Decorative evergreen, semi-evergreen or deciduous shrub, less often a low tree. The genus has about 1300 species.
Rhododendrons are propagated by seeds and vegetatively (by cuttings, dividing the bush, layering and grafting). The most acceptable method of propagation is by seed. Seeds for uniform sowing are mixed with fine sand and sown in dive boxes, pots or plates on the surface of the substrate. They do not sprinkle with earth (dense sowing is not recommended), water abundantly and cover with glass. The optimum regime of germination is 18–22 ° C, air humidity 90–100%. Crops are aired daily (remove glass). If the seeds hatch at least once, they will not sprout.
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With the advent of the first leaves, a pick is carried out (2 × 3 cm). Plants are buried in the substrate to the cotyledons, watered with rain or snow water at room temperature, tap water is kept for 24 hours in an open bowl. Three weeks later, twice a month, they are fed with full mineral fertilizer (PK) in a ratio of 3: 1: 2 (3-4 grams of the prepared mixture per 1 liter of water). In August, a second pick is carried out (4 × 5). In spring, boxes with picks are carried out at partial shade, and from September to February they are kept indoors at a temperature of 5-8 ° C. Watered sparingly. In late February - March, the temperature is raised to 18–20 ° С; in late May – early June, seedlings (16–18 months) are planted in open ground for growing. For the winter they cover with fir branches (5-7 cm) or dry leaves. In subsequent years, winter-hardy species do not harbor. Most species of rhododendrons bloom in the 4-6th year.
For sowing seeds and growing rhododendrons in open ground, mixtures are prepared: from heather, leaf soil and coarse sand (3: 1: 1), from sphagnum peat of upland bogs and half-ripened litter of pine-bilberry (2: 1), from turf land, sphagnum peat and coarse sand (1: 4: 1), although the ratio may be different. The soil should be acidic (pH 4-5.5), rich in humus, loose, air- and water-permeable. Alkaline soils are contraindicated.
After the first flowering, plants are planted in a permanent place. The best planting time is spring. The depth of the landing pits is 40-50 cm, the width is at least 70 cm. The root neck is left just above the soil level. Plants are abundantly watered, the soil is mulched with peat, finely chopped tree bark, fallen pine needles, rotted manure, sawdust with a layer of 3-5 cm. The soil is improved after 2-3 years: in the spring, a bucket of rotted manure or decomposed compost is brought into the tree trunk and closed to a shallow depth . In the early spring, a mixture of sulfuric acid ammonium, superphosphate, potassium sulfuric acid (2: 1: 1) 80 g per 1 m2 is applied dry. With the second feeding (end of May - beginning of June), the dose is reduced by half.
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