Gardens are always throwing up curiosities but one oddity that perplexes even seasoned gardeners is a plant that seems to change flower colour from one year to the next.
It was blue last year, the gardener will say pointing to a garden plant, and now it’s white (or yellow or pink). In most cases there’s a good explanation for what’s occurring in the garden, but to the average gardener it looks as if a plant has simply decided to change its floral wardrobe.
Flower colour is determined by the plant’s genetic make up but can be affected by external factors such the flower’s age, the season, a response to soil pH, chemical stimulus or poor nutrition.
Seed grown plants may be different from their parents. Some plants can also develop mutations – an odd-branch known as a ‘sport’ – with a different flower or leaf colour to the parent plant.
A plant that forms a ‘sport’ may be on its way to creating a new variety. The mutated growth (which may just be one shoot) could produce differently shaped or coloured leaves or a new flower colour. Sports may be found on common garden plants including azaleas and camellias.
Sports have given rise to new varieties when the odd- looking branch is detached from the parent plant, propagated and found to remain stable. Sports that are not stable may revert returning to the original appearance of the parent. Azalea ‘White Prince’ is a sport of the pink azalea ‘Rose Queen’. ‘White Prince’ may have both white and pink flowers on the one bush.
Ageing and chemicals
Some flowers change colour as they age – for example, a bloom may open white but gradually change to pink. Often the first colour is a signal to pollinators that the plant is full of nectar and pollen. Once the flower has been pollinated, its colour changes so it is no longer attractive to visiting insects.
Plants with flowers that change colour as they age include Hibiscus mutabilis, snowball bush (Viburnum opulus), yesterday today tomorrow (Brunsfelsia spp.) and Rosa ‘Mutablis’.
Flower colour may also be different at different times of the year. Repeat-flowering rose blooms for example may look different in spring and autumn due to the plant’s response to heat and cold.
Some changes are more dramatic. Petunia breeders are experimenting with a flower that changes colour between morning and evening. The flower changes from red to blue. For more on this fascinating project, known as Petunia Circadia, see more at Science Alert.
These dramatic changes are do to chemicals in the flower and lead to another reason for colour changes which is inhibition of certain chemicals in the flower. For example, a flower may appear blue or purple due to the presence of anthocyanin. If anthocyanin production is inhibited, the flower may appear white.
Anthocyanin inhibition could explain common blue and purple garden flowers such as agapanthus and iris, which sometimes confound gardeners by blooming white and is involved in the so-called Petunia Circadia experiment.
The big leaf hydrangea (Hydrangea macrophylla) is well known for changing its flower colour in response to soil pH. The part of the flower that changes is actually the sterile bracts around the true flower. The bracts may be pink, mauve, purple or blue as soil pH ranges from alkaline to acidic. The hydrangea responds to the availability of aluminium in the soil, which is determined by soil pH. The chemical in the flower that is affected is anthocyanin.
Adding lime to soil that naturally grows blue hydrangeas can change their colour to purple or pink as the soil becomes more alkaline. Adding aluminium sulfate to alkaline soils encourages pink hydrangeas to develop blue tones. To experiment with changing the colour of a hydrangea, grow a plant in a pot and add aluminium sulfate (sold as hydrangea bluing tonic) to the pot during spring. By summer, when the hydrangea blooms, its flower should be bluer than before if the pH of the potting mix has been lowered.
A big reason garden plants appear to change colour however is seedling variation. Many species have the potential to produce plants with a range of flower colours – for example pansies can be yellow, brown, orange white, pink, purple or blue. Plants raised from seeds from an orange pansy for example may flower yellow or even purple depending upon the breeding that lead to the original orange-flowered plant.
[This article first appeared in TasWeekend (October 22-23, 2016) in The Mercury.]