Identifying Underground Stems
Most people know what a flower bulb is. Images of an onion or a daffodil come immediately to mind. But the word bulb is often used as a catchphrase for any fleshy underground root, or stem. Identifying underground stems and differences between bulbs, tubers, corms and rhizomes on your perennials will help you identify and properly propagate your flowering plants.
First of all, the word for any fleshy, enlarged underground part of a plant is actually a stem, not a root. This word encompasses tubers, rhizomes, corms, and bulbs. The underground stem is a way for the plant to store up energy. The plant generates the energy during the growing season, through the leaves and photosynthesis. This fleshy underground stem stores the energy throughout the dormant season in preparation for the next season‘s growth.
Identifying underground stems is important not just for identification, but also plant propagation.
Plant Propagation
So why does identifying underground stems matter? All flowers produce seeds and new plants grow from seed right? Yes, it is true that all flowers produce seeds. (Non-flowering ferns produce spores) However, many flowering plants which produce seeds also have a secondary, asexual reproduction process, often called division.
For most bulbous plants, like the daffodil, sexual reproduction from a seed could take 5 or more years to get the seed to a mature enough state to actually produce a flower. The larger the underground stem, the longer it could take to mature. The amaryllis for example, when grown from seed, could take nine or ten years to flower. But division of the bulb could get you a whole new plant in a year. Division of tubers or rhizomes can multiply 5 fold or more over the course of one growing season.
This frankly is the easiest and fastest way to generate lots of new plants from your already existing plants. But you can’t do it without first identifying underground stems.
Another advantage – for gardeners – of asexual division over seeds is that the new plant will be exactly like its parent. Most gardeners have observed that seeds from a foxglove or a squash could produce a different offspring than the parent. This is because of cross-pollination, where a plant could be pollinated by another type of plant. The new plant has the genes of both the parent plants. It also allows for the baby plant to adapt to its environment. But for the gardener, it could result in a plant with a different color flower, or it could take on other characteristics.
In order to identify underground stems, we need to understand their different characteristics, and how they benefit the gardener.
Bulbs
Most bulbs are round or slightly oblong, and generally have a smooth appearance with a papery outer covering. Think of an onion, tulip or daffodil. Some bulbs have overlapping scales, like garlic or lillies. Both types contain the embryo of the new plant, and the scales protect and feed it.
Almost all them, with the exception of some lillies, are planted in the fall or early winter, and bloom the following spring. “Spring planted bulbs” that you see on nursery websites or in seed catalogues are generally not true bulbs, but are in fact tubers or corms. The word bulb is very often used as a catch-all for any plant with a fleshy underground stem.
Bulbs generate new bulbs at the basal plate of the bulb, which is near the edge of the roots. The roots serve to anchor the plant in the soil. There is evidence that the roots can even adjust the height of the bulb up or down in the soil to get just the right distance from the top of the soil depending upon the light and climate conditions.
Dividing Bulbs
The best time to divide most bulbs is when they are dormant; usually the fall or winter, before they start growing for the next season. The foliage should have died back before you dig. Remember that the plant needs to generate photosynthesis through its leaves to store up energy for the next season’s growth. Once the leaves are brown you are safe to divide.
To divide bulbs, gently dig out the bulbs from the soil. If relocating the plant, remove all of the bulbs from the area. If dividing to make more plants, remove all but a few of the bulbs. Bulbs which are overcrowded will generally smother each other in the competition for resources, so thinning them out periodically is healthy for them.
If the soil is hard or compacted, dig around the area making sure not to sever the bulbs. If you do sever a bulb, generally you can still replant it and it will grow back as long as you have most of the basal plate intact.
Read about how to regrow your store-bought onions for your backyard garden!
Depending upon when you dig them up, bulbs can last for several weeks to several months in storage before planting again. You’ll want to get them back in the ground before they start growing in early spring. If you start to see a shoot out the top, or even roots growing, it isn’t too late! You should still be able to plant the bulb and have flowers this season.
If you must divide when the plant is blooming or the foliage is still green, move the plant along with the soil surrounding it. And, try to put the plant back into the ground along with its original soil immediately – the same day.
Examples of Plants with Bulbs
- Daffodils / Paperwhite / Narcissus
- Tulips
- Dutch Iris
- Lillies
- Allium, Onion, Garlic, Shallots
- Amaryllis
- Scilla / Peruvian Lily
- Lycoris / Spider Lily
- Fritillaria
- Hyacinth
- Muscari / Grade Hyacinth
- Snowdrops
Tubers
Tubers are fat, rounded portions of the stem. The most definitive feature of tubers is that they have ‘eyes’. New growth will sprout from each eye, generating a new plant. Therefore if your one parent tuber has five eyes, you will have five new plants.
