Planting Bulbs Outdoors

Flowering bulbs are among the most beloved plants of all time. This group includes spring favorites such as tulips, daffodils, crocus, and hyacinths along with summer stand-bys such as gladioli, lilies, and dahlias.

What is a Bulb?
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 the 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 storage 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, such as iris, calla lilies, and cannas are horizontal stems capable of producing shoots and roots. Click here to view line drawings illustrating these differences.

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, are planted in the fall. 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, are planted in the spring. They are tender and do not survive cold winter conditions.

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 leaves and stems, and then flowers. 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. The differences among bloom times allow you to plan a bulb garden that will bloom for weeks and weeks!

If hardy bulbs don't get enough chilling, they may not bloom at all or if they do, their flower stalks may be stunted. Some regions in the United States are too warm to successfully grow hardy bulbs. If you want to try, you'll need to provide the chilling period by placing the bulbs in a refrigerator for a while before planting (see the Family Room feature on forcing bulbs). Paperwhite narcissus, a close relative of daffodils, is a good choice 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. They stow any food not used for daily living in their storage organs for next year’s growth, so it’s important not to cut back the leaves as soon as the bloom is spent. Once there's enough energy in the "lunch box," bulb leaves turn brown and die, and the bulb enters a dormant state through the summer, fall, and winter months until it is time to leaf out again the following spring.

Summer-Flowering or Tender Bulbs. These plants do not survive extremely cold winters, so we plant them in the spring after the last 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.

Planting Tips from the National Gardening Association Editors

• Unless you have the time and resources to provide extra care, choose bulb varieties with temperature, soil, and water requirements matching your conditions.
• Avoid buying bulbs that are rotting or dried out as they may not grow or bloom.
• Plant bulbs as soon as possible. If you have to delay planting, store the bulbs in paper bags (not plastic) in a well-ventilated, cool spot.
• In general, bulbs prefer a sunny location with well-drained soil. If your soil is heavy (or exceptionally sandy), add two or three inches of organic soil amendment (such as compost, ground bark, or peat moss) and cultivate the soil to a depth of six inches or more, incorporating the organic amendment as you turn the soil.
• Plant large bulbs at a depth of about twice the height of the bulb; plant small bulbs slightly deeper than twice their height. In regions with extremely cold winters, err on the side of planting deeper than normal.
• After planting, water the bed thoroughly to settle the soil. If desired, cover the bed with a 1-inch layer of organic mulch to deter future weeds and improve the soil.
• If gophers, mice, squirrels, or chipmunks are prevalent in your area, you might want to protect your bulb investment by planting them in a "cage" fashioned with chicken wire.
• After the bulbs have finished blooming, remove only the spent flowers. Allow the foliage to continue growing. Once it has turned completely yellow or brown, you can usually remove it by hand by giving it a gently tug. Cutting back the foliage while it is still green will seriously diminish the bulb's vigor the following year. Some gardeners disguise the withering bulb foliage with an overplanting of annuals.