Rhododendron Propagation Methods and Key Points of Soilless Culture Techniques

  Rhododendrons are world-famous flowers and one of China's top ten famous flowers. There are about 900 or more species of rhododendron in the world, and China has 530 or more. Because rhododendrons are short plants that bloom in all four seasons, with double flowers and extremely high ornamental value, they can often be seen in residential communities and plazas, and today the editor will talk about rhododendron propagation methods and the key points of soilless culture techniques.

  1 Morphological Characteristics

  Rhododendrons are evergreen, semi-evergreen, or deciduous shrubs or small trees, with plant height varying from 0.4 to 4 m. The young branches often bear very short brown or brownish hairs. Flowers are terminal or axillary, funnel-shaped or bell-shaped, with bracts, and the corolla diameter is 2 to 6 cm. There are many flower colors such as white, yellow, pink, orange, violet-red, and rose-red, with 5 to 10 stamens. The fruit is a capsule, with a 3- to 5-loculed ovary, and the seeds are tiny.

  2 Ecological Habits

  Rhododendrons prefer a sparse-shade environment and are averse to direct sun exposure. Their cold hardiness varies greatly depending on the place of origin. Rhododendrons are typical acid-soil plants and grow well in an environment with a hydrogen ion concentration of 31.63 to 316.3 μmol/L (pH 5.5 to 4.5). In gardens in northern China, there are still no open-ground plantings of rhododendrons in mass. In alkaline environments, rhododendrons develop yellow leaves due to lack of iron and manganese, and eventually shed them and die. The mechanism by which alkalinity leads to leaf chlorosis is still unclear to this day.

  3 Propagation Methods

  Propagation methods include seeding, cutting, layering, grafting, and division. Cutting and grafting can maintain the hereditary characteristics of the cultivar and can form a commercial production process; seeding is conducive to cross-breeding and selecting superior individual plants; for cuttings, use green shoots and select semi-lignified branches, and the survival rate is high when done during the plum rain season. The cutting medium is also one of the key factors affecting survival. Rhododendron cuttings require abundant nutrients around the base of the cutting, good aeration, and good moisture conditions, which can be provided by aeroponics and perlite culture. The grafting method can fully exploit the advantages of the rootstock root system and the fine characteristics of the scion, cultivating a single plant that combines the strengths of both. For the scion, take the tender tips of superior cultivars, remove the lower leaves, keep only 3 to 4 small leaves at the top, and cut the base into a wedge shape, with the cut surface 0.5 to 1.0 cm long. The rootstocks used include 'Guifei Zuijiu', 'Yu Hudie', and 'Zi Hudie'; generally the original species of Rhododendron mucronatum is not used.

  4 Soilless Culture Management

  4.1 Substrate

  4.1.1 Provide an acidic environment: such as pine needles, peat, and sawdust, all of which are acidic substrates.

  4.1.2 The roots in the substrate do not dry out from wind because the weather is dry: rhododendron roots are very likely to die from drying in the wind, so it is best to use a mixture of peat with vermiculite or perlite.

  4.1.3 Soil retention, fertilizer retention, and aeration: correctly handle the relationship among acidity, aeration, and water and fertilizer retention. When planting, consider that when the pot bottom does not leak, first of all the acidity of the substrate should be at a hydrogen ion concentration of 3.16 to 31.63 μmol/L (pH 5.5 to 4.5), while maintaining good aeration; nutrients and moisture can be ensured by watering and applying nutrient solution.

  4.2 Requirements for Nutrient Solution Culture

  4.2.1 pH: hydrogen ion concentration 3.16 to 31.63 μmol/L (pH 5.5 to 4.5), maintaining the most suitable environmental acidity for rhododendron roots.

  4.2.2 Complete nutrition: supply the various nutritional components required according to the needs of Western rhododendron growth, otherwise single-salt toxicity is easily caused.

  4.2.3 Large acid-base buffering capacity: the hydrogen ion concentration of the nutrient solution will not decrease noticeably due to the addition of alkaline substances

  4.3 Watering

  Western rhododendrons cannot tolerate alkalinity. Usually the groundwater used in cities has a hydrogen ion concentration lower than 100 nmol/L (pH higher than 7). Watering with groundwater easily causes rhododendron leaves to turn yellow, drop, and even die. Therefore, to avoid this situation, a water vat can be set up where rhododendrons are planted, and the water in the vat can be left in the sun for a few days. At the same time, vinegar essence or edible vinegar can also be used to adjust the hydrogen ion concentration of the water, about 1 spoon of edible vinegar mixed with 500 mL of water.

  4.4 Management

  The main management methods for rhododendrons are: they like shade and dislike direct sun, like coolness and dislike stuffiness, like acidity and dislike alkalinity, and like light fertilizer and dislike heavy fertilizer.

Key Points of Rhododendron Cultivation Techniques

Key Points of Rhododendron Cultivation Techniques

    1. Site selection and land preparation: The site should be chosen as warm and humid, with not too strong light, a cool climate, fertile soil, and humus-rich acidic soil, with pH below 7 or neutral soil with pH = 5-6, and soil with good drainage and aeration.

