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Is it a Bulb? Is it a Corm? (...and what is the difference?)

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 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.

9 thoughts on “Is it a Bulb? Is it a Corm? (...and what is the difference?)”

  • Jackie Powell

    Really enjoy sitting down with a big mug of coffee and reading your news/updates.Very entertaining, thankyou. I have a small garden now but would like to know what small shrubs to plant in clay soil, in an open position,between the field and us. Preferably evergreen as there are cows for about 3 months of the year! I don't want much do I?(We had a terrible experience with cows in a previous garden.) Many thanks, Jackie.

    • Julian

      Thank you very much indeed!

      Two quick observations:
      1. One man's clay is another's rich loam...
      2. You do not say for which 3 months you have cows...

      Given the above, yew and laurel will grow well. So will pyracantha, shrub honeysuckle and (although they are not supposed to) I have seen the upright cotoneasters like cotoneaster simonsii and cotoneaster franchettii doing well on clay.

      Not evergreen but capable of proving a good screen are smaller willows and the dogwoods with their lovely winter bark.

      And don't forget roses - they are great on clay and some of the shrub roses can make a thicket you cannot see through.

      Hope this helps

  • Bryan E. Isaac

    Why do daffodils not flower the following season after purchase and planting, and, will they ever flower again ?
    Thank you in anticipation, B E I

    • Julian

      Daffodil bulbs SHOULD always flower following planting PROVIDED the bulb size was sufficiently large when they were bought. Given a large enough bulb, there will be enough food stored to produce at least one flowering stalk. Bigger bulbs can produce several flowers. The only reason why that might not happen (that I can think of) is that the light levels are too low, but we have daffodils flowering in deep shade.

      Can you provide any more information?


  • Shirley Isaacs

    One year I planted many daffodils bulbs, and for a few years had a lovely display. However, not one has since come up and wonder why? - and could I replant bulbs in this area again?
    Many thanks.

    • Julian

      If you had a good show of daffodil bulbs that disappeared in the course of a single season I would be inclined to look at reasons such as:


      In which case, it would be worth while replanting a few bulbs (as a test) to see how they do before replanting en masse.

      Good luck

  • Jim Large

    I enjoyed your article on bulbs in your March newsletter. My garden has two lightly wooded areas with well established naturalised daffodils. They are always a picture and last year they were a carpet of yellow. However this year only about a quarter flowered and the rest came up blind despite tremendous leaf growth. I always dead head the flowers and leave the foliage to die back completely. I cannot think the problem is overcrowding because why would they all in two parts of the garden suddenly decide they were too overcrowded to flower. Last spring and early summer were very dry and I wonder if the ground was too dry to allow the bulbs to develop flowers for the following year. In the past I have never fed them but I am thinking of giving a high potash liquid feed in the next couple of weeks, perhaps soluble sulphate of potash. I would value your thoughts.

    • Julian

      Thanks for your daffodil bulbs post. I think that the weather last year is almost certainly the cause of so many blind bulbs appearing this spring. For about 6 weeks after flowering has stopped, bulbs need plenty of moisture and nourishment so they can plump up again to flower well the following season. A dry spring stops that. It can be countered in the short term by watering, and adding a little foliar feed cannot hurt. In the longer term I would recommend mulching your bulb areas with leaf mould or compost to improve the soil structure so that it will hold more moisture for longer and counteract the effects of dry spells at the wrong time of year.

      Hope this helps

  • Jim Large

    Many thanks for your thoughts. I had a feeling the dry spring and summer did not help. I will make sure the bulbs are kept watered, especially if we have another dry spring and will give them some foliar feed and mulch. I appreciate your response.

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