Rhododendron ARBOREUM

Rhododendron ARBOREUM - Tree rhododendron, Rhododendron arboreum, Azalea.

Rhododendron is a genus of woody plants in the heath family (Ericaceae), comprising 1,096 accepted species distributed in temperate regions of the Northern Hemisphere, in the mountains of tropical Asia, and in northeastern Australia. In nature, members of the genus occur in mountain forests, on damp slopes, peaty soils, rocky ledges, forest edges, and in subalpine scrub communities, where acidic soil reaction and moderate humidity are usually maintained. The genus also includes azaleas, which were previously segregated into an independent subgenus.

The genus is characterized by evergreen, semi-evergreen, or deciduous shrubs and small trees with a shallow fibrous root system. Leaves are arranged alternately, often clustered toward the ends of shoots, and can be leathery, elliptic, lanceolate, or obovate, with a smooth, pubescent, or lepidote surface depending on the group.

Flowers are usually large, funnelform, campanulate, or tubular, clustered in terminal inflorescences, colored in white, pink, purple, red, yellow, or lilac tones. After flowering, dry multi-seeded capsules opening by valves are formed, and the seeds are very small, dust-like, or narrowly winged, adapted to wind dispersal.

Tree rhododendron is a semi-evergreen tree with a well-developed main trunk with reddish-brown bark cracking into small flakes. Young branchlets have abundant gray pubescence. Leaves are narrowly elliptic to ovate, 6-19 cm (2.4-7.5 in) long and 2-5 cm (0.8-2 in) wide, with an entire margin, upper surface rugose-reticulate, lower surface whitish- or brownish-pubescent. Petioles are pubescent, sometimes becoming glabrous over time.

The inflorescence is dense, consisting of 10-20 flowers. The corolla is funnelform-campanulate, up to 4 cm (1.6 in) long, pink or crimson, occasionally white. The calyx is divided into 5 small triangular lobes. Stamens number 10, unequal, glabrous. Pistil glabrous, ovary covered with white, sometimes also glandular, pubescence. The fruit is a cylindrical capsule 15(30)×6 mm (0.6(1.2)×0.24 in).

Rhododendron seeds are very small, dust-like, light brown, maturing in dry oblong capsules that open by valves.

Sowing is carried out from late winter to spring or year-round with grow lights and a stable cool temperature. A sterile acidic substrate based on sifted peat, shredded sphagnum moss, perlite, and a small amount of washed sand is used. Seeds are sown superficially, not covered with soil, but only lightly pressed against the moist surface, as light is required for germination. The sowing is moistened from a spray bottle with a weak solution of a growth stimulant - SUPERthrive, HB-101, Aminozol, Epin, or Zircon are used.

The first sprouts usually appear in 10-14 days; the average germination period is 2-4 weeks; the maximum possible period under cool conditions and uneven seed germination reaches 6 weeks. After the appearance of the first leaves, the cover is opened gradually, and the temperature is reduced to 8-12 °C (46-54 °F) - this reduces the risk of fungal diseases in seedlings. Seedlings are watered via the tray: the container is filled with water until the substrate is thoroughly soaked, after which excess water is drained. Pricking out is performed after the formation of 2-3 true leaves, very carefully, preserving small roots and without burying the growing point. First flowering occurs in the 3rd to 4th year after sowing.

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For rhododendrons, bright indirect lighting or light partial shade is used, especially in regions with hot summers. Eastern and northeastern exposures are suitable, as well as spots under a sparse tree canopy where the plant is shielded from direct midday sun. With insufficient light, flowering weakens, while excess sun leads to leaf scorch, especially with dry air and root overheating.

During the period of active growth, a temperature of 16-24 °C (61-75 °F) is maintained; with sufficient air and soil humidity, a short-term increase to 28 °C (82 °F) is permissible. Overwintering is carried out in a cool bright place at 2-10 °C (36-50 °F), preferably without freezing of the root ball for a container plant. The minimum depends on age and growing conditions, but in pot culture roots are protected from temperatures below -5 °C (23 °F), as container substrate freezes faster than open ground.

Watering is carried out regularly with soft, slightly acidic water, without over-drying the root ball and without standing water. Soil is maintained evenly moist but not waterlogged, and excess water from the tray is always drained. Over-drying causes wilting and drying of leaf margins, while prolonged waterlogging leads to root rot. In hot weather, increasing air humidity and gently misting the foliage in the morning or evening is beneficial. Loosening the soil under the bushes is not performed due to the shallow root system.

For planting, an acidic, loose, moisture-retentive, and breathable substrate with a pH of about 4.5-5.5 is used, composed of peat moss, coniferous litter, bark, perlite, and a small amount of coarse sand. Drainage is laid at the bottom, but the mix itself must not be alkaline or calcareous. Pots are chosen wide and not too deep, as the root system is shallow. Repotting is carried out carefully, usually once every 2-3 years, without disturbing the root ball.

Fertilizer is applied from April to July once every 3-4 weeks with a weak solution of acidic complete fertilizer for rhododendrons, azaleas, or heaths. In the second half of summer, nitrogen fertilization is discontinued so that shoots have time to mature. Pruning is carried out minimally after flowering: faded inflorescences, damaged branches, and weak shoots are removed, while hard pruning is applied only to old plants and performed gradually.