When is a Bulb Not a Bulb?

Caladiums brighten our summer gardens.


By Kit Flynn and Lise Jenkins

Recently a garden group asked me to give a presentation on bulbs. When I replied that I really was a perennial gardener, rather than a bulb gardener, the chairman replied, “But you like to do research.” After he repeated this five times, I found myself unaccountably researching bulbs.

It turns out that the word “bulb” has several different meanings. The word refers to true bulbs — think of an onion with its layers of compressed leaves — but it also refers in the vernacular to a group of plants known as geophytes.

Now geophytes are those plants that have underground storage systems, including bulbs, corms, tubers, rhizomes and tuberous roots. Bulb catalogues will include specimens from these categories: Technically “Brent and Becky’s Bulbs” should be “Brent and Becky’s Geophytes,” a name that would probably drive many gardeners away in fright.

Most of us know that daffodils, tulips and lilies are bulbs but what about the other categories? Corms, consisting of solid stem bases, are flatter than bulbs but still have a basal plate where the roots emerge. Unlike bulbs, which happily produce new bulbs (a process we call “naturalization”), corms will wither away only to be replaced by a new corm. Plant five daffodils and soon you’ll have ten; plant three gladioli and you’ll still have just three.

Tubers (think of the potato) are thick, underground stems that lack the basal plate. Tubers simply get bigger without producing any offshoots. Caladiums are an example of a plant having tubers.

Rhizomes are different from tubers in that these swollen underground stems grow horizontally near the soil’s surface. Cannas and irises both have rhizomes. Stolons are stems that grow along the surface of the soil, plants with stolons are not geophytes.

Tuberous roots are seen in daylilies, dahlias and peonies. These are thick, fleshy roots that look as though they are tied together in a bundle. These plants grow larger but don’t naturalize. To get more of these plants the gardener simply divides them. 

Now the sad truth is that nature isn’t interested in these manmade divisions. Consequently, there are plants, such as colocasias, that cannot be categorized. Sometimes you’ll see colocasias linked to corms, sometimes to rhizomes and sometimes to tubers. The only thing botanists agree on is that colocasias aren’t bulbs.

So, you ask, why should you care if a plant has a rhizome or a corm? In planning a garden, this is important. For example, many plants with rhizomes like to travel. Plant a canna and chances are that not only will the clump size increase, it will start to climb over that prize rose shrub you have. Fortunately, plants with rhizomes are easy to pull out but you might get tired of curbing it. Forewarned is forearmed.

Perhaps you dream of a field filled with daffodils and crocuses. The daffodils, being bulbs, will increase in number whereas the crocuses will not. Crocosmias will trip you up because sometimes they’re referred to as bulbs and sometimes as corms. Certainly “Lucifer” has acted as a bulb in my garden, increasing in number rather quickly.

There is a lot of conflicting information out there on how to plant geophytes. When you purchase your geophytes, be sure to look for planting information. Tubers generally are planted deeper than rhizomes. Lily bulbs want more depth than daffodil bulbs. Tuberous roots should be totally covered by soil. The giant crinum bulbs detest being moved once planted, as do peonies. Move the house before you move these two plants.

The one thing that experts agree on is this: all geophytes need well-draining soil. They will drown in solid clay soil so soil amending is a must if you are to have any success with this group of plants.

If you have managed to read this far, congratulations. You have just taken part of Botany 101. 

Absent from their gardens, Kit and Lise enjoy roaming our region exploring the intersection of horticulture and suburban living. More on Instagram @AbsenteeGardener or email: info@absentee-gardener.com

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