Bulbs
Bulbs
Learn how to plant bulbs, and about the structure of a true bulb.
Learn how to plant bulbs, and about the structure of a true bulb.
Like, Totally Tuberous, Dude!
True Bulbs
Activity: Each child learns about the structure of a true bulb and plants a bulb in the garden.
Goal: To learn about plant parts and bulb structure, to learn how to plant a bulb.
Supplies: Bulbs, tools for planting bulbs.
How to proceed
Read over the bulb facts below prior to your lesson and use the information as a basis for discussion with your class.
With the class:
Make sure the pointed end of the bulb points up and the dangling roots point down. Look over Bulb Facts before planting so you can look at their bulb before planting them.
It has been very dry, so you made need to dampen the soil a few days before planting your bulbs.
Depending on the soil consistency at the site, an adult may need to help the children plant by loosening the soil in the area. This will make it easier for them to dig the bulb holes with their forks or trowels.
You will need to dig each hole three times as deep as the tulip bulb is tall. There should be twice as much soil over the tip of the bulb as height of the bulb, so if your tulip bulb measures 2 ½ inches tall, dig your hole 8 inches deep, so you’ll have 5 inches of soil above the bulb. Set the bulb so the pointy end is facing up. Don’t worry if you get some upside down. They should flower anyhow, but it will take them longer to come through the ground in spring and they may not be as tall as they should. Exactness isn’t crucial; they’ll adjust.
If it is raining or the ground is too wet, you may have to delay your planting for another day.
Bulb Facts
What is a bulb?
The word bulb loosely describes plants that grow from an underground mass of food
storage tissue. There are many bulb or bulblike plants that hold a reserve of nutrients in a
thickened underground storage organ.
A true bulb, like a daffodil or tulip, contains a complete plant inside. Bulbs are plant
packages that contain a flower bud or buds, leaves, stem, sleeping roots, and food storage
structures. Other types of underground plants that many people believe are bulbs but are
not are corms (e.g. gladiolus), tubers (ranunculus), tuberous roots (dahlia) and rhizomes
(iris and calla lily). Consult the attached bulb types sheet for more information.
Bulb structure
When a bulb starts to sprout, a flowering plant grows up and roots grow down.
The main body of a bulb is formed by thickened, modified leaves. The outermost scale
leaves that are thin and brown protect the bulb against invasive soil microorganisms and
insects. The fleshy, interior leaves are food storage for the plant.
The central flower bud is found in the central core of the bulb. The bud contains
immature foliage leaves that will eventually emerge from the bulb along with the flower
bud.
The small, disk-like structure at the base of the bulb is the stem of the plant. Numerous
roots grow from the underside of the stem. The leaves and bud are attached to the upper
side of the stem.
Small axillary buds may grow outward from the stem. These buds enlarge to form new
bulbs and can be split apart (e.g., dividing your bulbs) to form separate plants.
Bulb adaptation and habitat
Bulbs are native to many areas of the world that have no summer rains. Because of their
storage structures, they are able to become dormant for much of the year and thus
withstand periods of drought or cold temperatures. When favorable conditions arise, they
are ready to use their storage reserves and grow rapidly. Most bulbs bloom only once a
year.
Planting and caring for bulbs
Bulbs prefer soils that drain well. They will rot if they are saturated in water over a
period of time, although they do need adequate moisture to maintain healthy roots. In
heavy soils, a good rule of thumb is to plant the bulb as deep as it is tall, or three times as
deep as the bulb’s greatest diameter. In areas subject to hard freezes or with more porous
soils, bulbs may be planted deeper. Some bulbs even have contractile roots that enable
them to pull themselves deeper!
After a bulb has finished flowering, its supply of nutrients is depleted and must be
replenished by photosynthesis in the remaining leaves. Therefore, to help insure repeated
blooming by bulbs, it is important to leave the foliage on the plant until it has died back
on its own. A clever gardener may disguise the unsightly foliage by overplanting a bulb
bed with annual plants. Some bulbs, like tulips, may not reliably repeat bloom in our
area because they have a chilling requirement and need a certain amount of cold weather
True bulbs
True bulbs, such as daffodils, tulips, hyacinths, and snowdrops, often have a papery skin or tunic on the outside, much like an onion. Bulbs with a papery covering are called tunicate bulbs. The tunic helps protect the bulb from drying out when it’s resting or waiting to be planted. However, some true bulbs, such as lilies, don’t have a tunic. These bulbs dry out faster and are more easily bruised.
All true bulbs share the following characteristics:
They’re more or less rounded, sort of ball-like, and narrow to a point on the top. Leaves and flower stems appear from this point.
With or without a tunic, true bulbs have a flat part, called a basal plate, at the bottom. That’s where roots grow and also where shoots and scales are attached.
True bulbs have new bulbs, called offsets, which form from the basal plate. When they get big enough, these offsets, or daughter bulbs, produce flowers on their own.
True bulbs are made up of rings, called scales, which are modified leaves that store food. Cut apart a true bulb, such as a hyacinth, at the right time of year, and you can find a miniature flower inside, just waiting to begin growing. Perennial true bulbs add new rings each year, from the inside. Old rings on the outside are used up, but the true bulb itself persists from year to year.
If any of the characteristics that identify true bulbs are missing, the plant isn’t a true bulb. Instead, it’s a corm, tuber, tuberous root, or rhizome. Popular corms include crocosmia, gladiolus, freesia, and crocus.
Corms have these traits:
Corms have a tunic. The tunic may be fibrous, what botanists call netted or reticulate, or the tunic may be smoother, with distinct rings, what botanists call annulate. Some crocuses have reticulate tunics, and others are annulate, which is one way you can tell crocus species apart.
Corms have a basal plate at the bottom and one or more growing points at the top. Bulbs and corms both have a definite vertical orientation.
Corms are undifferentiated, uniform, and contain no rings when cut apart. Corms are stem tissue, modified and developed to store food.
The corm you plant is used up for growing the flower. Before it withers away at the end of the growing season, however, a brand new corm (sometimes several new corms) forms and replaces the mother corm. The new corm contains the food reserve for the dormant crocus or gladiolus until it’s time to grow again.
You may not know it, but you might already be familiar with a popular tuber: the potato. Other tubers include tuberous begonia and cyclamen.
Tubers have these features:
Tubers have no tunic.
Tubers lack a basal plate. Most tubers root from the bottom.
Tubers have several growing points, called eyes. More organized tubers, such as caladiums or tuberous begonias, have their eyes at the top. Some tubers, such as anemones, aren’t so orderly. Distinguishing the top from the bottom of the tuber may be difficult.
Tubers are made of modified, undifferentiated stem or enlarged hypocotyl tissue. They have no highly specific internal structure.
Tubers don’t make offsets or produce new tubers. Tubers usually just get bigger each year, making more growing points.
Tuberous roots are modified, enlarged, specialized roots that store food, and are used up during the growing season to be replaced by new storage units. The tuberous roots cluster together, joined to the
bottom of a stem. The stem contains the new growing point for the next year — a piece of root alonewon’t grow.
Examples of tuberous roots are dahlias, daylilies, and sweet potatoes.
Rhizomes are stems that grow sideways rather than up, running along the surface of the soil or just below it. Plants that use rhizomes for food storage have fatter, more bulblike rhizomes, covered with a dry base of leaves. Rhizomes branch out, and each new portion develops roots and a shoot of its own.
Familiar rhizomes include iris, lily-of-the-valley, canna, and ginger (Zingiber officinale).