Hydrangeas

Hydrangeas

Hydrangeas are available in hundreds of varieties; they tolerate shade and are relatively low maintenance. As a long flowering shrub they are an excellent garden centerpiece. Hydrangeas bloom from early June into September. Their blooms are also excellent long lasting cut flowers. Hydrangea quercifolia (Oak leaf) even have scented flowers. There are three main flower types available; mophead, lacecap and paniculata. Most mopheads and lacecaps have naturally pink flowers even on slightly acidic soil; however many types can be manipulated into blues and purples. White hydrangeas will always open white; though their flowers may turn to a pink or red in the fall. Red cultivars will never turn blue, always staying a red or purple tone. A specimen’s ‘true’ colour can only be seen if the plant is grown on neutral soil.

Light Requirements

Hydrangeas bloom and grow well with morning sun and afternoon shade. They do not do not do well in heavy shade as their blooms will be few and unlikely to fully develop. Oak leaf types grow well in sun or shade however blooms will last longer if they get some afternoon shade and leaves will colour up best in the fall if they receive some sun. Paniculata hydrangeas can take full sun if they receive enough moisture; they require 5 hours of sun per day to set bloom.

Moisture and Hardiness

Hydrangeas need to be regularly watered in hot dry conditions. They can wilt quickly if they do not receive enough moisture; however they recover swiftly once watered.

Pruning

Hydrangea macrophylla does not respond well to pruning as flowers occur on 2nd year growth. As a result it is best to plant the specimen in a location where it can reach its full size. If you must prune do so by removing 1/3 of the stems almost at the base of the plant. Choose the oldest stems to remove. This rejuvenates the plant without being too radical. This practice may result in bigger blooms.

Hydrangea paniculata responds well to annual pruning producing larger flowers on vigorous growing stems. Cut the stems back to two or three buds each just after winter has ended. Paniculata does not require pruning, and left unpruned will produce more flowers at a smaller size. Deadheading may be done on any type of Hydrangea but is not necessary.

Fertilizing

Hydrangeas should not be fertilized with mushroom manure. Steer manure may be applied shortly after winter ends. Manure be applied between the beginning April and the end June. Blue Hydrangeas should be fertilized using a fertilizer with high levels of potassium and low levels of phosphorus to maintain a blue colour. Blue Hydrangeas should not be fertilized with superphosphates or bone meal as these may alter the flower colour. Pink Hydrangeas should be fertilized with a fertilizer a high phosphorus number as phosphorus seems to prevent hydrangeas from accessing aluminum in the soil.

Colour Manipulation

The colour of most mop heads and lace caps can be changed to be anything from pink to blue. Altering the soil around your hydrangea will facilitate this change. Flower colour is determined by the plant’s access to aluminum sulfate in the soil. If the soil is too basic the calcium in the soil will bind to the aluminum sulfate making it unavailable to the plant. Fall flower colour is not influenced by soil pH, however shad slows down fall colouring especially in H. serrata. Before changing the soil pH in an area consider the plants surrounding your Hydrangea as they could be adversely effected by the pH change. Altering the soil to change flower colour is not a onetime event, these conditions have to be maintained to keep the flower colour you desire. Newly planted specimens should be acclimated for a full year before you attempt to change their colour. Chemical additives could be fatal to a young plant. It may take up to two growing seasons to reach the colour you desire.

Turning your Hydrangea BLUE

Even in an acid soil if there is no aluminum sulfate present your flowers will not be blue. Also if the soil is very basic no amount of aluminum sulfate will turn your flowers blue. Plants near a concrete foundation or walkway may never turn blue because of the lime leaching out of the concrete. A pH of 5.2 – 5.5 is required for blue flowers. Aluminum sulfate should be added in September to encourage blue flowers for the following season (depending on plant size) 15-50g per plant dissolved in water applied into the soil at the base of the plant. Be careful not to touch the leaves as this solution might harm them. If your Hydrangea does not stay blue, dig holes around the plant, remove the soil and replace it with a soil with a pH below 4.5.

Turning your Hydrangea PINK

To turn your Hydrangea pink add dolomite lime to your soil to raise your pH to between 6.0 – 6.5. At this pH the Hydrangea cannot access the aluminum sulfate but will not suffer any other nutrient deficiencies or chlorosis. Adding 4 oz of life at the base of the plant will alter the soil by one pH point. Apply the lime in 4 oz increments dissolved in one gallon of water. Adding nitrogen and phosphorus will inhibit aluminum sulfate uptake encouraging pink bloom.

pH and Corresponding Flower Colour

4.5   Deep vivid blue
5       Medium blue
5.5   Lavender purple
6      Purplish pink
6.5  Mauve pink
6.8  Medium pink
7      Deep vivid pink

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