Towering chrysanthemums
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.
The Biking Gardener 
Reblogged this on Anita Dawes and Jaye Marie.
I am going to plant chrysanths next year, as I need some strong, reliable and tough additions in my garden…
I have never taken to chrysanthemums though they are beautiful flowers.
Many of the cut flower cultivars are very different from garden varieties (cultivars). Furthermore, some of the ‘potted mums’ are very different from cultivars that were developed for other uses. This applies to many sorts of cut flowers and potted plants, from roses to hydrangeas to azaleas and so on. We grew ‘landscape’ azaleas, but only a single cultivar of ‘florist’ azalea (which was later discontinued because it was difficult to grow within our facilities). Some of the chrysanthemums that were developed for cut flowers get so tall and lanky that they need support in the garden, or should be cut back to get shrubbier. They are designed to generate long stems for floriculture applications. They can develop big single flowers if disbudded, or sprays of smaller flowers if pinched, so can be somewhat accommodating. Nonetheless, for some cultivars of chrysanthemum, genetics are as important as cultural conditions.
I think one has to pinch or cut back rigorously through the summer to achieve that short bushy shape. I never manage to do that, so my yellow mum is more than 1 m tall, and has to be staked. It’s blooming well, though.
Chrysanthemums are very malleable and can be shaped and pruned during growth and you are right that lots of pinching while they are growing vegetatively can restrict their size. I don’t always get mine pinched in time but, like you, am always happy to see their flowers, at whatever height.