Above L-R: Crocosmia, Cyclamen, Tulip ‘Queen of the Night’
When it’s a tuber… a rhizome… or a corm. We often refer to ‘bulbs’ in general to include plants such as crocus, cyclamen, crocosmia or ranunculi which in fact all originate from corms. These, along with dahlias (from tubers), spring-flowering anemones (from tubers or rhizomes), and Iris germanica (from rhizomes) use underground starch storage organs as their method of sustained nutrition throughout winter.
So long as you plant them at the right time and in the right conditions, it doesn’t matter too much what you call them. But if you’re looking to propagate these plants and want an idea of how quickly and how far they’re likely to spread of their own accord, it’s useful to know which category they fall into:
BULBS
A bulb is multi-layered in structure (think about an onion when you cut through it) and is in fact a modified shoot/leaf that has evolved to become a food storage organ for the plant. The leaf scales within a bulb help protect it from the extremes of winter as well as giving some defence from pests such as mice or eelworms. If you dig up bulbs from a previous season you may see ‘child bulbs’ forming at the base of the main bulb. These will eventually break off to produce new plants or you can carefully break them off yourself and plant them, along with replanting the parent bulb if it’s still healthy.
The best time to divide spring bulbs is 6-8 weeks after flowering has finished. Wait until the foliage has died back and become yellow and floppy as this gives the plant the opportunity to reabsorb nutrients from the leave back into the bulb for next season’s growing. You can also divide bulbs in the autumn but this usually means digging them up and sometimes they’re hard to find!
Popular flowering plants grown from bulbs include: Tulips, Narcissus, Hyacinths and Alliums Other members of the onion and garlic family can be grown for their flowers as well as food.
CORMS
Corms look similar to bulbs but are usually smaller. They are underground adaptations of the plant stem and although having dry scaly outer leaves, they don’t grow layers of scale leaves as bulbs do. Instead, they form new corms – ‘cormlets’ – each year, which grow on the side of the parent corm and can be broken off to propagate them. Corms also grow specialised roots which enable them to adjust their position in the soil to maximise their chance of blooming success. Clever, eh?
Popular flowering plants grown form corms include: spring or autumn crocuses*, Crocosmia, Gladiolus, and the alien-looking Ranunculus corms.
* Not to be confused with Colchicum autumnale which is a herbaceous perennial.
Above L-R: Ranunculus corms, Crocus corms, Dahlia tubers
RHIZOMES
Rhizomes are stems which grow horizontally just under the surface of the soil or sometimes on top of it. Adventitious roots allow the rhizome to take up nutrients from the soil and its branching mechanism allows it to grow and spread quickly. Iris germanica (bearded iris), Bistorta (persicaria) and Canna lilies all grow from rhizomes and can easily be divided as they spread to make new standalone plants. The rhizomes can be tough and you may need a small saw or bread knife to divide them but they can survive this rough treatment if replanted promptly. Some plants take rhizomatous growth to extremes and should be contained or avoided altogether – eg. bamboo, horsetail and Japanese knotweed.
TUBERS
When we talk about growing flowers from tubers, dahlias are probably the first plant that springs to mind. Other flowering tubers are Cyclamen, spring Anemones and Peonies. Tubers are root adaptations and sometimes referred to as ‘roots’ by nurseries. Large tubers, such as dahlia plants, can be divided in spring with a sharp knife, making sure to include at least one ‘eye’ which will form the new shoots. If that’s got you thinking about chitting potatoes then you’re on the right lines – potatoes also grow from tubers, multiplying underground to make multiple tubers which when ready can be dug up and cooked.
Peonies should only be divided once mature and only every few years if the plant has become crowded as this may delay flowering for a year or two.
Want to learn more about propagation (making new plants, for free)? Our online, interactive Roots to Shoots course runs for 6 months every year. Join the WaitList to be notified when the next course opens for registration.