    2. Propagation methods

    (1) Seed propagation method: The vast majority of rhododendrons can set seed and be collected for seed, and only double-flowered ones do not set seed. Generally, the seed maturity period is 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, and at this time they should be collected at any time. Those that have not split but have turned brown should all be collected, spread out to cool in a well-ventilated indoor place, allowed to split naturally, then the fruit shells and other impurities removed, 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, the sowing time is March to April, using pot sowing, because the seeds are small. Wash the inside and outside of the pot clean, dry it in the sun, sterilize it, and sterilize the soil as well. Fill the pot with acidic soil that has good permeability, is moist and fertile, and is rich in organic matter. To achieve uniform emergence, mix the seeds with some fine soil, scatter them into the pot, then cover with a thin layer of fine soil. Water by the seepage watering method so that water seeps 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 high and low changes, and strong light exposure. The seedlings grow very slowly, and only grow 2-3 true leaves by May to June. At this time, carry out the first transplanting indoors, with a plant and row spacing of 2-3 cm. When the seedlings are 2-3 cm tall (about November), large seedlings are transplanted one plant per 10 cm pot, and small seedlings three plants per pot. Use a fine spray pot to water and apply dilute fertilizer water. In the spring of the second year after sowing, move them out of the flower room and place them under a shade shed for care. In June, shift them into 13.3 cm pots. In the third year, the plants are 20 cm tall, already have several branches, and some flower buds appear; shift them into 16.7 cm pots. Afterward, repot year by year according to the size of the plants.

    (2) Cutting: The cutting method is the most commonly used propagation method for rhododendrons. Apart from a very few that cannot root, all can survive by cutting. It is easy to collect material from, simple to operate, and can preserve the fine characteristics of the cultivar. Cuttings can be taken throughout the year, with the best time from May to June, followed by August to September. However, because cultivars differ, the times vary: Western rhododendron from late May to early June, Rhododendron mucronatum in early June, and Eastern rhododendron and Summer rhododendron from mid-June to late June.

    Selection of cuttings: For cuttings, choose vigorous growth, free from pests and diseases, current-year new shoots, semi-mature branches, already lignified, and terminal shoots on the outer part of the canopy are best. In principle, the requirement is: if the mother plant has 3 new shoots, take 1 as a cutting; if 5, take 2 as cuttings. The lengths are respectively: Western rhododendron 5-7 cm, Eastern rhododendron and Summer rhododendron 6-8 cm, Rhododendron mucronatum 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, inserted as soon as collected.

    Pot cutting method: Wash and disinfect the pot, fill with potting soil, place coarse-grained soil in the bottom layer 1/3 as a base; fill 2/3 with fine soil, make holes, insert the cuttings 1/2-1/3 deep, with a spacing of 3 cm, and water by the seepage watering method with a fine-hole spray pot.

    Ground cutting method: Use bricks to enclose the surroundings at a height of 15-20 cm and a width of 80-100 cm, fill the inside with coarse-grained soil, then fine soil, in the same way as

    pot cutting. Both of the above two methods should be under semi-shade conditions. Build a shade shed to prevent side sun exposure, hang curtains, and keep air humidity at

    85%. Pay attention to water management, and always keep the cuttings fresh without 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 appearance is not very attractive, so it is not very suitable for large-scale production. The method is: during the growing season, select the approach scion and rootstock well, and at an appropriate site, at a smooth and node-free place, make one cut on each, with the cut surface 3-4 cm long and deep to the xylem. Align the cambium layers of the two, tie tightly with a plastic bag, and after 4-5 months the wound heals. Cut off the scion to separate it from the mother plant, and cut off the rootstock above the graft union, so that it becomes an independent plant. This method has a reliable survival rate, quickly becomes 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 to June, when the plants grow vigorously, there are many scion resources, the scions are moderately old and tender, healing is fast, and the survival rate is as high as more than 90%. For rootstock selection, use Rhododendron mucronatum that has been cut-propagated for 2 years and has not yet forked. For the scion, choose one with high ornamental value, free from pests and diseases, 3 cm long, keep 3-4 fine leaves on the upper part, remove all lower leaves, cut into a wedge shape 0.5-1 cm long, cut off the rootstock new shoot at 2-3 cm from the base, and make a vertical split in the center, slightly deeper than the scion cut surface. Insert the scion so that one side or both sides have the cambium layers of the two matching. Tie tightly with a plastic bag, cover the scion and graft union with a plastic bag, tie a transparent bag below the graft union to prevent rain from soaking it and to 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 Rhododendron mucronatum as the rootstock, making an oblique cut to the xylem at 6-7 cm from the base, 0.5-1 cm long. The requirements for the scion are the same as for cleft grafting, but the front is made one long and one short; insert the long side facing inward and the short side facing outward, cover and cut well, align the cambium layers, tie well and put on a bag, and place in a shaded place.

