Detailed Explanation of Key Techniques for Forcing Hydrangea macrophylla into Bloom

Hydrangea macrophylla, also known as hydrangea, powder-ball flower, or zi yang hua, is an ornamental shrub in the family Saxifragaceae, genus Hydrangea. Potted varieties grow about 30cm tall, with opposite leaves and flower heads up to 20cm in diameter, nearly spherical, borne at the tips of branches, in white, pink, and blue-purple; the flowering period is from May to July, and with proper cultivation it can be extended to the end of September. Its flowers are large, brilliantly colored, and gracefully shaped, making it highly favored by people. The market development prospects are broad.

I. Characteristics

Hydrangea macrophylla is native to China and has now become a famous ornamental woody flower in the Yangtze River Basin. Hydrangea macrophylla has relatively strong adaptability; it is a subtropical plant, intolerant of extreme heat and also averse to severe cold. In areas north of the Yangtze River, it must be moved indoors for winter at temperatures above 5°C. It likes light and a humid, cool environment. The growing soil is required to be rich in organic matter and moist, but avoid prolonged waterlogging, otherwise it easily causes root rot. Its drought tolerance is relatively weak. It has strong adaptability to soil, and the pH of the soil directly affects flower color: when the pH value is 4 to 6, the flowers are blue; when the pH value is above 7.5, the flowers appear red. Therefore, Hydrangea macrophylla can serve as an indicator plant for determining soil acidity and alkalinity.

II. Cultivation and Management

1. Fertilizer and Water Management Use leaf mold: sand (1:1) with a small amount of organic fertilizer added as the growing medium. Hydrangea macrophylla likes fertilizer; during the peak growing season, apply a dilute liquid fertilizer mainly based on nitrogen fertilizer once every half month, and apply phosphorus and potassium fertilizer once each before and after flowering. Especially when fertilizer is urgently needed during the flowering period, spray a 0.2% potassium dihydrogen phosphate solution on the leaves. Because Hydrangea macrophylla has large leaves, the water transpiration from the leaf surface is relatively vigorous, and its water demand is relatively large; water can be sprayed on the leaves to reduce the transpiration rate. During the growing season, generally water thoroughly once a day, and spray the leaves once every 1 to 2 days. In the north, a small amount of ferrous sulfate can be added to the water to keep the potting soil slightly acidic. Before and after flower bud appearance, the water demand increases significantly, so water 1 to 2 times daily. In winter, watering should follow the principle of "water when dry, stop when wet." Before Frost's Descent, gradually reduce the amount of watering to make the branches grow robustly, which is beneficial for winter dormancy. In summer, Hydrangea macrophylla needs to be placed in a semi-shaded place to prevent direct scorching sunlight; otherwise, the leaves will turn yellow and scorch, affecting growth and ornamental value.

2. Pruning After flowering, prune and shape in time, thin out excessive branches at the base (which can be used as cutting material), and keep 2 to 3 buds at the base to maintain a good plant shape. When new shoots extend 10cm, pinch once to promote branching, which helps produce luxuriant foliage. Control light appropriately.

3. Pest and Disease Control Hydrangea macrophylla has relatively strong disease resistance, and the frequency of pests and diseases is very low. During the growing period, leaf blight, anthracnose, and scale insects occasionally occur. Diseases can occur throughout the year in greenhouses. In production, pay attention to ventilation and light transmission, rationally thin branches, and promptly remove diseased and withered leaves. In the early stage of disease onset, spray with chlorothalonil at 800 times or 50% thiophanate-methyl wettable powder at 500 times. For scale insect control, in addition to strengthening ventilation, spray once with 80% dichlorvos at 1000 times or 50% fenitrothion emulsifiable concentrate at 1000 times during the nymph hatching period.

4. Propagation Methods Hydrangea macrophylla is mostly propagated by cuttings, division, and layering. Cuttings are generally taken from May to June, in combination with early spring pruning and post-flowering shaping. Select tender shoots or branches that have basically become woody; the cuttings should be 10 to 15cm long, retain 2 to 3 leaves at the top and cut off 1/3 to 1/2 of each, quickly dip in ABT rooting powder 100ppm talcum powder slurry, then insert into a seedbed with river sand or vermiculite as the substrate, and provide appropriate shade. After 1 month, they can root, and after 2 months they can be potted and planted. Apply fertilizer 1 to 2 times each month, and they can bloom for ornamental use in the same year. Division is best chosen during the vigorous growth season; move 2 to 3 of them to another pot, and they will take shape in 2 months. Layering can be carried out throughout the year.

III. Forcing Cultivation of Hydrangea macrophylla

1. Principles of Forcing Cultivation Hydrangea macrophylla is a short-day plant. With more than 10 hours of darkness per day, flower buds can form in about 6 weeks. For plants used for forcing cultivation, after a 6-to-8-week cold period (5 to 7°C), place the plants at 20°C to force flowering. When flower buds are visible, lower the temperature to 16°C, and after maintaining for 2 weeks, they can bloom. Under low temperature (13 to 18°C) and short-day conditions (10 hours), Hydrangea macrophylla differentiates flower buds quickly; above 20°C, flower buds are not easy to differentiate. Combining this characteristic allows purposeful regulation of the flowering period.

2. Early-stage Treatment During flower bud differentiation, strengthen light, and appropriately spray phosphorus and potassium fertilizers and daminozide to ensure water supply. After flower bud differentiation, gradually reduce watering to promote full maturation of the branches. At the end of October or early November, remove the leaves and move the plants into a cold room (5 to 8°C), control watering and fertilization, maintain a semi-dry state, and promote full dormancy. After 6 to 8 weeks, forcing cultivation can be carried out, with the time chosen from December to January of the following year.

3. Key Technical Points of Forcing Cultivation In the initial stage, the temperature should not be too high. Generally, first raise the temperature to 13 to 15°C, then gradually increase it to about 20°C. During this period, sufficient light, moist soil, and relatively high air temperature are required. In the north, double-layer film can be used to preserve heat and moisture. When flower buds are visible and the flower heads elongate, lower the temperature to about 16°C, reduce air humidity, and maintain a liquid fertilizer application once a week, using a mixture of nitrogen-phosphorus-potassium concentrated fertilizer 10:30:20 and potassium nitrate each at 1000 times for irrigation, while also increasing soil calcium, which can be irrigated with calcium nitrate aqueous solution. Apply 15-15-15 compound fertilizer and peanut bran once each to the soil (with an interval of 10 to 15 days). In addition, to promote flowering, spray 10 to 40ppm gibberellin (GA3) 1 to 2 times. After about 20 days, they can bloom (calculated from the time flower buds are visible). If the cultivated hydrangea is a pink variety, southern regions should pay attention to adjusting soil pH to prevent the flower color from turning blue or purple. Soil pH can be adjusted by irrigation with a dilute light calcium carbonate aqueous solution, preferably adjusted to 6.0 to 6.5. Northern regions should pay attention to applying alum fertilizer water 1 to 2 times to prevent poor growth and flowering.

Forced-blooming hydrangeas come onto the market right during New Year's Day and the Spring Festival. The large, delicate, and beautiful flowers bloom among the lush green leaves, bringing a strong sense of spring and abundant festivity to the holiday.