Bulbs And Other Rooting Structures
Ron Cornwell and Floyd Giles
The term "bulb" is used by most people to refer to plants
that have underground, fleshy storage structures. Only some
of the plants commonly called bulbs actually are bulbs.
The definition of a bulb is any plant that stores its complete
life cycle in an underground storage structure.
The primary function of these underground storage structures
is to store nutrient reserves to ensure the plants survival.
Bulbs or bulb like plants are usually perennials. They
have a period of growth and flowering. This is followed
by a period of dormancy where they die back to ground level
at the end of each growing season. For spring bulbs, the
end of the growing season is in late spring or early summer.
Spring bulbs start to grow again in the fall and flower
the following growing season.
Bulbs can be broken down into five types of storage structures.
These include: true bulbs, corms, tubers, tuberous roots
and rhizomes. A sixth category of fleshy roots has been
added here for the purpose of showing the structure. Daylilies
and peonies, which are popular plants with gardeners, are
examples of this type.
TRUE BULBS
The true bulb has five major parts. It contains the basal
plate (bottom of bulb from which roots grow), fleshy scales
(primary storage tissue), tunic (skin-like covering that
protects the fleshy scales), the shoot (consisting of developing
flower and leaf buds), and lateral buds (develop into bulblets
or offsets).
True bulbs are divided into tunicate bulbs and imbricate
bulbs. A tunicate bulb has a paper-like covering or tunic
that protects the scales from drying and from mechanical
injury.
Good examples of tunicate bulbs include: tulips,
daffodils, hyacinths, grape hyacinths (muscari), and alliums.
Many plants such as daffodils form new bulbs around the
original bulb. These bulbs, called offsets, develop from
buds within the base of the mother bulb and produce new
plants. When these bulbs become overcrowded, the flowers
start to diminish in size. This is an indication that it
is time to dig up and divide the bulbs.
An example of the imbricate bulb is the lily.
The imbricate bulb does not have the tunic (papery covering)
to protect the fleshy scales. Imbricate bulbs must be kept
constantly moist before planting so they are not injured
by the scales drying out.
Lilies can be propagated from bulbils that
develop in the leaf axils of the plant. They can also be
propagated from bulblets that develop at the base of fleshy
lily scales if maintained in a moist sand medium. It will
take more than one year for the bulbils or bulblets to become
flower size.
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CORMS
A corm is a swollen stem base that is modified
into a mass of storage tissue. A corm does not have visible
storage rings when cut in half. This distinguishes it from
a true bulb.
The corm contains a basal plate (bottom of
bulb from which roots develop), thin tunic and a growing
point. Examples of plants that develop from corms include
gladiolus, crocus, and autumn crocus.
When gladiolus corms are dug in the fall,
they should be separated into well developed corms, to be
stored for replanting, and poorly developed corms which
the gardener may want to discard. The newly dug bulbs will
have cormels that are pea size formed around the top of
the old corm. The remains of the old corm will be directly
beneath the newly formed bulb. When the bulb is cleaned
up and the old stem removed, the growing point of the bulb
will be evident. The cormels can be saved and replanted
in the back of the garden until they reach flowering size.
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TUBERS
A tuber differs from the true bulb and the corm by not
having a basal plate from which roots develop and not having
a protective tunic covering.
The caladium tuber has buds scattered over the tuber surface
from which shoots and roots develop.
Examples of plants that develop from tubers include caladiums,
oxalis and anemones, and the common vegetable, the potato.
(The potato does flower.) Back to top.
TUBEROUS ROOTS
The tuberous root differs from other root structures by
the nutrient reserves being stored in an actual root instead
of an enlarged stem.
The dahlia reproduces from buds at the top end of the
root or base of the stem.
The tuberous root of a dahlia should not be divided before
placing in storage in the fall but should be divided at
planting time. The root should be divided into sections
with an eye bearing portion of the stem left with each section
of the root.
The tuberous-rooted begonia reproduces from
buds on top of the round, flat tuber. Back
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RHIZOMES
Rhizomes differ from other storage structures
by growing horizontally under the surface of the soil. On
some plants, this type of rooting structure can be very
invasive.
The iris is propagated by cutting the rhizomes
into sections leaving a fan of leaves with each section
of root.
The lily-of-the-valley rhizome spreads horizontally
underground and produces pips which develop into new plants.
This plant is increased in the landscape by digging in
the fall and removing pips with developed roots for replanting.
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FLESHY ROOTS
Plants that have fleshy roots store nutrient
reserves in the fleshy roots.
Both peonies and daylilies can be propagated
by dividing. The root clumps of peonies should be divided
in the fall leaving at least three crown buds with each
clump.
The daylily can be divided in the fall or
spring into plantlets with a single fan of leaves.
The daylily has a fleshy root system with
some varieties having what might be considered a rhizome
type root system. Daylilies are hardy herbaceous plants
with a perennial growth habit. They have clumps of rich
green, smooth foliage that dies back during the winter.
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