diseases can affect the tree: fire blight, which is bacterial, and sooty mold, which is fungal. Against pests, use a treatment suited to the insect involved. Overwatering leads to root rot.

Soil mix

Scarlet firethorn has no special needs: a free-draining standard bonsai mix suits it. It tolerates slightly acidic, neutral or alkaline soil. What matters most is that water drains freely, because its roots dislike standing water.

Watering

Keep the root ball slightly moist, without a fixed schedule: check the soil with a fingertip and water as soon as the surface starts to dry, until water runs out of the drainage holes. During flowering and fruit set, in summer, the tree needs plenty of water: a root ball that is too dry at that time causes the berries to drop early. In winter, water much less, but never let the root ball dry out completely. Avoid the opposite excess as well, which causes root rot.

Feeding: during the growing season, apply a solid organic fertilizer every four weeks or a liquid fertilizer every week. Choose a balanced feed with a little extra phosphorus and especially potassium, which encourages flowering and fruiting.

Pruning

Scarlet firethorn tolerates hard pruning and buds back from old wood when it is healthy. The only real difficulty is keeping the flowers: they form mainly on shoots from the previous year. Keep as much two-year-old wood as possible, and be careful with hard pruning in late summer and with removing the short flowering spurs.

• Structural pruning: thin out branches that are too dense and remove deadwood in spring or late summer.
• Maintenance pruning: after flowering, cut new shoots back to two leaves once they have lengthened. Most pruning is done with scissors.
• Large leaves: remove them at any time. In early summer, partial defoliation, which means removing all the large leaves, encourages ramification.
• Fruit and spent flowers: remove old berries early in spring. On a weak or freshly repotted tree, cut off spent flowers so it regains vigor, even if that means giving up fruit for the year.

Styles: firethorn is often grown as a cascade. It also suits the informal upright style, with a single or multiple trunks, in every size.

Wiring

The thorns make wiring tricky: remove those in the way before applying wire, or use guy wires, stretched between the branch and the pot or a sturdy root, with the bark protected. Young shoots bend easily and respond well to wire. Older branches become stiff and brittle, sometimes from two years old: bend them gently, pushing on the inside of the curve, and do not bend the same spot repeatedly.

Choose wire about one-third the thickness of the branch, coiled at about 45°. Hardened wood takes a long time to hold its new position. Check the wire often during the growing season and remove it by cutting it at each turn, without unwinding it, before it marks the bark. Do not wire a weakened or freshly repotted tree.

Repotting

Repot a young tree every two years and an older specimen every three to five years, early in spring, as the buds begin to extend. The tree tolerates root pruning, but heavy disturbance of the root ball can reduce the next flowering: on a tree grown for its flowers, stay moderate.

Propagation:

• Cuttings: take semi-ripe cuttings in summer.
• Seed: sow outdoors in fall; the seed needs about three months of cold to germinate, and cultivars usually do not come true from seed.
• Air layering: the third possible method.