What are the key cultivation techniques for rhododendrons?
1. Site selection and soil preparation: Choose a warm and humid site with not-too-strong light, a cool climate, fertile soil, and humus-rich acidic soil with a pH below 7, or neutral soil with a pH of 5-6, and soil with good drainage and aeration.
2. Propagation methods
(1) Seed propagation method: The vast majority of rhododendrons can set seed for collection; only double-flowered varieties do not set seed. Generally, the seed maturity period runs from October to January each year. When the fruit skin turns from green to yellow and then brown, the top of the fruit splits open and the seeds begin to scatter; at this point they must be collected at any time. Fruits that have not yet split but have turned brown are all collected, placed indoors in a well-ventilated spot and spread out to cool, so that they split open naturally; then the fruit shells and other impurities are removed, and the seeds are put into paper bags or cloth bags and stored in a cool, ventilated place. If greenhouse conditions are available, sowing immediately after collection gives a high germination rate. Generally, sowing time is March-April, using pot sowing, because the seeds are small. Wash the pots inside and out, dry them in the sun, sterilize and disinfect them, and also sterilize and disinfect the soil. Fill the pots with acidic soil that has good permeability, is moist and fertile, and rich in organic matter. To ensure even emergence, mix the seeds with some fine soil, scatter them into the pot, cover with a thin layer of fine soil, and water by the seepage method so the water soaks into the pot. Place the pot on the front windowsill and cover with a layer of glass or plastic film, the purpose being to raise the temperature inside the pot. After the seedlings emerge, gradually reduce the covering time; because the seedlings are tender and small, pay attention to temperature changes, sudden highs and lows, and strong light exposure. The seedlings grow very slowly, and only in May-June do they produce 2-3 true leaves. At this time, do the first transplanting indoors, with a plant and row spacing of 2-3 cm. When the seedlings are 2-3 cm tall (around November), transplant large seedlings one plant per 10 cm pot, and plant small seedlings three per pot. Water with a fine watering can and with dilute fertilizer water. After sowing, in the spring of the second year, move them out of the flower house and place them under a shade frame for care. In June, transfer to 13.3 cm pots. In the third year, the plants are 20 cm tall, have already produced several branches and some flower buds, and are transferred to 16.7 cm pots. After that, repot each year according to the size of the plant.
(2) Cuttings: Cuttage is the most commonly used propagation method for rhododendrons. Except for a very few that cannot root, all can survive by cuttings; they are easy to collect, simple to handle, and can preserve the fine characteristics of the variety. Cuttings can be taken throughout the year, with the best time being May-June, followed by August-September. However, because varieties differ, the times vary: Western azaleas from late May to early June, hairy azaleas in early June, and Eastern azaleas and summer azaleas from mid-June to late June.
Selection of cuttings: Choose vigorous, pest- and disease-free new shoots of the current year, semi-mature branches that have already become woody; terminal shoots on the outer part of the canopy are best. In principle, if the mother plant has 3 new shoots, take 1 as a cutting; if it has 5, take 2 as cuttings. The lengths are respectively: Western azaleas 5-7 cm, Eastern azaleas and summer azaleas 6-8 cm, hairy azaleas 8-10 cm. For overly long branches, the tip can be cut off, the lower leaves removed, and 4-5 small leaves kept at the upper end; insert immediately after collection.
Pot cutting method: Wash and disinfect the pot, fill with potting soil; put coarse-grained soil in the bottom 1/3 as a base layer; fill 2/3 with fine soil, make holes, insert the cuttings to 1/2-1/3 of their length, with a spacing of 3 cm, and water by seepage with a fine-holed watering can.
Ground cutting method: Use bricks to enclose the sides to a height of 15-20 cm and a width of 80-100 cm, fill inside with coarse-grained soil and fine soil, the method being the same as;
pot cutting. Both of the above methods should be kept under semi-shade conditions. Set up a shade frame to prevent side sunlight, hang curtains, and keep air humidity at
85%. Pay attention to water management, always keeping the cuttings fresh and not dropping leaves; in this way they can root in one month.
