A Detailed Guide to Hydrangea Cultivation: Planting Methods and Care Techniques

Hydrangea (Hydrangea macrophylla), also known as bigleaf hydrangea, snowball flower, and Ajisai, is an ornamental shrub of the genus Hydrangea in the family Saxifragaceae. It is an important greenhouse pot flower and a warm-region garden flower, widely cultivated around the world. Potted varieties grow about 30cm tall, with opposite leaves that are obovate or elliptical and coarsely serrated along the margins; leaf thickness varies by variety. The inflorescence is terminal, up to 20cm in diameter, nearly spherical, and corymb-shaped. The ornamental part is usually the enlarged petal-like sepals, 4 sepals, mostly sterile flowers, in white, purple, red, and pink; the flowering period is from May to July.

1 Propagation

In production, hydrangeas are mostly propagated by division, layering, and cuttings.

1.1 Propagation by Division

This is best done in early spring before bud break. When dividing, first tip the mother plant out of the pot, shake off most of the old potting soil, expose the new buds and the direction in which the sucker roots spread, and separate the roots that have become tangled together, injuring the roots as little as possible. Use a sharp knife to cut apart the part connecting the sucker seedlings to the mother plant, and plant them separately.

1.2 Propagation by Layering

This can be done in all four seasons. Choose robust, semi-lignified branches, and use a knife to score the phloem at the lower part near a node. Wrap the scored part well with moist moss, peat soil, or potting soil, then wrap the outside with plastic film and tie it tightly at the top and bottom. If the temperature is suitable, roots will form in about 1 month. Cut below the layered part, remove the plastic film, and pot it up.

1.3 Propagation by Cuttings

Cuttings can be propagated in the greenhouse in all four seasons. Generally, this is done in May to June, in combination with early spring pruning and post-flowering shaping. Choose tender, lignified, or semi-lignified branches free of pests and diseases, 10-15cm long, keep the top 2-3 leaves and cut off 1/3-1/2 of each, quickly dip them in ABT rooting powder 250mg/L talc slurry, then insert them into a seedbed with river sand, vermiculite, or perlite as the substrate. Shade appropriately, keep the leaf surface moist; rooting begins in about 18d, after which gradually reduce the frequency of misting and increase light. In about 1 month they can be transplanted.

2 Cultivation Management

2.1 Substrate

Prepare a soil that hydrangeas like: loose, fertile, and well-drained. Mix equal amounts of vinegar chaff (mainly rice husk), cow manure, peat, and garden soil evenly, and leave it to fully decompose before use.

2.2 Transplanting and Shaping

After transplanting, shade the seedlings appropriately and mist the leaves to help them recover. When new leaves grow, pinch the tips to promote branching, promptly thin out overly dense and overlapping branches, and keep 3-5 branches to obtain a good plant shape.

2.3 Temperature

The suitable growth temperature is 15-27°C. North of the Yangtze River, plants must be moved into a greenhouse above 5°C to overwinter. In hot summers, cool the plants by shading, increasing ventilation, and misting the leaves.

2.4 Light

Hydrangeas prefer a semi-shaded environment. Too strong light can cause leaf chlorosis and whitening in mild cases, and leaf scorch and withering in severe cases. Under moderate light, the plants grow robustly and the leaves are deep green. In hot summers, use a 75% shade net for shading and adjust the light by raising and lowering it. On sunny days, draw the shade net at 10 a.m. and open it at 4 p.m.; on cloudy and rainy days, remove the shade net entirely. The shade net should be removed gradually to avoid scorching the plant leaves with overly strong light.

2.5 Watering

Do not water too much; it is best to keep the soil constantly moist. During dry, hot summers, mist the leaves to reduce the transpiration rate. In winter, water on the principle of "see dry, see wet"; control watering during dormancy and keep the soil semi-dry. Before and after bud formation, water demand increases significantly, so water 1-2 times daily.

2.6 Fertilization

During the growing period, apply decomposed alum fertilizer water once every 10-15d (water 400-500L, soybean cake 10-20kg, pig manure 20-30kg, ferrous sulfate 5-6kg), and spray the leaves once every 15d with 0.1% urea or 0.1%-0.2% ferrous sulfate solution. During flower bud differentiation, appropriately increase P and K fertilizers. During dormancy, stop fertilizing.

2.7 Control of Main Pests and Diseases

(1) Leaf spot. It mainly damages leaves; the spots are round to polygonal, brown or dark gray, with purplish-brown or nearly blackish-brown margins. At the initial stage, spray 65% zineb wettable powder 500 times dilution or Bordeaux mixture (1:1:200), once every 7d, for 2-3 consecutive treatments.

(2) Rust. It mainly damages leaves; larger pale yellow to rust-brown spore masses appear on the leaf surface, and diseased leaves dry up and break apart. In the disease season, spray 15% triadimefon wettable powder 800 times dilution.

(3) Damping-off. It mainly damages leaves and stems; water-soaked yellow-brown spots appear on leaves near the ground and spread to the stems, causing leaves to dry up and stems to turn black and rot. At the initial stage, spray 75% chlorothalonil wettable powder 800 times dilution, or 50% thiram wettable powder 500 times dilution.

(4) Powdery mildew. It mainly damages leaves and can infect stems in severe cases. A layer of light gray mold appears on the leaf surface, gradually turning light brown. During the disease period, spray 15% triadimefon wettable powder 1000 times dilution, or 70% thiophanate-methyl wettable powder 1000 times dilution, once every 7d, for 2-3 consecutive sprays.

(5) Scale insects. They mainly damage young tender stems and leaves, causing leaves to yellow and branches to dry up. When few occur, gently brush them off with a soft brush, then rinse clean with water. For chemical control, it is best to spray 80% dichlorvos 1000 times dilution or 50% fenitrothion emulsifiable concentrate 1000 times dilution during the nymph hatching period.

2.8 Other

Strictly strengthen quarantine to prevent diseased plants from entering the nursery. Always pay attention to environmental sanitation, promptly remove surrounding weeds, and improve ventilation and light penetration. Promptly remove diseased leaves, pull out diseased plants, and destroy them.

3 Forcing Cultivation

Hydrangea is a short-day plant. With more than 10h of darkness daily and a night temperature of 11-18°C, it takes about 42d to form flower buds. After flower bud differentiation, gradually reduce watering to promote full maturity of the branches. Remove the leaves and move the plants into a cold room (5-7°C), control watering and fertilization, maintain a semi-dry state, and promote full dormancy; only after 42-56d can forcing cultivation begin. At the initial stage of forcing, the temperature must not be too high; generally, first warm to 13-15°C, then gradually raise it to about 20°C. During this period, sufficient light, moist soil, and relatively high air humidity are required. When visible flower buds and inflorescences elongate, lower the temperature to about 16°C and reduce air humidity; flowering can occur in about 20d. During the flowering period, keep water adequate and light sufficient. Hydrangea flowers are huge; to avoid lodging when blooming, set up supports and tie the plants to improve the ornamental effect.

4 Applications

Hydrangea is a shade-tolerant flowering shrub and an excellent ornamental tree species. In warm southern regions, it can be planted on the north side of buildings, in shaded courtyard areas, under trees, on trellises, and at forest edges. As a potted plant, it is often used for indoor viewing in halls and studies. Its roots, leaves, and flowers can be used medicinally to treat malaria and palpitations from heart heat. The flower color of pink varieties is affected by soil pH: in acidic soil the flowers are blue, and in alkaline soil they are red, so it can serve as an indicator plant for soil acidity and alkalinity.