Hydrangeas, as a genus, cover a wide range of growth habits, sizes, and flower color. This is due in large part to the rna ny different species, but it is also a product of the plant's ease of cross-pollination. We will briefly cover all the different species that grow in our area, but will focus mainly on the Macrophyllas (Big Leaf) as those can be the most frustrating and are the subject of most questions.
Arborescens also known as Smooth Hydrangea, is our native species and will grow in a wide range of conditions. It prefers rich, moist, well-drained soils but will adapt to almost any soil. Partial shade is preferred, but it will grow in full sun if adequate moisture is supplied. Soil pH doesn't seem to affect the plants. The flowers are white and appear from early to mid June into fall. The most common cultivar is 'Annabelle' and is noted for its huge flower heads (up to 1' across) . The Arborescens flower on new wood and can, therefore, be pruned just about anytime. ObViously, pruning during the bud-set to flowering period will affect flower production.
These are the climbers of the family and cling by small root-like holds similar to some of the ivies. Climbing hydrangeas are sometimes slow to develop, but if planted in a high-organic, well-drained soil and in partial shade, they are worth the wait. The flowers are lacy and white in early to midsummer but one the best characteristics is the beautiful exfoliating (peeling) bark that shows itself during the dormant period
The Panicle Hydrangeas have been in American gardens for generations haVing first been introduced from Asia in the 1860's. For years, the most recognized of these was the Pee Gee; however, in recent years many new and improved varieties have made their way into the market. The flowers of most varieties of Panjculata are white, changing to some degree of pink and appear somewhere between late spring and late summer depending upon variety. For best results, plant these in full sun to partial shade in any well-drained soil. This is another species that flowers on new wood, similar to the Arborescens. We usually prune these during the dormant period to maintain the desired shape and to remove spent flowers.
Oakleaf Hydrangeas hold a special place in the heart of many gardeners. They're Widely used because of their adaptability to soil conditions and tolerance to either sun or shade, although some shade is best. There are now many cultivars of the species and they vary in size from 2'_ 3' up to 10'+. The incredible floral display in summer is followed by fall color ranging from red to burgundy. The attractive cinnamon-colored exfoliating bark shows itself best in Winter. ThiS species blooms on new growth coming off of old wood so pruning heavy could reduce flower production.
For many, the Bigleaf Hydrangeas are a great source of frustration. However, with the new varieties now on the market, there's really no need for this. If a few gUidelines are followed, great success Is easily achieved.
Pruning and Winter Die-Back
With the old-fashioned varieties, this can be the biggest reason for flowering problems. If you prune heavily or your plants die back to the ground in the Winter, the likelihood of getting flowers the next summer isn't good. This is because these varieties flower on new growth coming off of old wood.
If all of the old wood is pruned away or killed, the plant won't flower that summer. The easiest way to negate the issue is to choose varieties that bloom on both old and new wood. The most well-known of these is the Endless Sum mer Collection as well as a variety called Penny Mac.
Soil pH and Fertility
Flower and leaf color of Macrophylla Hydrangeas are both determined indirectly by soil pH because of its affects on aluminum and iron intake.
Flower color is determined by the amount of aluminum the plant takes up. The lower the soil pH, the more aluminum will be available and the result will be more blue in the flowers. Higher pH will result in less aluminum availability and the more likely the flowers will be pink.
Yellowing of the leaves is usually a result of an iron deficiency. Iron, like aluminum, is more readily available in lower pH situations.
The addition of aluminum sulphate to the soil will result in blue flowers and dark green leaves.
Sulphur will lower the pH which will make both the aluminum and iron more readily available. If you prefer to have pink flowers, a general purpose fertilizer with a low amount of sulphur and no aluminum would be advised.
It is also worth mentioning that too much nitrogen and not enough phosphorous could create an imbalance that would result in poor flower production.