1. Taxonomy and Origin
2. Morphology
3. Edaphoclimatic Requirements
4. Propagation
5. Growing techniques
6. Pests, illnesses and physiopathology

1. TAXONOMÍA Y ORIGEN

Family	Saxifragaceae
Genus	Hydrangea
Specie	H. hortensis or H. macrophylla or H. opuloides
	H. quercifolia
	H. paniculata
	H. arborescens
	H. sargentiana
Scientific name	Hydrangea hortensis
	Hydrangea quercifolia
	Hydrangea paniculata
	Hydrangea arborescens
	Hydrangea sargentiana
Common name	Hydrangea

Hydrangea (Hydrangea spp.) is a shrub belonging to the saxifragaceous family. The most important species are characterized by inflorescences.

This genus comprises of 90 species. The most cultivated is Hydrangea hortensis which is the most common specie of hydrangea.

2. MORPHOLOGY

- It is branched and in some cases, is a climbing shrub. It shows wide variability of sizes, from dwarf to huge plants with several meters high.

- Stems: The stems are sturdy but little woody and cylindrical.

- Leaves: The leaves have jagged edges and opposite in most species. In some varieties, they can be lobed, oval, and acuminate and can be very long. In every stem node, three leaves are distributed.

- Flowers: The flowers are arranged in inflorescences with several shafts that support each flower. They are clustered in large terminal corymbs on the stem of the previous year. The individual flower is relatively small. Around it, there are a series of bracts which give colour to the plant. The range of colours is extensive (red, pink, purple, white, blue, etc.). Hydrangeas bloom from spring until autumn. Flowers develop from buds formed the previous year.

There are two types of flowers:
- Sterile: colourful with petaloid tetramer calyx and located on the periphery of the inflorescence.
- Hermaphrodites: Small size and without ornamental value.

Two types of plants by the number of flower buds:
- Unifloral: A single flower bud.
- Plurifloral: Various flower buds. They can be one or two depending on the required time for flowering.

Las características morfológicas de las especies más cultivadas son:

- H. hortensis (Common Hydrogenea): Compact rounded shrub, native of Japan. It grows between 1.5-2 meters. It has cylindrical and slightly woody stems. It is deciduous and has large leaves with toothed margins. It has sterile flowers with large sepals and clustered in inflorescences..

- H. quercifolia (Hortensia leaves in oak colour) rounded and irregular growth similar to oak leaves. It is native to the northern part of the American continent. The heads of the flowers are initially white but as time progresses, they turn to pinkish colours until they finally turn to brown. The foliage also undergoes changes in colour according to how the fall progresses.

- H. paniculata: It is a large hydrangea (can reach up to six meters high). It comes from East Asia. Its peculiarity is that it has pyramidal heads of flowers with panicles composed of flowers with or without bracts. It has white flowers with pink tones. It flowers between the months of July to October.

- H. arborescens: Shrub with large ovate-lanceolate leaves. It produces large white inflorescences in July.

- H. sargentiana: Inflorescences with fertile flowers in lilac colour and sterile white flowers. Its leaves are dark green on top and light green on the underside. It is coated with abundant fuzz.

3. EDAPHOCLIMATIC REQUIREMENTS

- Temperature: For the formation of flower buds, it is necessary that the temperature is below 18°C for at least 6 weeks. Subsequently, a six-week period at 5°C is required to start flower forcing. Generally, it requires a cool environment with high humidity and mild temperatures. It should not be exposed to direct sunlight. It is necessary to protect the plant because it does not tolerate frost and temperatures below 4°C.

- Flower Forcing: it consists in maintaining the plants in dormant state. It is carried out at temperatures above 15°C for 70-90 days. If flower forcing is too much, it may reduce the quality of flowers. At first it is important that the relative humidity is high, so it will be convenient to water and sprayed the surroundings. During this period nitrogen fertilization is necessary.

- Light: The hydrangea cannot live under the sun all year round except in coastal areas with mild climate. Shades must be available especially in the middle of the day and if it is possible, it should be a partial shade.

- Humidity and Watering: The plant requires large amount of water and constant humidity. Waterlogging should be avoided since it favours the emergence of fungal diseases and root suffocation.

- Substrate: The organic matter content should be high. Substrate should drain well, as the plant suffers with waterlogging. Soil pH influences the colours of flowers: blue (pH 4.5-5) or pink (pH 6-6.5).

4. PROPAGATION

The propagation of the hydrangeas can be carried out by vegetative propagation (cuttings) or micropropagation.

- Vegetative propagation: Performed from cuttings 8-10cm although smaller cuttings can be used if there is no sufficient vegetative material. 1cm from each side of the leaf is to be taken and the stem is to be divided into two parts longitudinally. These cuttings are to be placed on the substrate. If background heat (18-20°C) is used, the transplant may be carried out at 30-40 days.
If the plants are two years old plurifloral, they are propagated from April to June and January to March, if they are one year old. The unifloral plants are propagated later.

- Micropropagation: consists of removing the apical meristem and it is grown in a medium.

5. GROWING TEHNIQUES

- Pruning: Pruning hydrangeas is a quite delicate task.

Outdoor hydrangeas:
- Cold weather: it is pruned at the end of winter season. Leaves and branches located in the basal part of the plant should be removed.
- Warm Climate: pruning is after flowering. In this way, the outbreak of lateral buds for next year is enhanced.

Indoor hydrangeas: It is pruned in September (Northern Hemisphere) leaving 2buds/branch.
Types of pruning:
- Dry Pruning: all hydrangea stems are cut almost at ground level. It takes 6-9 months until the plant resumes flower production
- Hail Pruning: stems with flower buds are removed. The objective of this pruning is to delay production for few months.
- Selection Pruning: unwanted vegetal materials are removed in order to enhance plant vigour.

- Fertilization vs. Flower Coloration:

For pinkish flowers, fertilization should be rich in nitrogen and phosphorus and low in potassium. This can be achieved with alkaline fertilizers; calcium nitrate and, to a lesser extent, calcium nitrate.

If blue flowers are desired, fertilization should be rich in potassium and low in nitrogen and phosphorus. Moreover, blue flowering requires input of acidifying fertilizers such as ammonium sulphate, ammonium nitrate, potassium sulphate, etc. Aluminium sulphate can also be added if the above measures are insufficient.

6. PESTS, ILLNESSES AND PHYSIOPATHOLOGY

6.1. Pests

The main pests that affect hydrangea are: Red Spider, cottony cushion scale, thrips and aphids.

6.2. Illnesses

Among the fungi that cause disease in this plant stand the following:

- Botrytis cinerea: It causes rotting of leaves, buds and shoots.
- Ascochyta hychengeae: Produces more or less regular green or greyish brown spots on the leaves.
- Oidium hortensiae: Causes whitish gray spots on the undersides of the leaves. The upper part of the leaf turns to dark green.

The nematode Dytylenchus dipsaci causes widening of the stems, which are shortened or twisted. The leaves are small and twisted. Curative treatments are not effective, so preventive control is preferable: sterilizing the soil and removing infected plants.

Plant illnesses caused by Fusarium sp., Cercospora hydrangea, etc can also be mentioned.

6.3. Physiopathology

- Leaves with curled brown edges: caused by lack of water supply.
- Chlorosis on leaves and flower buds: caused by too much calcareous soil.

Author: Infoagro