One of the lovely sights in summer and September where I live is the hydrangea blossoms. It’s hard for me to pass by a shrub without approaching it to look at the flowers more closely and admire their beauty. The blooms are large, showy, and often very colourful. I enjoy photographing them as well as observing them. I found all of the hydrangeas shown in this article near my home.
The two varieties of the plant that I most often see are mophead and lacecap hydrangeas. The scientific name of both of these forms is Hydrangea macrophylla. The plants are sometimes known as bigleaf hydrangeas. Since the flower heads are inflorescences, they contain multiple flowers, or florets as they’re sometimes called. The plant grows as a deciduous shrub. It’s native to Asia but has been introduced to many other parts of the world.
In mophead hydrangeas, the florets are gathered in a dense and attractive bunch that resembles a pom-pom. Each floret in a flower head contains four coloured sepals that some people might assume are petals. The real petals are small and are located in the centre of the whorl of sepals. When they haven’t opened up, the centres look somewhat like the broad end of a hatpin, as can be seen in the photo above. When they do open, the stamens or male reproductive structures become visible.
Despite the presence of the stamens, the majority of the florets in the flower head are infertile. If a person gentles pushes the outer florets aside, however, they may see a few inconspicuous fertile florets in the middle of the pom-pom.
Lacecap hydrangeas have the shape of a flat disk and also contain two types of flowers. The small inner ones have five petals and are fertile. The bigger outer florets look like those of mophead florets and form a circle around the inner disk. These florets are sterile.
I observe and admire the mophead and lacecap hydrangeas that I discover in parks and landscaped areas near my home, but I’ve never grown them myself. They are said to be easy to cultivate, but I’m content to look at the gorgeous specimens in my neighbourhood.
Plants in the genus Hydrangea have white, pale green, pink, lavender, or blue flowers. Most species bear white flowers. Coloured ones are frequently chosen for gardens, however. Some–but not all–hydrangeas can change their flower colour over time. The species that are most often chosen for gardens and landscaping have the ability to produce coloured flowers if a sufficient concentration of aluminum ions is present in the soil
The colour change in the flowers is interesting chemically. The presence of aluminum ions that are separated from compounds and able to enter the roots of the plant depend on the acidity of the soil. If the soil is acidic, aluminum ions enter the plants and the flowers tend to be blue in colour. If the soil is basic, the aluminum ions are “locked up” in a soil compound named aluminum hydroxide and are unable to enter the hydrangea plants. In this case, the flowers are pink.
Anthocyanins are pigments in flowers and fruits. An anthocyanin called delphinidin-3-glucoside is involved in the hydrangea’s colour change. When no free aluminum ions are present, the anthocyanin molecules and the sepals that contain them are a shade of red or pink. When aluminum ions join to delphinidin-3-glucoside, the anthocyanin molecules and the sepals become blue.
The colour change behaviour in hydrangeas is opposite to that seen in litmus paper. You may have learned in school that litmus paper turns pink in an acid and blue in a base. Hydrangea flowers turn a shade of blue when growing in an acidic soil and a shade of pink in a more basic one. Intermediate colours and deeper or lighter shades of a colour can be seen depending on how mobile the aluminum ions are. Some shrubs have flowers of multiple colours.
Anyone who is interested in controlling the colour of their hydrangeas should do some research and look at reliable sources. I’m not going to share instructions because I’ve never performed the process. Changing the chemical content of the soil needs to be done carefully so that it doesn’t harm the plant. A soil test is required before any chemicals are added. A colour change may take six months or more to achieve.
It’s important to find out whether the particular type of hydrangea growing in the garden is genetically capable of changing colour. Not all types are. Hydrangeas with white flowers can’t change colour. Some hydrangeas may change their colour naturally as soil conditions change without any effort by a gardener. Once the flowers change to a desired colour, the soil pH will need to be kept constant in order to maintain the colour.
Those many people focus on the appearance of a hydrangea’s flowers, the leaves of the plant can be attractive, too. They are oval and have a pointed tip and toothed edges. Their veins are prominent. The leaves may be speckled or edged with red, even in summer. Some become an attractive red colour in the fall.
Hydrangeas are popular ornamental plants, with good reason. Some bloom from late spring to early fall, as many do in my part of the world. Even a general category of the plants, such as mophead hydrangeas, contains different cultivars having different features. A wonderful selection is available for plant lovers. If you don’t want to grow them in your garden, you may very well find ones near your home. They are worth looking out for.
- Hydrangea Selection, Pruning, and Care from the Virginia Cooperative Extension (a partnership between Virginia Tech and Virginia State University)
- Changing the colour of hydrangeas from the American Scientist
- An anthocyanin and hydrangea colour from the Horticultural Research Institute Journal