Rhododendron Propagation Methods and Key Management Techniques
Propagation methods: Rhododendrons can be propagated by seed sowing, cuttings, grafting, layering and other methods.
1. Seed propagation: Rhododendrons have a high seed-setting rate, produce many seeds and have a high germination rate, and the seedlings have strong adaptability and vitality, but they grow slowly, are difficult to cultivate, and the offspring are not necessarily ideal, so seed propagation is rarely used in production. It is used only in the following cases: first, for raising rootstock; second, for obtaining new varieties through hybrid breeding; third, when excellent wild species need to be introduced.
(1) Seed collection: When the fruit turns from green to brown, pick the fruit as soon as the upper part begins to split, dry it indoors, and once the valves have all opened the seeds can be collected and stored in a dry place indoors.
(2) Sowing: Sow in pots in spring. Use a shallow pot, wooden box or ground bed, with shredded sphagnum moss as the sowing medium; the moist sphagnum moss should be pressed firm to 3/4 of its original thickness, and a 3mm thick loose layer of sphagnum moss scattered over the surface, then water thoroughly with a fine watering can; because the seeds are small, broadcast them. After sowing, there is no need to cover with soil; cover with glass and place a layer of newspaper on top, keep at 15-20°C, and germination will occur in 2-3 weeks.
(3) Management: After germination, the covering can be removed and ventilation gradually increased to improve seedling resistance. Keep the potting soil moist. When the seedlings have 2-3 true leaves and are 1.5-2.5cm tall, they can be transplanted at a spacing of 4-5cm; after autumn, when they are 3-4cm tall, they can be transplanted once more. During the seedling stage, avoid adverse conditions such as strong light and strong wind.
2. Cutting propagation
(1) Cutting preparation: Take current-year semi-lignified branches with short internodes and sturdy growth as cuttings, about 10cm long, remove the lower leaves and keep 2-3 upper leaves; some need the growing tip removed, with the lower cut made just below a node.
(2) Cutting: For Western azaleas, take cuttings from late May to early June; for Mao azaleas, from early to mid-June; for Summer azaleas, from mid to late June. The rooting medium can be low-pathogen, sterilized orchid soil, Huangshan soil, river sand, vermiculite, perlite, etc. For small quantities, pot cuttings can be used; for large quantities, bed cuttings can be used. First make holes in the medium with a wooden stick, then insert the cuttings to a depth of 1/3 or 1/2 of the cutting length. After insertion, spray with fine mist until thoroughly wet. For pot cuttings, cover the pot with a plastic bag and tie it tightly around the edges to maintain air humidity and temperature, and keep the potting soil moist at all times.
(3) Management: After insertion, prevent direct sunlight, but do not shade too heavily; insufficient light leads to slow, weak growth. Keep the temperature at 25-30°C, mist frequently to raise air humidity, and avoid waterlogging. Roots will form in about one month; pot up the following year, and they can bloom in the third year.
3. Grafting propagation
The grafting method generally uses cleft grafting at the tip of softwood shoots, in May-June. Choose Mao white azalea (Rhododendron mucronatum) or its varieties as rootstock, such as 'Maoye Qinglian', 'Yu Hudie', 'Zi Hudie', etc. Select 2-year-old single-stem plants as rootstock. From the mother plant of Western azalea, cut 3-4cm young shoots, remove the lower leaves, keep 2-3 upper leaves, and cut the base into a smooth wedge 0.5-1cm long. Cut off the current-year new shoot of the rootstock at 3-4cm, remove the leaves, make a vertical cut about 1cm from the flat cut, insert the scion, align the cambium, tie tightly with plastic tape, and after grafting cover with a bag to keep warm and moist.
Other methods include cut grafting, approach grafting, bud grafting, etc.
4. Layering propagation generally uses air layering, carried out after the flowers of spring and summer azaleas fade; roots usually form in 3 months, with a relatively high survival rate.
Cultivation management: Because there are many rhododendron varieties, some can be planted in the open ground and also grown in pots, while some can only be grown in pots, such as Western azaleas. The following are introduced separately:
1. Open-ground cultivation, such as Mao azaleas, East azaleas, Summer azaleas, etc.
Choose acidic soil with deep, fertile layers and a pH of 4-5.5. If the soil is unsuitable, it must be improved. According to ecological habits, shade measures are needed to avoid sun exposure at the planting site in summer. Watering should be sufficient, but avoid waterlogging. When winter comes, deciduous azaleas enter dormancy and should be given one thorough watering, after which the frequency of watering can be reduced. Evergreen azaleas should be watered normally in winter. When winter comes, measures should be taken to protect open-ground azaleas from cold so they can overwinter safely. During the growth period of azaleas, shaping should be combined with pruning, such as pinching and branch trimming. Pinching is mostly done on young plants, artificially removing the terminal buds or flower buds to save nutrients and promote early sprouting of new branches below. Branch trimming is used to tidy the tree shape, and can remove pest-damaged branches, overlapping branches, crossing branches, bare branches and winter freeze-damaged branches. To make azaleas grow robustly, topdressing should be combined to make flowering abundant and flower colors vivid; when applying fertilizer to the roots, pay attention to the influence of soil pH. Foliar topdressing can be done on clear evenings; the concentration should not be too high, spraying should be thorough and even, and rainy or windy weather should be avoided.
2. Pot cultivation, such as Western azaleas. For potted Western azaleas, the pot should not be too large, because azaleas have fibrous roots, a relatively shallow root system and slow growth; if the pot is too large, fertilizer and water are easily out of control, which is unfavorable for growth. It is best to use unglazed clay pots with strong water and air permeability. The potting soil should be acidic soil with good drainage, loose layers, fertility and a pH between 5.5-7, such as pine needle soil, leaf mold, orchid soil, sawdust, etc. Potted azaleas are generally potted when moved outdoors in spring or brought indoors in winter. First place a coarse drainage layer at the bottom of the pot, then fill in the growing medium; support the seedling with one hand and fill soil with the other. Do not fill too much soil; leave space for watering. After potting, first place in a cool place and strengthen misting and watering management so the plants recover quickly; after 5-7 days they can be placed in a suitable position for normal management. For watering, clean and hygienic river water or pond water free of alkalinity is best. If tap water is used, first store it in a water vat for 1-2 days so the chlorine is released and the water temperature is close to the soil temperature. The amount of watering should be determined according to weather conditions, the dryness and wetness of the potting soil, and the needs of the growth and development stage. For fertilizing Western azaleas, soybean cake water, fishy water, etc. are best; apply light fertilizer frequently, and appropriately add ferrous sulfate when fertilizing to adjust the soil pH. Pruning of Western azaleas is mainly thinning pruning. For young plants of about 3 years, in order to accelerate tree shape formation, flower buds are often removed so they do not bloom, and pinching and removal of new shoots are carried out to promote sprouting of new branches. For mature trees, thin out pest-damaged branches, excess branches, crossing branches, etc.