I live in downtown Toronto (Cabbagetown) and I have many irises in my backyard which gets full sun ll day. All of my irises come up purple and yet I only planted white, pink, and black. One year I planted a blue and it bloomed blue that year then went purple. So, I gave it to a neighbour and it blooms blue in her garden. Do I need to add something to my soil in the spring to get the colours I bought?
Thank you for contacting Toronto Master Gardeners about colour fluctuation in your iris bed. The Iridaceae family has numerous subgenera, species, sections, and cultivars. The hybridization of irises is a complex industry and offers myriad iris colours and plant structures to the gardener. Your question about irises turning purple has no simple answer. Below are a few suggestions of what might be occurring in your garden.
1. You mention that there have been many irises in your garden. Older, more dominant purple irises may be resurfacing. Even a small piece of an earlier rhizome will grow quickly and take over existing rhizomes. Using a barrier and space around newly planted irises to prevent intermixing with other rhizomes could help maintain plant integrity.
2. Seeds from existing irises, pollinated by bees, may have germinated in your soil. New growth irises from seed are not true to the colour of the parent. Deadheading the irises after bloom will prevent it.
3. A sport or offset may arise and possess colour that is not true to its parent. In a plant sport, a genetic mutation takes place from faulty chromosomes in the parent plant. This results in a segment of the plant being distinctly different. This is noticeable in leaves where a section is variegated. Generally in garden irises, this is not a regular occurrence.
4. Irises easily establish themselves, when transplanted. Sometimes irises skip a year of blooming. If they are dug up, planted in augmented soil by another gardener, then the original bloom colour can return.
5. Certain cultivars are far more vigorous growers than others and in closely planted beds, the more vigorous growers will almost inevitably choke out the less aggressive irises over time.
6. Tie bands around exceptional bloom colours each year to distinguish which rhizomes are responsible for that particular colour. Divide rhizomes 6 – 8 weeks after blooming and carefully plant with up to 2 feet separating individual colours. Use bone meal in the soil when replanting.
Below are detailed websites with scientific explanations of propagation and diversification of colour in plants. Also specific methods of planting rhizomes. With any new irises you acquire, perhaps giving each colour group plenty of space might be the best strategy. Good luck with your irises.
Iris Cultivation and Colour Change
Colour change in irises – American Iris Society
Dividing and Transplanting Irises