The differences between bulbs, corms and tubers.
When is a bulb not a bulb? When it’s a corm (or a tuber!). In fact, the word ‘bulb’ is used to refer to all Geophytes. These are underground bulbous organs, used to store the nutrients and water required for the plants to feed themselves when dormant and not actively growing.
Bulbs, corms and tubers share some of the same characteristics but differ in interesting and important ways. Bulbous plants include a diverse range of species such as agapanthus, alliums, cannas, crocuses, daffodils, hyacinths, lilies, dahlias, daffodils, tulips and many more. Some of these are fully hardy and can be left in their growing position almost indefinitely, and others need more careful handling to ensure they make it through the winter.
Knowing about these differences can help you to understand the best way to care for your plants during their growing and flowering periods and also after they have finished blooming to ensure they return the following year.
Tulips, Daffodils, Lilies and Alliums are all examples of ‘true bulbs’. They consist of layered ‘scales’ which attach to the basal plate at the bottom of the bulb, and a surrounding tunic which holds together and protects them (Lilies are true bulbs but do not have this layer of protection so they need extra care when handling). The scales are a type of modified leaf which store the nutrients gathered over the growing season. They actually shrink as the bulb grows and flowers, and then replenish themselves before the bulb becomes dormant again over the winter.
Corms often look outwardly similar to true bulbs, but are usually flatter and more rounded and inside are completely solid being a stem rather than a scaled structure.
The biggest difference is that corms shrink and shrivel as the plant grows and uses up all the stored energy to produce its leaves, stem and flowers. The energy generated from these is then used by the plant to develop and grow a new corm, which will be the basis of next years flowers.
For the purposes of this article we’re actually talking about tuberous roots, of which Dahlia tubers are one of the best known examples. These are simply swollen sections of root which absorb nutrients and water from the soil and act as a food store for the plant.
Unlike potatoes which are a stem tubers, Dahlia tubers cannot grow on their own, and must be attached to a portion of the crown with at least one growing point, or eye, to produce a shoot. Dahlia tubers can be tricky to overwinter and are prone to rotting in damp or waterlogged soil or getting damaged by frost. Many people with well draining soil do leave them in the ground over winter and use a thick mulch to protect from frosts. Otherwise, they can be lifted, dried and stored over winter ready for planting again the following spring.
Crocus sativus corm.
Dahlia 'Peaches' tuber.
One common feature of all the tuberous roots covered here is that the energy for the bulb to grow and flower again the following season is produced via photosynthesis from the foliage grown during the year. Therefore it’s important not to remove foliage by cutting or mowing until it has died back naturally. You really do need to let the bulb squeeze out and store every last bit of energy it can to guarantee it will be healthy and flower again the following year. Removing deadheading or removing any seed pods after flowering is a great way to divert all the plants energy into the bulb to be stored rather than used to develop seeds.