Key Points of Commercial Forcing Cultivation Techniques for Hydrangea
After undergoing the necessary cold treatment, hydrangeas are usually shipped and @#@136@#@ from late autumn to early winter. Like dormant plants, they can be planted in 4-6 inch pots, or pre-planted in 4-inch pots like bare-root plants. Newly introduced plants should first have their root growth stimulated (about 2 weeks) before final planting. The optimum temperature for hydrangeas to begin growing is a soil temperature of 15.6°C-16.7°C, while maintaining a slightly lower air temperature of about 15°C, which promotes root activity before bud break. Keeping the soil slightly dry before transplanting prevents root rot and promotes root growth. Do not fertilize before the roots begin to grow and before transplanting.
One of the main problems encountered in hydrangea production is that rooting is relatively difficult, and this condition makes plants susceptible to water stress in the later stages of forcing cultivation. At transplanting, the roots should be divided twice (forming an "X" shape at the bottom of the root system, that is, four sections), and then these four sections should be separated and planted directly into the pot.
Flower Color Control
If pure purple and blue flower heads are to be maintained, the key is to have the plant gradually develop the desired color and continue coloring during forcing cultivation. Fertilizing during the early summer growth stage can affect coloring during forcing, and changing the speed of coloring at this stage may produce purplish-red markings.
Whether hydrangeas (including white varieties) ultimately form purple or blue flower heads depends on the amount of available aluminum. When aluminum content is low, purple flowers form, while at high concentrations, blue flowers form. Therefore, by regulating the aluminum content, flower color can be controlled.
The colors of various cultivars differ greatly. Some are suitable for inducing purple, while others are suitable for inducing blue. In production, the flower color that best meets market demand should be selected. Although there are already more than 500 hydrangea cultivars, only a small portion are produced in the United States.
For purple flowers, the application of aluminum fertilizer is prohibited, and aluminum-containing soil and fertilizers must not be used in the growing medium. A relatively high concentration of phosphorus fertilizer can be applied; phosphorus hinders aluminum uptake and helps purple color formation. During cultivation, 1.36-2.04 kg of superphosphate can be applied per cubic meter of growing medium; using monoammonium phosphate alone can also increase phosphorus content and inhibit aluminum uptake.
The pH of the growing medium should be 6.0-6.2 (aluminum is more available when pH is low), and should not exceed 6.4, otherwise iron-deficiency chlorosis will occur. If the pH of the irrigation water is higher than 6.5, use an acidifier to lower it to 6.3. Using phosphoric acid can increase the phosphorus content in the growing medium, which can help form purple flower heads. Do not apply high concentrations of potassium, otherwise hydrangeas will form blue flower heads.
Recommended cultivars: cultivars with flower colors from deep purple to red include 'Bottstein' and 'Schenkenburg'; moderately purple ones include 'Merritt's Surpreme', 'Kasteln', 'Red Star', 'Rose Supreme', and 'Enziandom'. Some cultivars such as 'Mathilde Gutges' and 'Brestenburg' cannot form stable or pure purple and can only be used to induce blue flowers.
Blue Flowers: Although dormant plants that produce blue flowers have been treated with aluminum potassium sulfate before shipping, aluminum must also be applied during forcing cultivation. Immediately after transplanting, drench with aluminum potassium sulfate. Add 1 kg of aluminum potassium sulfate to every 100 liters of water, and apply 227 ml per 6-inch pot. The growing medium must be thoroughly saturated; wetting only the surface soil will damage the roots. Fertilize once every 10-14 days. Ten days after each fertilization, measure the pH of the medium once; if it is higher than 5.6, apply aluminum potassium sulfate again. Do this throughout the entire forcing cultivation stage. Aluminum potassium sulfate not only provides aluminum but also keeps the medium at a low pH (5.2-5.5), which is necessary for forcing blue hydrangeas. If the pH of the irrigation water is higher than 5.8, acidify it to 5.3. For growing blue hydrangeas, it is best to use 35% sulfuric acid as the acidifier, because it does not provide phosphorus and is not as corrosive as concentrated sulfuric acid and nitric acid.
