Gardening is a simple and joyous activity, but when you dip into the science of horticulture, things can become a bit murky – like the terminology used for flower bulbs!
Bulbs, corms, rhizomes, and tubers are all thrown into the mix, and they seem to serve the same function of anchoring plants and producing roots and shoots from underground storage organs.
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So is it really important to know the difference?
Well, if you’re partial to spectacular floral displays, it is.
Not to the point of needing a degree in botany, but knowing your fleshy tubers from your corms is helpful for many reasons.
Because while they might share common terminology, they all have different planting requirements, cultivation needs, and differ in cold hardiness.
And when you can give your underground rootstock the best conditions for planting and growth, you’ll enjoy healthy, vigorous plants with an abundance of flowers. And that is important!
So if you’re ready for a little terminology tutorial, let’s dig into flower bulb types and their characteristics.
Here’s what’s ahead:
Types of Underground Storage Organs
“Bulbs” is a blanket term used to describe a variety of different geophytes – plants that have specialized underground organs for nutrient storage, produce growth buds and roots, and allow them to survive in challenging environments.
The term generally covers true bulbs, corms, fleshy roots, rhizomes, tubers, and tuberous roots.
Collectively, they come in a range of shapes and sizes from small teardrops to hefty globes to gnarly stick-like structures, and encompass an array of plant types from crocuses to peonies.
These underground storage organs also contain all the genetic information needed for growth, producing foliage, flower scapes, and roots.
And they help plants to survive in adverse or inclement conditions, providing protection from grazing animals, bush fires, excessive heat or cold, and drought.
Many have contractile roots that expand and contract in response to moisture availability and temperature changes, moving the host organs to more appropriate soil depths to keep them healthy.
Here’s a brief outline of the most common and popular types of underground rootstock – some benefit from lifting and winter storage while others can safely overwinter in ground.
True Bulbs
True bulbs typically have a rounded or teardrop shape and are mostly made up of adapted leaf tissue called scales.
Cutting a true bulb in half reveals that the scales grow in rings radiating from a central growing point – an onion is a good example of a true bulb’s fleshy scales.
Along with the fleshy scales, other notable characteristics include a root-sprouting basal plate and a prominent growing tip from which leaves and flowers emerge.
True bulbs are typically planted at a depth of two to four times their height, and the basal plate is always placed at the bottom of a planting hole, with roots growing downward.
They also propagate by producing lateral or side buds known as bulbils, bulblets, or offshoots.
Many true bulbs have a papery tunic that protects the fleshy scales and helps to retain moisture – these types are referred to as tunicate.
Tunicate species include:
Varieties that lack a tunic are called imbricate and have a scaly appearance.
Imbricate types are more easily damaged and they need to be kept lightly moist while in storage and before planting.
Imbricate varieties include English bluebells (Hyacinthoides non-scripta), fritillaries (Fritillaria spp.) and lilies (Lilium spp.).
Many types of true bulbs are spring-flowering and cold hardy, going dormant in summer’s heat. Others, like lilies, are summer-flowering and have moderate cold hardiness.
Corms
Corms are small, rounded bulbs often with a slightly gnarly or squished appearance that consist of a modified stem base that stores sugars.
Corms lack the rings of fleshy scales that typify true bulbs, but many varieties, like crocus and gladiolus, have a protective papery tunic, a basal plate that the roots grow from, and a pointed growing tip.
These types are planted like true bulbs, with the basal plate downwards and at a similar depth of two to four times the corm’s height.
Other types of corm develop a thin rind or skin instead of a tunic, and lack a basal plate or growing tip, like those of Grecian windflowers (Anemone blanda).
These are planted two to four inches deep and it doesn’t matter which way they go in the ground, the stems and foliage always grow towards the light.
Corms may be cold hardy or cold tender, depending on the variety.
More examples of flowering plants with corms include liatris (Liatris spicata) and autumn crocus (Colchicum spp.).
Fleshy Roots
Fleshy roots are modified tap roots that are used for nutrient storage and typically have a swollen or elongated appearance that bulges in the middle.
They also feature a cap or thick top section where stems are attached to the roots, and fine roots at the bottom or along the main root’s length.
Fleshy roots are planted with the crown fairly close to the surface and the roots spreading down and outwards, and they produce new shoots that form clumps.
They have neither a basal plate nor papery tunic but a thin skin that offers protection.
Plants with fleshy roots can be hardy or tender, depending on the species.
Some popular flowering plants with fleshy roots include Oriental poppy (Papaver orientale), peonies (Paeonia spp.) and ranunculus (Ranunculus spp.).
Rhizomes
Rhizomes are modified, swollen stems that grow horizontally along or just below the soil surface.
Rhizomes don’t have a basal plate or tunic, but they do have a thin skin, and primarily serve as storage structures for vegetative reproduction and spreading.
They’re planted close to or at the soil surface, developing roots on the underside with foliage and buds sprouting from the topside.
Lateral shoots form new rhizomes, enabling them to spread and colonize readily.
Garden favorites with rhizomes include canna (Canna spp.), ginger (Zingiber spp.), hosta (Hosta spp.), iris (Iris spp.), lily of the valley (Convallaria spp.), and rhubarb (Rheum spp.).
Rhizomes can be tender or hardy.
Tubers and Tuberous Roots
Tubers and tuberous roots are both storage organs, plump fleshy structures that hold water and nutrients and neither have a basal plate or papery tunic, but often have a thin skin for protection.
However, tuberous plants are typically short-lived, like potatoes, while plants with tuberous roots are clumping and have a longer life, such as dahlias (Dahlia spp.).
They’re also propagated differently.
Tubers are cut into sections with each section having growth eyes that produce new plants, in the case of potatoes.
Tuberous roots are typically divided with each division having tubers and a section of the crown.
The crown develops buds, or eyes, that contain foliage and flower stems while roots form at the distal or bottom end, like dahlias.
Some garden standards that produce tuberous roots like dahlias include daylilies (Hemerocallis spp.) and sweet potatoes (Ipomoea spp.).
Examples of tubers include potatoes (Solanum tuberosum) and caladiums (Caladium spp.).
Cut tubers are typically planted at a depth of three to five inches, and clumps of tuberous roots are planted with the crown at a similar depth, with variations according to plant types.
And like others in the collective, some tubers and tuberous roots are cold tender while others are hardy.
Gorgeous Geophytes
Gorgeous geophytes like true bulbs, corms, and tubers have another thing in common – they all need a dormant period to rest after flowering.
For some, like daffodils, summer is the dormant season while others, like peonies, rest in winter.
If you garden in cooler climates, you may have to dig and store your bulbs for winter.
Others, like grape hyacinth, need a cold period to set flowers. When planting, make a note in your garden journal for ones that are tender and need to be lifted for winter.
You may be surprised at how many “bulbs” you already have!
And knowing the characteristics of the different types of underground rootstock is the best way to ensure reliable, thriving plants with an abundance of flowers for years to come.
What types of bulbs are growing in your garden? Let us know in the comments section below!
And if you’re looking for more flowering bulb know-how, you’ll want to add these guides to your reading list next.
