This is one of a new series of posts based on real questions I have been asked. Feel free to add to the answers or include your ideas and experiences
i recently had an enquiry about chyrsanthemums. The person had grow a cutting from a stem in a bunch of chysanthemums and planted it in the garden where it finally bloomed in November but had grown to more than 2m high. This is different to the ‘pot mums’ you can buy for autumn display in the garden.
It opens up a whole can of worms because chrysanthemums, which have been cultivated for millennia, are a complicated lot and they are classified in many different groups. Those used for commercial cut flowers and potted flowers for temporary house plants (as opposed to autumn potted plants for garden use), that are available all year round, are late-flowering chrysanthemums. They are more or less hardy but they are grown under glass because they flower so late and the flowers are easily damaged by cold and wet. They flower so late because they are short-day plants. The flower buds do not start to form until the plants are exposed to light periods where the ‘days’ are considerably shorter than nights. This is the same principle that makes poinsettias and Christmas cacti flower in winter. Famously, and well known, if you want your poinsettia to bloom the second year in the home you have to pop them in a cupboard every night from October to keep them away from artificial light so they have days shorter than 12 hours. Once buds start to initiate (or more notably the bracts) they can be brought back into ‘normal’ conditions.
To get these chrysanthemums to bloom all year, the plants, whether grown in soil in beds for cut flower or in pots, are screened every night to keep out light and extend the night periods. The size of the plants depends on when they are planted, before the flowers are initiated. It is all very scientific and manipulated so, for pot plants just 20cm high the plants may only be weeks old while for cut flowers it may be months, so they have time to develop stems before flowers.
The same variety could be used for both short and tall plants. So if you plant out your pot of ‘houseplant’ chrysanths, even though the plants in the pot were only 20cm high, in the garden they will grow all summer and the shoot tips will keep growing and not initiate flower buds until late autumn. And if you have streetlights above the plant or security lights that break the dark period, flowering will be even later.