(3) Graft propagation
① Approach grafting: Approach grafting is rather troublesome, and after survival the shape is not very attractive, so it is not very suitable for large-scale production. The method is: in the growing season, select a good scion and rootstock, and at a suitable spot, smooth and without nodes, make one cut on each, with the cut surface 3-4 cm long, reaching the xylem. Align the cambium layers of the two, tie tightly with plastic bags, and after 4-5 months the wound heals. Cut off the scion to separate it from the mother body, cut off the rootstock above the graft union, and it becomes an independent plant. This method is reliable for survival, quickly forms a tree, and gives quick results. If it does not survive, the loss to the plant is not great.
② Softwood cleft grafting: Cleft grafting is mostly carried out in May-June, when the plants grow vigorously, there are many scion resources, the old and tender degree is moderate, healing is rapid, and the survival rate is as high as more than 90%. For the rootstock, choose 2-year-old hairy azalea cuttings that have not yet branched. For the scion, choose one with high ornamental value and no pests or diseases, 3 cm long, retaining 3-4 fine leaves at the upper end and removing all lower leaves, and cut it into a wedge shape 0.5-1 cm long. Cut off the new shoot of the rootstock at 2-3 cm from the base, and make a vertical cleft in the center, slightly deeper than the scion cut surface. Insert the scion so that the cambium layers on one or both sides match. Tie tightly with plastic bags, cover the scion and graft union with a plastic bag, and tie the transparent bag below the graft union to prevent rain and maintain temperature. Usually keep the temperature inside the bag, prevent direct sunlight, give a little fertilizer water, promptly cut off sprouts from the rootstock, and after 2 months, when it has survived, remove the bag.
③ Softwood side grafting: Side grafting is done from mid-May to late May, still using hairy azalea as the rootstock. At 6-7 cm from the base, make a slanting cut to the xylem, 0.5-1 cm long. The requirements for the scion are the same as for cleft grafting, but the front is one long and one short; insert the long side toward the inside and the short side facing outward, cover and trim well, align the cambium layers, tie and bag it, and place it in a shady spot.
(4) Layering propagation: Layering propagation is mostly carried out during May-June and is divided into two methods: one is air layering, and the other is ground layering.
① Air layering: Choose 2-3-year-old branches, and at a suitable spot cut a ring, reaching the xylem, 0.5 cm wide, and remove the bark. Then take a prepared bamboo tube, 8 cm long and 2-3 cm in diameter, with a node on one side, split it open from the middle, and at the node cut a semicircular hole according to the thickness of the plant stem. Fit the two semicircular bamboo tubes around the cut. Fill the tube with acidic soil, water it, and keep the soil in the tube moist. After 3 months, many fibrous roots grow inside, and it can then be cut from the mother plant, potted, and cultivated.
② Ground layering: Ground layering uses shoots sprouting from field-grown rhododendrons. At a suitable spot, cut a ring, remove the bark, and bury it in the soil. Water after 3 days and keep the soil moist. After 3 months, when fibrous roots are found to have grown, separate it from the mother plant and pot it for cultivation.
3. Field management
(1) Soil treatment: At present, garden ornamental rhododendrons are still mainly grown in pots, which helps meet the ecological conditions of different varieties. In cultivation, the soil requirement is mainly acidic soil, and it is best if the pH must be below 6. No matter which kind of soil is used, before cultivation it needs soil disinfection, full decomposition, fermentation, and alternating high- and low-temperature treatment; only after one year can it be used. Among these, the best soil is forest humus soil, or self-prepared pine needle soil, with dead branches and fallen leaves, plus a small amount of horse manure and sheep manure; after collection, add 1/3 fertile field soil, or soil from the foot of a mountain, add cake fertilizer, human excrement and urine, pig manure, chicken and duck manure, superphosphate, etc., and then add 1/3 sandy loam soil and Huangshan soil. Leaf mold soil (dead branches and fallen leaves, with added excrement and urine) is second best.
