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Hydrangeas

 

hydrangeas 

Hydrangeas uniquely contribute both color and form to our landscapes and one can hardly go wrong planting any of them.

 

There are about 23 species of these plants, many imported from Asia, but a few are native to the Americas. Five are commonly available and used in our landscapes. Books have been written about the many hundreds of cultivars of each of these more common species.

 

Below is a brief outline of the five common species. For more information look to Dr. Michael Dirr’s books on hydrangeas and plants in general.

 

Type of Hydrangea

Plant structure

Blossom

Comments

H. macrophylia

 

“Mophead, Big Leaf”

Many, many cultivars

3 to 6’ (+), huge leaves 4-8”. Rounded shrub with long stems which rarely branch.

Usually Blue or pink, some white. Large flower clusters. Species bloom on past year buds, some cultivars bloom on new growth as well

Hardiness zone 6-9.

Part shade, must be watered in summer

H. macrophylia

 

“Lacecap”

Same as above

Typically have clusters of larger sterile flowers surrounding small fertile ones.

Same as above

H. arborescens

 

“Smooth Hydrangea, Annabelle, Snowball”

3 to 5’, low growing, clumpy and will spread over a wide area with suckers

Green to white to brown. Booms on last year’s buds as well as current season new growth.

Hardiness zone 4-9.

Prefers part shade but will grow in full sun if watered.

 

H. paniculata

 

“Pee Gee”, There are many other cultivars

10-20’, grows fast spreads into small tree or large shrub

White panicles changing to pink. Blooms on current years buds

Hardiness zone 3-8.

Most sun tolerant of these five species

H. quercifolia

 

“Oak leaf Hydrangea”

Several cultivars available

4-8’, spreads by suckers. Large leaves shaped like red oaks

Some cultivars are shorter.

Large white panicles changing to pink then brown

Hardiness zone 5-9. Needs part shade, water.

 

 

General Care: In Oklahoma all of the hydrangeas do best with afternoon shade. It is essential for some. They also drink a lot of water in order to make those huge leaves. They will need summer irrigation in additional to our typical rainfall to remain healthy. Fertilizing with a slow released predominately nitrogen fertilizer in spring is appropriate. Fertilizer choice is always best based on a current soil test.

 

Pruning: Mopheads, lacecaps and oakleaf hydrangeas typically bloom on buds formed during the previous late summer and fall. If pruning is needed, do so after the flush of blooms in spring is completed.

 

Panicle and smooth hydrangeas bloom on current year’s growth and may be pruned, if needed, late summer until early spring before new growth begins.

 

Any older and dense hydrangea (or any shrub) will benefit from removing 1/3 of the older stems back to the ground in winter or early spring.

 

Changing Blossom Color: Macrophylia hydrangeas—the mopheads and lacecaps—typically have blue to reddish to pink blossoms. The color of their blossoms depends on the amount of aluminum in the blossom. Inadequate aluminum causes them to be pink, whereas abundant aluminum produces blue petals.

 

Our soils generally have ample amounts of aluminum, soil deficiency is not the problem. The limiting factor is that these hydrangeas absorb aluminum only if the soil is acidic (low pH). A pH of 5 to 5.5 is best for blue color, anything above 6 tends to produce pink.

 

A soil test will measure the pH or acid content and help decide what to do. In the absence of a soil test and if you wish to turn pink blossoms to blue you should acidify the soil. Aluminum sulfate adds both freely available aluminum and acidifies the soil. Use ½ to 1 ounce of aluminum sulfate added to a gallon of water and apply to the root zone as a drench. This may be repeated 2-3 times during the growing season. Also, the use of acidic organic material such as peat moss will help lower the PH.

 

If you wish to turn blue flowers to pink, reduce the acidity (increase the pH) of soil using garden grade lime. Apply one cup to the root zone twice a year during the first year and observe results. More applications may be needed, but lime raises pH very slowly (months); if overdone it can interfere with nutrient absorption.

 

Lastly, be aware that too much phosphorus in the soil will block aluminum absorption and produce pink flowers. In Tulsa, soil test from landscapes previously fertilized have shown that over 75% have either enough or, more often, too much phosphorus. Once excessive phosphorus is in the soil, it takes years to correct. Here you must accept that the blossoms will be pink unless you replace the soil.