At transplanting, use phosphorus-free base fertilizer and chemical fertilizer, and apply a high concentration of potassium to promote blue color formation. With each irrigation, apply ammonium nitrate and potassium nitrate to provide nitrogen and potassium.
Height Control
Most hydrangeas, especially tall cultivars, need height control during forcing cultivation. Spraying 2500 ppm (most cultivars) to 5000 ppm (tall cultivars, especially 'Rose Supreme') B9 can effectively control height (Table 1). In addition, spraying 50 ppm Bonzi is also effective for height control. When 3-5 small leaves have unfolded, that is, 2-4 weeks after forcing begins, start applying growth inhibitors. Under low light, apply once every 10-14 days. Stop application when flower buds reach 0.75 inch in diameter or before the flower head form matures.
Temperature and Timing
During forcing cultivation, the rate of hydrangea development is directly related to the average daily temperature, and to a certain extent the speed of forcing can be controlled by adjusting temperature (Table 2). Generally, at a night temperature of 15.6°C and a day temperature of 21°C on cloudy days or 24°C on sunny days, the sepals can be colored after 80-100 days of forcing. When coloring begins, the temperature should be lowered to a night temperature of 12°C and a day temperature of 18°C to enhance flower coloring. Try to avoid prolonged high temperatures during forcing, because excessively high temperatures cause plants to be short, flowers to become smaller, color to become lighter, and quality to be poorer than plants grown at cool temperatures.
Hardening Off and Postharvest Handling
At the early stage of sepal coloring, reduce the fertilization rate by half to control vegetative growth and make the plants strong. Fully colored flowers are very delicate, so in the last few weeks of cultivation, especially during "Mother's Day", appropriate shading should be provided to prevent excessively high temperatures. At the same time, gradually reduce watering, and do not let the flowers wilt.
If the indoor air is humid and relatively cool, and direct sunlight is avoided, hydrangeas can maintain a relatively long postharvest life. For retailers and home growers, water management is critical; wilting will seriously affect hydrangea growth, and once dried out, they cannot fully recover.
Pests and Diseases
Pests and diseases in greenhouses vary from place to place. Consult your local service center for prevention and control measures. The main pests and diseases of hydrangea are listed below.
Insects: aphids, true bugs, leaf tiers, scarab beetles, scale insects, grass bugs, thrips, citrus whiteflies, etc.
Mites: red spider mites
Other pests: slugs, snails
Bacteria: bacterial wilt
Fungi: rust blister disease, bud rot, gray mold, leaf spot, powdery mildew, root rot, stem rot.
Nematodes: foliar nematodes, root-knot nematodes, stem nematodes.
Viruses: alfalfa mosaic virus, cucumber mosaic virus, hydrangea mosaic virus, hydrangea ringspot virus, tobacco ringspot virus, tobacco necrosis virus, tomato ringspot virus, tomato spotted wilt virus.
Hydrangea Market Outlook
For most hydrangea producers, supplying flowers for "Valentine's Day" has become very common, and there is a trend toward increase.
Another trend is the increase in foliage-type hydrangea cultivars. For example, a new cultivar 'Kasteln' recently appeared; this cultivar was unknown a few years ago and has now become a major cultivar in late-season forcing cultivation.
In subsequent years, 'Pia' appeared; this cultivar is extremely dwarf, has purple flowers, and is very cold hardy in winter. Other possible new cultivars also include the "lacecap" series variants, such as 'Libelle', 'Tawbe', and 'maculata', all of which have variegated leaves. The flower heads of "Lacecap" are more open than the "florist" series and have broad development prospects.
(Note: This article is applicable material for the United States and is for reference only in China. Unit conversion: 1 inch = 2.54 cm)