(2) Transplanting into pots and repotting: Before transplanting, if the pot has no drainage hole, cover it crosswise with several pieces of tile, fill a layer of coarse-grained soil, then fill with fine soil that has been stored for a year. Plant one seedling in the middle of the pot so the roots spread out, add soil toward the pot until the root crown, press firm, and shake the pot to level the soil surface. Spray finely until water drips from the potted flowers. After a 7-10 day recovery period, place it in front of and behind the house, and it should be placed in a semi-shady spot in the greenhouse.
Repotting is done because after the plant grows larger, it can no longer meet its ecological conditions, so new culture soil must be used. Take the plant out of the old pot, remove some of the shrunken soil, spread the surrounding roots out, and cut off diseased roots; the operation is the same as above. Generally repot once every 2-3 years. For large plants, once every 3-5 years; very large ones may not be repotted, and technical measures such as topdressing are used instead, applied in the space around the rhododendron, keeping the relative humidity above 70%.
(3) Watering: For watering rhododendrons, rainwater, river water, lake water, pond water, and @#@233@#@ water are best. Do not use alkaline water, well water, or tap water. If they must be used, adjust the temperature of well water to be similar to the air temperature; tap water must be dechlorinated and left for several days to volatilize before use. For alkaline water, add sulfuric acid to adjust the pH. The amount of watering should vary according to different growth stages: less during the dormant period. In Jiangnan from November to February, and from March to June when rhododendrons bloom, they grow vigorously and need more watering; on sunny days, water once a day, and in some cases supplement water in the evening. During the plum rain season, pay attention to drainage. In July-August, check at all times, and water whenever the pot is dry, while also spraying the leaves with water. In winter, water in the morning; in summer, water in the morning; in spring and autumn, water at any time except noon. In a heated greenhouse indoors, water once every 2-3 days; in an unheated one, water once every 4-5 days. In late February, as the temperature rises, the flower buds begin to swell and the leaf buds also begin to sprout, so the water amount should be appropriately increased.
(4) Fertilization: Fertilization includes base fertilizers, long-lasting hooves, shell pieces, bone meal, cake fertilizer, dried manure, etc., mixed with the soil when potting or repotting. Topdressing uses human excrement and urine, chemical fertilizers, and quick-acting fertilizers. Rhododendrons need different fertilizers at different growth stages. Before flowering, apply more phosphorus fertilizer to promote flowering, making the flowers larger, the petals thicker, the color better, and the flowering period longer; apply once every 10 days, 2-3 times. Stop fertilizing during the flowering period. After flowering, in order to restore tree vigor and promote shoot growth and leaf development, apply nitrogen fertilizer. In the high-temperature season (July-August), growth stops, so it is not easy to fertilize. Before moving indoors in autumn is the bud differentiation period, so apply more phosphorus fertilizer, once every 7-10 days. Stop fertilizing during the winter dormant period.
The principles to master in fertilization: the fertilizer must be fully decomposed; apply frequently and thinly; never apply fertilizer on the leaves; apply on sunny days, when the potting soil is dry, and in the evening; water the next morning.
(5) Pruning: In order to maintain the beauty of the rhododendron's tree shape, every year prune excessive branches and water sprouts, buds, shoots, and other parts. When the rhododendron tree grows to a certain height, remove the terminal buds to control height and stimulate side branches. After pinching, many side branches grow again in a whorled pattern. When the side branches grow to a certain height, pinch again. Then the next level of side branches sprouts.
Bud removal: When the flower buds grow to a certain degree, remove the tips of the buds to prevent too many flowers from opening and wasting nutrients. At the same time, wipe off adventitious buds. In order to maintain the beauty of the tree shape, the tree vigor should be strengthened, branches renewed, ventilation conditions adjusted, and weak branches, diseased branches, dead branches, crossing branches, overlapping branches, overly dense branches, and water sprouts cut off from the base. For old plants, pruning should be carried out for rejuvenation. Before the new buds sprout in early spring, cut off the upper part of the branches, leaving about 30 cm. Do not cut all at once; do it in stages, cutting back 1/3-1/5 each time, completing it in 3-5 years. This way it does not affect flower viewing, and after pruning, with careful management and fertilization, the plant can maintain vigorous growth for a relatively long time without decline.