Satsuki azalea

Identification and origin

Rhododendron indicum belongs to the Ericaceae family. Despite its epithet, which means “of India,” it is native to Japan and has become naturalized in places such as the Azores, China and Korea. It was long known as Azalea indica.

It is a small, dense, spreading shrub with a slow growth rate, usually reaching 2–3 ft tall and as wide, sometimes 3.5 ft. Twigs and leaf stalks carry stiff reddish-brown hairs, and the leaves are crowded in tiers at the shoot tips. Narrowly lance-shaped, finely toothed and up to about 2 in long, they are glossy dark green above and paler beneath. The foliage combines evergreen leaves with seasonal ones, which take on red or purple tints in fall and winter.

The funnel-shaped flowers, 1–2 in across, are borne singly or in pairs. They are scarlet red, hot pink to burgundy, more rarely white, with darker spots on the upper lobe. They open from late spring into early summer, sometimes with a second flush in fall, and are followed by small capsules.

In Japan, satsuki azaleas have been selected and hybridized for centuries. They bloom later than most azaleas and often carry flowers of different colors on the same plant. Cultivars include ‘Macrantha’, widely sold, ‘Coral Bells’, a dwarf with pink flowers, and ‘Formosa’, lavender pink and more sun tolerant.

Do not confuse it with the “florist’s azalea” sold in bloom for the holidays: that is Rhododendron simsii, often mislabeled R. indicum.

Light

It prefers part shade: two to six hours of direct sun a day, or the filtered light of high shade, such as under tall pines. Avoid long hours of hot full sun and drying winds: flowers last less long and azalea lace bugs become more troublesome. Heavy shade, on the other hand, leads to poor flowering and weak growth. A sheltered east-, north- or west-facing spot suits it well.

Watering

Its fine, shallow roots must never dry out, yet they rot in waterlogged soil. Do not water by the calendar: pull back a little mulch and feel the soil. If the top layer of soil is dry, water slowly and deeply, until the soil is moist to 6–8 in, ideally with drip irrigation or a soaker hose rather than overhead, which encourages disease.

Flower buds form in summer: even a short dry spell at that time can make them abort or drop unopened the following spring. Be especially attentive during dry periods in the second half of summer. Use rainwater when you can: hard tap water reduces the acidity around the roots.

Potting mix

It needs acidic soil, with a pH of 4.5 to 6, rich in organic matter, moist but well drained; heavy, wet soils do not suit it. In clay, work in composted pine bark and plant on a slight mound. Spread an airy mulch of conifer bark, pine needles or leaf mold 2–3 in deep, renewed each spring, and do not cultivate around the base. In a container, use a loam-based, peat-free potting mix for acid-loving plants.

Its fertilizer needs are low: amended soil and an organic mulch are often enough. If needed, apply an acid-forming azalea fertilizer lightly from late spring to early summer, never in late fall. Too much fertilizer burns roots and leaves.

Temperature

The species is hardy in USDA zones 7a to 8b and is reported to withstand lows of about -0 °F; in the UK it is rated H5, meaning winters of 5–14 °F. In colder regions, mulch heavily and plant against a wall, sheltered from winter winds. In severe cold the leaves droop, then recover when it warms up. Avoid frost pockets and early morning sun after a frosty night: a sudden thaw can split the bark.

Repotting

In the ground, plant in fall or early spring, without burying the plant: the roots should be just covered.

In a container, potting mixes break down over time and eventually starve the roots of air. Repot every other year, in early spring, as soon as growth begins. In the year between, replace the top 2 in of mix. To keep the same pot, trim off up to a third of the roots.

Pruning

It needs little pruning: remove dead wood and cut off spent flowers as soon as blooming ends.

• Thinning: cut an overlong, damaged or diseased branch back to where it joins another, preferably right after flowering so next year’s bloom is spared.
• Renewal: an old, leggy plant can be cut back to 6–12 in above the ground before spring growth, at the cost of one season without flowers. Then pinch the new shoots when they reach 6–12 in.
• Avoid: late pruning, which removes the buds formed in summer.

Propagation

• Cuttings: semi-ripe cuttings in the second half of summer; many cultivars root quickly.
• Layering: allow fifteen months to two years for roots to form.
• Seed: only the true species comes true from seed. Sow on the surface, uncovered, with gentle bottom heat.

Common problems

The same symptom can have several causes: yellowing can come from alkaline soil, excess water, compacted soil or too much fertilizer. Look at the shrub and its soil before acting.

• Azalea lace bug: a small insect that feeds on leaf undersides; the upper surface becomes speckled with pale dots, then whitish, and the undersides are soiled with black droppings. Treat as soon as the first generation appears, with insecticidal soap on the young nymphs. ‘Macrantha’ is resistant.
• Other pests: mites, vine weevil, whiteflies, leafhoppers, scale insects and aphids.
• Root rot (Phytophthora): leaf loss, reduced vigor, branch dieback and wilting. Improve drainage and avoid overwatering and overfertilizing.
• Azalea leaf gall: fleshy, pale green swellings that replace leaves or flowers, then turn white with spores. Cut them off before that stage.
• Other diseases: petal blight, leaf spots, powdery mildew, honey fungus.
• Iron chlorosis: in alkaline soil, leaves turn yellow between veins that stay green.
• Leaf drop: after drought or waterlogging; some older leaves also drop normally in spring and summer.

Toxicity

Satsuki azalea is toxic to dogs, cats and horses. It contains grayanotoxins, which can cause vomiting (except in horses), diarrhea, weakness and cardiac failure.

It is also poisonous to people, with a low severity rating: all parts are involved, but eating moderate amounts of nectar or leaves poses little hazard. It does not cause contact dermatitis. Wear gloves when handling it, keep it out of reach of children and pets and, if any is swallowed, contact a poison control center or a veterinarian.