A subcategory of tubers are plants with tuberous roots, which just means that the tuberous area is derived from root rather than from stem. Tuberous roots are elongated rather than rounded, but like tubers are fat and have eyes on each fleshy underground piece. Dahlias and sweet potatoes are both excellent examples of tuberous roots.
Dividing Tubers
Tubers reproduce rapidly so you will have multiple tubers at the end of a growing season where you previously had one. Also, to make even more plants, true tubers can be cut into sections with at least one eye on it prior to replanting. Division of tubers is best done at the end of the growing season, after the green of the plant is flopping over or has died back. Removal of the tubers is easy in very loose soil. If your soil is not loose, gently dig around the area to loosen it a bit first. Carefully pull out the tubers from the soil. Using your fingers to sift through the soil is helpful so as not to damage the tuber.
Tuberous roots will also multiply and can be divided into sections of the root structure so long as there is one crown or eye per new plant. The new plant will grow from the top end and the roots will grow from the bottom end. Just like tubers, tuberous roots are best dug up after the foliage has completely died back, however division is best done just before replanting in the spring.. When dividing, remove the entire tuberous root structure. Usually the structure will be attached at the top, where the stems had grown and have now died back. Divide the roots into sections with at least one eye per section.
Examples of Plants with Tubers
- Potato
- Caladium
- Elephant Ears
- Cyclamen
- Anemone (these look like corms, but are actually tubers)
- Begonia
- Dahlia (tuberous roots)
- Ranunculus (tuberous roots)
- Peony (tuberous roots)
- Sweet potato (tuberous roots)
- Daylilly (tuberous roots)
Rhizomes
Rhizomes are also fattened portions of the stem, growing underground (or, as in the case of iris, right at the ground level). They also have eyes like tubers and tuberous roots do. However unlike tubers and tuberous roots, these large fleshy underground stems grow horizontally in the ground.
Rhizomes are a bit variable; they can be quite fat like the iris pictured above, or like a Calla Lily, or they can be thinner like bamboo or Bermuda grass. Also, like tubers, they do not have to be whole when divided. As long as you have at least one eye per plant, you can successfully regenerate a new plant.
Dividing Rhizomes
Usually you will want to divide ornamental grasses and bamboos in the spring but fat rhizomes like peonies, canna and calla lillies in the fall. However, in my experience, Bearded iris will transplant just fine at any time of year.
Fat rhizomes can be stored for several weeks or months in a cool dark place after removal from the ground, just like bulbs. They can be planted right away or planted in the winter or in spring, depending upon your growing zone.
Examples of Plants with Rhizomes
- Bearded Iris, German Iris
- Calla Lily
- Ginger
- Bamboo
- Bermuda Grass
- Canna
- Lilly of the Valley
Corms
The corm is probably the least known underground stem type plant, and is most often confused for a small bulb. This is because they look the most like bulbs, but they do not have the fleshy scales a bulb has. Rather, they are drier and composed of solid tissues. But they can be the same shape as a bulb, or they might be a bit flattened on the top.
Some corms multiply just like bulbs, with the parent corm in the center and baby cormlets generating from the root area. Some other types of corm will have the parent die back after the cormlet generates, so you only have the new plants but not the parent.
Dividing corms
Treat corms the same as you would when dividing bulbs. The best time to divide most corms is when they are dormant; usually the fall or winter, before they start growing for the next season. The foliage should have died back before you dig.
To divide corms, gently dig them out from the soil. If relocating the plant, remove all of the corms from the area. If dividing to make more plants, remove all but a few of them. If the soil is hard or compacted, dig around the area to loosen the soil a bit before removing the corms.
Depending upon when you dig them up, like bulbs, corms can last for several weeks to several months in storage before planting again. You’ll want to get them back in the ground before they start growing in early spring. If you start to see a shoot out the top, or even roots growing, it isn’t too late! You should still be able to plant it to have flowers this season.
If you must divide when the plant is blooming or the foliage is still green, move the plant along with the soil surrounding it. And, try to put the plant back into the ground along with its original soil immediately – the same day.
Examples of Plants with Corms
- Gladiolas
- Acidanthera
- Crocus
- Crocosmia
- Liatris
- Sparaxis
- Taro
- Celeriac
Be it bulbs, tubers, rhizomes or corms, with a little practice you will be an expert at identifying underground stems.
Today I harvested the rhizomes of my two ginger plants. Both were started at same time, and from the same parent rhizome.
On one plant, in each of seven of the many long thick roots, far from the rhizomes, there is a small in-line “radish” / “rutabaga”. Smooth, uniformly shaped, of same color as root. I took a photo.