Background

The word "bulb" is loosely used by gardeners to describe plants growing from an underground mass of food storage tissues. Bulbs store enough food to enable plants to grow and flower without added nutrients during the first year. To help students to relate to this concept, tell them the storage tissue is like the bulb’s lunch box, packed with enough food for the whole growing season!

Botanically speaking, a bulb is a modified stem containing a complete miniature plant, including embryonic leaf, stem, and flower parts, and surrounded by fleshy scales (which provide food for the young plant) and a basal plate (which produces roots). Bulbs are also surrounded by a thin protective layer called a tunic. Tulips, daffodils, and onions are true bulbs. If you slice a bulb in half horizontally, you'll see rings formed by the scales, and if you are looking at one close to planting time, you'll see a small plant in the center (an onion left in the refrigerator for too long makes a great example).

Some other plants we call bulbs, such as crocus, iris, and dahlias, aren't true bulbs because they have different botanical structures than those listed above. Technically, crocus and gladiolus are corms. What's the difference? A corm stores most of its food in an enlarged basal plate rather than in its scales. Tubers, such as dahlias and tuberous begonias, sprout new stems and roots from “eyes” on their surface. And rhizomes, which are horizontal stems, include iris, calla lilies, and cannas. Click here to view line drawings illustrating these differences.

Dahlia tubers

To a botanist, the distinction between these storage organs is very important and can provide information for scientific classification and offer clues about the origin and evolution of plants. But gardeners approach plants in a more practical sense. Because most of the plants with underground storage capacity rely on similar planting instructions, gardeners lump them all together as bulbs.

Bulb Life Cycles
Bulbs are divided into two categories based on when they bloom. Spring-flowering bulbs, such as daffodils and tulips, we plant in the fall for spring bloom. They are also called hardy bulbs because they survive cold winter conditions. In fact, they need exposure to cold temperatures in order to flower properly. Summer-flowering bulbs, including dahlias, begonias and gladiolus, we plant in the spring for summer bloom. They are tender and do not survive cold winter conditions.

Daffodil shoots are among the first green signs of spring.

Spring-Flowering or Hardy Bulbs. The exact timing for planting hardy bulbs varies by region, but they need to be in the ground before it freezes. Bulbs develop their roots soon after planting, and then lie dormant during much of the winter. When the weather begins to warm, they draw on that "lunch box" of stored energy to produce flowers and leaves. Some bulbs, such as crocus, require fewer cold hours and less warming to bloom, so their flowers emerge in early spring. Others, including tulips, need a longer cooling period and warmer temperatures to emerge, so they bloom later in the spring. These differences among bloom times allow you to plan a bulb garden that will bloom for weeks and weeks!

If they don't get enough chilling, hardy bulbs may sprout leaves when the weather warms, but if they bloom at all, the flower stalks will likely be stunted. There are regions in the United States that are too warm to grow all bulbs successfully. If you must grow them, you'll need to provide chilling by placing bulbs in a refrigerator for a while before planting (see Forcing Bulbs, below). The Paperwhite narcissus, closely related to daffodils, is a good spring bloomer for warmer climates because most do not require a long cold treatment.

Like all plants, bulbs take in nutrients and produce new food through photosynthesis. The food not used for daily living they stow in their storage organs for next year’s growth. Once there's enough energy in the "lunch box," leaves will turn brown and die. The bulb will enter a dormant state through the summer, fall, and winter months until it is time to leaf out again the following spring.

Those homely dahlia tubers above give rise to these beauties!

Summer-Flowering or Tender Bulbs. These plants do not survive extremely cold winters, so we plant them in the spring after chance of hard frost has passed, and enjoy their blossoms in the summer. (However, there are summer-flowering lilies that are hardy enough to survive winters in some areas.)

After planting, they grow much the same way as spring-flowering bulbs, and produce and store food for next year. Most keep their leaves and continue to photosynthesize until fall temperatures turn cool. In northern climates, gardeners must dig up tender bulbs and store them in a dark place that stays cool enough to keep them from sprouting prematurely.

Where winters are warm enough, it's safe to leave tender bulbs right in the ground. It is important to know the hardiness zone of your bulbs to determine the proper care.

Making New Plants
When pollinated, most bulb flowers will produce seeds, and you can use them to grow new plants. So why don't you see daffodil or tulip seeds at the garden center? Because it takes at least 5 years, and sometimes longer, for seedlings to mature and produce flowers. (Do you and your students want to wait that long?)

Many bulbs also reproduce via bulblets or offsets, tiny bulblets that form on the "mother" bulb. These baby bulbs grow over time, and once they have stored enough energy you can separate them from the mother bulb to grow new plants. These mature more quickly than seedlings.

In the case of rhizomes and tubers, you can propagate new plants via division. This involves cutting them into pieces that include both roots and "eyes" (stem buds).

Forcing Bulbs
Planting bulbs in your outdoor garden is not only a great way to introduce these fascinating plants to kids, it is also a way to develop skills in patience and delayed gratification. However, if you don't want to wait (or can't!) for winter to pass, or if you live in an area that does not receive enough cold weather for spring-blooming bulbs, you can force them.

Forcing is the process of exposing bulbs to necessary cold temperatures (usually accomplished by placing them in a refrigerator) and then planting them in an environment that makes them think it is time to grow. Here are directions on forcing bulbs from GrowLab: A Complete Guide to Gardening in the Classroom:

How to Force Bulbs
1. Purchase hardy bulbs for forcing in the fall. This is when they are commonly available from garden centers and seed companies. If you won’t be planting them right away, store bulbs in a cool (40° to 50° F), dry, dark spot. Because bulbs are living things, don't leave them unplanted for long -- try to plant them by the end of October.

2. Plant bulbs in 6-inch pots filled with moist soilless mix (three bulbs per pot). Bury the bulbs to their tips, with pointed ends facing up. You don't need to add fertilizer because they have their own food stored within.

3. Place bulbs in a cold place (garage, cold frame, or refrigerator -- where temperatures remain between 35° and 45° F) for a minimum of eight weeks. This cold treatment simulates the winter conditions necessary for the formation of roots.

4. Next, move the containers inside and put them under normal classroom light for two weeks. Then put them under your indoor garden lights or onto a windowsill. They should bloom in two to four weeks. Once they have begun blooming, remove them from the bright lights of the indoor garden to encourage a longer bloom period.

5. If you want to try saving your bulbs for another year, you must leave the foliage on after the blossoms have faded and fertilize the plants regularly. It’s during this time that a bulb produces and stores food for next year’s flower. Rather than attempting to force bulbs the following year, you can plant them outside. It will still be two to three years before they’ll produce another good bloom.

Check out Lesson 1: Bulb Botany and Lesson 2: Chill Out! for ideas on ways to incorporate bulb activities into your classroom curriculum. Also check out the Plant of the Month spotlighting the tulip (have you ever heard of “tulipmania?”) and the Resources page for links to additional information.

Copyright© 2005 National Gardening Association