Is it a Bulb? Is it a Corm?

And what is the difference anyway?

Why do gardeners love flowering bulbs so much? If you think of some of our favourite flowers through the seasons—snowdrops in winter, daffodils in spring, lilies in summer, the Christmas amaryllis and the now so trendy alliums — then bulbs are an integral part of our gardening culture. But what is a bulb, or a corm, for that matter…and why are they so successful?

A bulb is simply a central stem surrounded by concentric circles of tightly packed, specialised leaves. Each leaf is packed with nutrients. These include the starches and minerals that act as a reserve during winter and will be used to fuel the growth of stem and flower in spring. At the top of each bulb there is a terminal bud from which the shoot develops to form your beautiful flower in spring.

As green leaves grow away from their base in the bulb, appearing above ground, and the flower also grows, a lot of the winter reserves in the bulb are consumed—ponder on the size of an amaryllis flower and stem compared to the original bulb—and the bulb shrinks. In the meantime, the roots are working double time to try to keep that gorgeous flower maintained AND to replenish the bulb’s leaf bases with water and nutrients.

Eventually the flowers fade, shrivel and die. At this point they should be deadheaded because, if a flower has been fertilised, even more energy reserves will be summoned from the bulb to help form seed. Gradually, the nutrients that remain in both the stem and leaves drain back into the base of the leaves to keep the bulb healthy over the summer and form the basis for next year’s flower display, and the leaves turn yellow. So, when you are instructed not to mow until the leaves have died back (six weeks after flowering should actually be sufficient) this is not a deliberate ploy to make your life more difficult as you weave between clumps, but a guarantee of bigger and better flowers next year.

Less obviously (because it’s usually underground at this point) a bulb over time develops small lateral buds on its surface. These grow into separate bulbs with their own set of concentric leaves and, ultimately, the power to flower. This is how daffodils naturalise in grass, or snowdrops spread into rivers of white in your garden. This is vegetative reproduction. The great advantage of this is that the new bulb will produce exactly the same flower as the parent plant—a Dolly the Sheep clone, in fact. You will not suddenly get some rogue garish yellow daff in your sea of pearly whites.

Gardeners love the fact that, without any effort on their part at all, bulbs go forth and multiply reliably with no mutant throwbacks. As a strategy for the plant, this vegetative reproduction is less risky and energy-consuming, if more plodding, than dispersing fertilised seeds far and wide. This is because the new bulbs are being formed where the parent bulb is already flourishing, happily giving the new bulbs a real chance of surviving too, whereas the scatter-gun approach of sexual reproduction means that a fertilised seed could land anywhere and fail to germinate to form a new plant. As importantly, the bulbs grow close together so that there is no chance for a competitor plant to get in between them and pinch the available resources.

From a gardener’s perspective this means several things, good and bad. On the positive side you have an ever-increasing stock of something appearing year in and year out. On the negative side bulbs are very hard to be rid of because even a tiny bit of bulb with a bud can re-colonise an area—or they are so deep you cannot reach them. For three years I have been digging out Camassia leichtlinii bulbs, which produce a sensational blue or white tall flower, and moving them from my lawn (where clearly they are superfluous to requirements) to underplant quince trees in unmown grass. Each year, in spite of my best efforts, they come back even more strongly in my lawn and a few pop up under the quince trees!

The second consideration is that, after a few years of clump-forming, your bulbs will become congested and require more space to flower successfully and continue to reproduce. You need to divide the clumps when flowering is over, remove some of the individual bulbs and replant them to form their own colony. Give them a little bonemeal as a nutribullet for the summer and observe the correct planting depth, which means planting them at the same depth as they were before.

And finally…what is the difference between a bulb and a corm? A corm is very similar to a bulb but it stores all of its starchy nutrients in the stem itself and any leaves you see are just papery wrappers to protect the stem. Were you to cut one in half, you’d find it solid and hard, without the layers found in bulbs. Most corms behave just as bulbs do and plants that are cormous (just so you know!) include crocuses, cyclamen, crocosmias, gladioli and freesias.

Regardless of which you choose to plant in your garden, it is a fact that our gardens would be much poorer without the fantastic flowers that emerge from these slightly unprepossessing beginnings.