“What is the Difference between bulbs, corms, tubers, rhizomes?” I hear you ask. “I’m not even sure what they all are!”
Allow me to enlighten you with botanical facts and simple explanations….and some examples too.
Bulbs, Corms, Tubers, Rhizomes – Botanical Facts – What sort of plants they are?
These are all perennial plants. which means they have a life expectancy of more than two years. Generally speaking, they are among the short-lived perennials, ie up to about ten years.
They mainly reproduce by vegetative means rather than by seeds. Other perennials reproduce by seeds as well as vegetative methods.
Bulbs, corms, tubers, rhizomes share other qualities. For example, they are all storage facilities for the plant, storing starches, proteins and other nutrients. In other perennials this function would be carried out over winter by the root system.
With the exception of the tender tuberous perennials like dahlias the vast majority of these plants are easy to care for. The initial soil preparation and planting is important for some more than others but none of it should be too onerous. And one of the delights of using these plants is that you can fill your garden with flowers for most, if not all of the year, depending on where you live, of course. But a lot of the UK can have flowers from bulbs, corms, tubers, rhizomes for most of the year.
Bulbs, Corms, Tubers, Rhizomes – Botanical Facts and Differences
Most perennial plants have a root mass which develops over the years underground. Above ground, woody perennials have a growing permanent structure. Whilst herbaceous perennials tend to have soft top growth which dies back over winter.
However, the leafy top growth of bulbs, corms, tubers, rhizomes generally dies back after flowering, regardless of the season. The nutrients which had been stored were used up by the flowers and foliage as they grew. After the flowers die, the leaves continue to photosynthesise, making ‘food’ which is then stored in the bulb, corm, tuber or rhizome until the next year.
True bulbs are underground shoots that have modified leaves. The shoot lies vertically in the ground with a thickened stem, called the basal plate at the bottom. Thickened leaves which act as storage organs grow from the top of this basal plate; roots from underneath.
Shoots and flower will develop from both a central bud and from axillary, or side buds, situated between the leaves. These buds will develop into bulbs in the following year.
What are sometimes referred to as ‘bulbs’ by gardeners and plant nurseries may not meet this botanical definition. The term is used as a shorthand way of referring to bulbs, corms, tubers, rhizomes.
A corm is similar to a bulb, but is a swollen underground stem in which the plant’s food is stored. Shoots and flowers grow from a main bud on this swollen stem. The previous year’s leaves are the papery coat that you’ll see on a corm. The new corm develops on top of the old corm; this will eventually fall off.
As with bulbs, it may be that one or more axillary buds also develop. They will produce new corms in the following year.
There are both stem tubers and root tubers; which are slightly different.
Stem tubers develop at the end of rhizomes; ie underground stems. They store food over winter and then develop into new, independent plants with fresh top growth the following year.
Root tubers are storage organs formed at the ends of roots. Unlike stem tubers they rarely become independent plants but are part of the parent plant’s method for survival. The tubers themselves are bi-ennial (survive for two years) but the process allows the plant to renew itself.
Essentially, these are underground stems. Understanding this helps to understand how and why rhizomatous plants grow. Which in turn means you can give the plant what it needs to thrive in your garden.
The underground stem of a rhizome sends out both roots and shoots from the nodes along the stem. These are the ‘nobbly bits’ that you can feel or see.
Occasionally, there are rhizomatous perennials which do not have a concentrated store of starch in their rhizomes. Nettles, Urtica dioica, would be an example of an perennial which reproduces and spreads via rhizomes.
Bulbs, Corms, Tubers, Rhizomes – Examples that may be Growing in Your Garden
- Daffodils, Narcissus
- Snowdrops, Galanthus
- Onions and ornamental Alliums
- Sweet potatoes, Ipomoea batatas
Just to confuse the issue, whilst Iris are rhizomatous plants, their rhizomes prefer to bake in the sun. So you’ll find these near the soil surface rather than underground.
Although colloquially called a banana tree, the hardy banana, Musa banjoo is actually a rhizomatous perennial. What you see growing upwards is a pseudostem, formed from leaves which wrap around each other.
If your brain is reeling slightly after this whistle stop tour of the differences between bulbs, corms, tubers, rhizomes, don’t panic.
Let us ignore the vegetables above for a moment, and consider the ornamental ones. The reasons why you might need to know the differences are: –
- Whether you can plant them in the ground and they’ll look after themselves for years
- Will they reproduce themselves?
- Do they need to be planted deep in the ground or near the surface?
- Will they need to be dug up in the autumn, stored and planted again the next spring?
- Are they easy maintenance garden plants?
A couple of these points are covered above, but for more details, follow the links below. And then expand the range of plants in your garden…
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