Detailed Explanation of Rhododendron Cutting Propagation Methods and Techniques

Plant propagation methods have always been divided into two major categories: sexual and asexual. At present, most large-scale rhododendron production uses the cutting method, which belongs to vegetative propagation. This is one of the methods that ensures the offspring are completely identical to the parent, and is a commercial route for mass-propagating identical so-called clones.

1. Propagation subjects:

Roughly speaking, the species with the largest leaves require bottom heat during cutting, and the air humidity must be high. From the end of June, closed cutting beds are used, and the branches must be fully mature; only about two-year-old branches are suitable. For the short, creeping alpine rhododendrons from high mountains, cuttings are taken in autumn during propagation, and no bottom heat is needed during insertion; rooting occurs only under conditions of appropriate ventilation. Species with medium-sized leaves fall exactly between the two extremes in their requirements.

2. Taking cuttings:

1. Timing of taking cuttings:

Generally speaking, for evergreen rhododendron varieties with medium or large leaves, taking cuttings from September 20 to the end of October can achieve gratifying results.

As for the various varieties of the azalea group that root easily, from mid-June to mid-July in summer, with an outdoor air temperature of 20°C and a soil temperature of 30°C, semi-mature wood cuttings are the best timing. For autumn and winter cuttings, mature wood is used and cuttings are taken indoors, and the survival rate is very high.

2. Requirements for cuttings:

Use mother plants that are mature, robust, free of pests and diseases, and highly resistant; it is best to take branches from mother plants grown under shade frames or in locations with only half shade each day. Two days before taking cuttings, the mother plant should be thoroughly watered. Then take cuttings on a sunny morning or on a cloudy day, and use various methods to keep the cuttings from wilting. The length of the cuttings should generally reach 10 cm, and the branches should be of medium thickness. If the cuttings have flower buds, this will affect rooting, so the flower buds must be removed before insertion.

3. Preparation of cuttings:

1. Trimming of cuttings:

The length of the cuttings should be standardized (about 10 cm), with the lower cut just below a node. If the leaves are too large, cut off 1/3 to 1/2. Keeping 3 leaves on each cutting is sufficient.

2. Special treatment method for cuttings:

On the lower side of the cutting, use a sharp knife to shave off a strip of about 4 cm, including the cortex, cambium, and xylem. The depth and length shaved off on each cutting should be consistent.

3. Ensure a balance between substrate moisture and aeration.

4. Treatment with rooting hormones:

1. Indolebutyric acid (IBA) has a very obvious effect on promoting rooting. Through experimentation, a 150 ppm (equal to 0.015%) aqueous solution of indolebutyric acid can be used to soak the cuttings for 18 hours. The cuttings should be immersed in this concentration of solution at room temperature indoors, and the immersion depth should preferably not exceed 5 cm.

2. Cuttings can also be treated with B9. "B9" is a growth regulator; use a 2500-5000 ppm (i.e., 0.25-0.5) aqueous solution, soak for 5-10 seconds, and after treatment insert into the substrate.

5. Preparation of the substrate:

1. There are many basic types of substrate for rhododendron cuttings, but the commonly used one is one part sand plus one part peat, which gives very good cutting results. The peat should not be too finely crushed; if it is too dry, add water to moisten it before use. Choose coarse sand, wash it before use, and it must not contain silt or other impurities. The best sand particle size is 3 mm; even 6 mm gravel can be used, especially under full-light mist conditions, where large particles drain well. Some people have suggested that using a mixture of 1/3 coarse sand and 2/3 peat for rooting rhododendron cuttings also gives good results.

2. Requirements for substrate pH and depth:

The pH of the substrate should be between PH 4 and 4.5. After mixing the substrate, place it into pots, shallow boxes, or bed frames prepared for cuttings, with a depth of no less than 7.5 cm. To prevent moss from growing, a layer of sand can be added.

3. After one batch of cuttings has rooted, the substrate should be replaced with new material; it should be completely replaced after at most two consecutive uses to prevent parasitic miscellaneous fungi.

4. Used substrate, pots, bed frames, etc. must all be disinfected; the most convenient method is high-temperature steam.

6. Requirements for substrate temperature and light:

1. Cutting rhododendrons in a hotbed is sometimes called the "hot cutting method." For large-scale cutting beds in greenhouses, a spacing of 10 x 10 cm is generally used. After insertion, do not press the soil firmly; let it settle naturally after irrigation, then cover with plastic film so that the internal humidity reaches saturation. In the greenhouse, it is relatively convenient to heat the bottom with hot water pipes; the substrate reaching 20-22°C is sufficient. After about 3 months, once roots have emerged, the bottom heat can be lowered to 18°C.

2. Light requirements: With regard to light, when cuttings are taken outdoors, direct sunlight must be prevented, but excessive shading is also not allowed.

Misting also has a certain shading effect. If the misting frequency is high, even though each misting is not long, shading may be unnecessary; this is the "full-light mist cutting method." However, the substrate drainage must be good. In summer, using the mist cutting method, because sunlight and moisture are sufficient and photosynthesis is vigorous, rooting is very good. Cuttings under artificial light do not need shading; try to bring light and humidity to the best effect under artificial control. Either "cold light sources" (i.e., fluorescent lamps) or "hot light sources" (i.e., incandescent lamps) can be used, and it is best to use the two equally. At a distance of 25-30 cm from the plants, illuminating for 18-24 hours per day gives ideal results. It is worth mentioning that using a flash lamp at night, with 1 second of light every 60 seconds on the cutting bed, for 8-24 hours per day, also gives very good results, and the electricity cost can be greatly saved.

7. Management after rooting:

Semi-mature wood cuttings will certainly have rooted well by August. Before transplanting, part of the window or shed can be opened to gradually begin ventilation and adaptation to the external environment, in preparation for transplanting. After about 7-10 days of hardening, they can be transplanted. First transplant into small pots or shallow boxes 8 cm in diameter, or they can be planted directly in the ground, but they must pass through the initial stage in a semi-shaded environment. The soil in the pots and shallow boxes must be loose, with half peat and half sand, plus half leaf mold. Because winter is approaching, the newly rooted seedlings must be protected through their first winter. In order to make the future plant shape short and round, the new shoot tips should be pruned at this time to promote the growth of side branches.

In the second spring, prepare the open-field planting beds early. After leveling the ground, make beds running east-west, with a specification of 10 x 2 (m), spread a 10 cm thick layer of peat on the surface, and then evenly turn it into the topsoil. For planting spacing, large-leaved species can use 25 x 25 cm, while those with small leaves can be reduced accordingly. After planting, spread a 5 cm thick layer of leaves, pine needles, or wood shavings on the ground to reduce evaporation. For shading methods, shade nets, wood strips, bamboo blinds, etc. can all be used, but the light transmittance should suitably be 50%.

In the second year, pinching back should still be carried out 1-2 times. Rhododendrons like moisture and fear high temperatures, so sprinkler irrigation in summer should be carried out frequently. Adding fungicides, insecticides, fertilizers, and herbicides to the irrigation water has already become very popular, as it can both lower the temperature and apply fertilizer, achieving two goals at once.