    (4) Layering propagation: Layering propagation is mostly carried out from May to June and is divided into two methods: one is air layering, and the other is ground layering.

    ① Air layering: Select 2-3 year old branches, at an appropriate site, cut a ring, deep to the xylem, with a width of 0.5 cm, remove the bark, then take a prepared bamboo tube, 8 cm long, 2-3 cm in diameter, with a node on one side, split it open from the middle, dig a semicircular small hole at the node according to the thickness of the plant stem, and fit the two semicircular bamboo tubes around the cut place. 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 off from the mother plant and potted for cultivation.

    ② Ground layering: Ground layering uses shoots sprouting from field-planted rhododendrons. At an appropriate site, cut a ring, remove the bark, 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, rhododendrons for garden ornament are still mainly potted, and this method helps meet the ecological conditions of different cultivars. In cultivation, the soil requirement is mainly acidic soil, and it is best that pH must be below 6. No matter which kind of soil is used, before cultivation, soil disinfection is needed, with thorough decomposition and fermentation and alternating high and low temperature treatment, and it can only be used after one year. Among these, the best soil is forest humus soil, or self-prepared pine needle soil, with dead branches and fallen leaves, adding a small amount of horse manure and sheep manure, then adding 1/3 fertile field soil, or soil from the foot of a mountain, adding cake fertilizer, human manure and urine, pig manure, chicken and duck manure, superphosphate, etc., and then adding 1/3 sandy loam soil and Huangshan soil; leaf mold soil (dead branches and fallen leaves, with human manure and urine added) 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 aged for a year, plant one seedling in the middle of the pot so that the roots spread out, add soil to the pot up to the root crown, press firmly, and shake the pot to level the soil surface. Spray finely until water drips from the potted flowers, and after a 7-10 day pot-covering stage, place it in front of and behind the house, and it should be placed in a semi-shaded place in the greenhouse.

    Repotting is because after the plant grows larger, it can no longer satisfy the ecological conditions, so new culture soil must be replaced. Take the plant out of the old pot, remove some of the compacted 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, every 3-5 years; extra-large ones may not be repotted, and technical measures such as topdressing are carried out, applying in the space of the rhododendron and keeping relative humidity above 70%.

    (3) Watering: For watering rhododendrons, rainwater, river water, lake water, pond water, and fish-tank water are best. Alkaline water, well water, and tap water are forbidden. If they really must be used, treat well water to a temperature similar to the air temperature; tap water must be dechlorinated and left for several days to volatilize before use. Alkaline water should have sulfuric acid added to adjust pH. The amount of watering should vary according to different growth stages: less during dormancy; in Jiangnan from November to February, and from March to June when rhododendrons bloom and grow vigorously, water more, once a day on sunny days, and in some cases replenish water in the evening. During the plum rain season, pay attention to drainage. In July to August, check at all times; when the pot is dry, water it, and at the same time spray the leaves. In winter, water in the morning; in summer, water in the morning; in spring and autumn, water at any time except noon. In indoor heated greenhouses, water once every 2-3 days; in unheated ones, once every 4-5 days. In late February, as the temperature rises, flower buds begin to swell and leaf buds also begin to sprout, so the water amount should be appropriately increased.

    (4) Fertilization: Fertilization includes base fertilizer, and long-acting hoof pieces, horn pieces, bone meal, cake fertilizer, dried manure, etc., mixed with the soil when potting or repotting. For topdressing, use human manure 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, once every 10 days, applying 2-3 times. Stop fertilizing during the flowering period. After flowering, in order to restore tree vigor and promote new shoot growth and leaf growth, apply nitrogen fertilizer. In the high-temperature season (July to August), growth stops, and it is not easy to fertilize. In autumn, before moving indoors, it is the bud differentiation period, so apply more phosphorus fertilizer, once every 7-10 days. Stop fertilizing during the winter dormant period.

    Principles to grasp in fertilization: the fertilizer must be fully decomposed, apply dilute fertilizer frequently, do not apply fertilizer on the leaves, apply on sunny days, apply when the potting soil is dry, apply in the evening, and water early the next day.

    (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 bud to control height and stimulate side branches. After pinching, many side branches grow again, in a whorled shape. 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 extent, remove the tips of the flower buds to prevent too many flowers from blooming and wasting nutrients. At the same time, wipe off adventitious buds. In order to maintain the beauty of the tree shape, tree vigor should be strengthened, branches renewed, ventilation conditions adjusted, and those weak branches, diseased branches, dead branches, crossing branches, overlapping branches, overly dense branches, and water sprouts should be cut off from the base. For old plants, pruning should be carried out for rejuvenation; before the new buds sprout in early spring, leave about 30 cm of the branches and cut off the upper part. Do not cut it all at once; carry it out in stages, cutting 1/3-1/5 each time, shortening them, and completing it in 3-5 years. In this way, flower viewing is not affected, and after pruning, with careful management and fertilization, the plants can be kept growing vigorously without decline for a relatively long time.