Growing Bulbs
Plants that grow from bulbs are technically called geophytes. They store energy in swollen underground organs, which can be divided into four different categories: true bulbs, rhizomes, tubers, and corms. These specialized root structures not only store energy, but also participate in asexual reproduction. Some geophytes are frost tender and typically grown as annuals, while others are well adapted to cold climates and reliably return each year as perennials.
Flowering bulbs include spring-blooming, summer-blooming and indoor-forcing bulbs. Spring-blooming bulbs should be planted in the fall before you want them to bloom. Many will require a period of chilling to ensure a complete bloom cycle, although gardeners in warmer climates can purchase bulbs that have gone through a prechilling before shipping. Summer-blooming bulbs like dahlias, lilies, or cannas are best planted in the spring when the soil has warmed and daytime temperatures are reliably above 50 degrees Fahrenheit.
When buying bulbs, look for firm, unblemished specimens. Any bulbs that have sprouted should be planted as quickly as possible. Avoid tubers that appear shriveled and dried out or feel squishy like an overripe avocado; they will never recover no matter how carefully they are planted.
Types of Bulbs
True bulbs are made of layers of modified leaves surrounding a shoot. Examples of true bulbs include tulips, daffodils, alliums, and hyacinths. Lilies are also true bulbs, although they lack a papery covering and have a scaled look. The foliage continues to grow and feed the bulb after blooming as the plant prepares for winter dormancy.
Rhizomes are enlarged lateral roots that tend to run just below the soil surface. Two of the best known rhizomatous plants are Lily of the Valley and many irises. These plants multiply quickly and are winter hardy. Cannas and tropical ginger also grow from rhizomes, but are cold tender and may need winter protection in USDA growing zones 7 and lower.
Tubers have large, often egg or pear-shaped, underground roots, and are the most susceptible bulb to rot. Some of the most popular tuberous plants include dahlias, caladiums, certain begonias, and peonies. The more tender and tropical plants like caladium and dahlia may need to be extracted from their garden beds before winter and stored in a dark, dry, frost-free location until the average temperature in the spring rises above 50 degrees Fahrenheit.
Corms are modified stems that form hard and typically flattened bulbs with roots growing from one side. They multiply as cormels under the soil and can take years to grow large enough to flower. Popular examples of these plants include gladiolus, crocus, and ranunculus.
Planting Bulbs
Always plant bulbs in well-draining soil, whether it be in a garden bed, raised bed, or a container. Amend heavy soils with organic material such as finely shredded arborist chips or compost. This will improve the drainage and texture of the soil. Bulbs also prefer deep soil with no tree or shrub roots to compete with for nutrients and water.
Most bulb species grow best in full sun (6 hours or more of direct sun exposure), although many appreciate some shade from mid-day on in hotter climates. The general rule of thumb for planting true bulbs and corms is to bury the bulb at least 2-3 times deeper than its size, with the pointy side up. Rhizomes and tubers should be planted more shallowly, and some plants like bearded iris need to have their rhizome partially exposed to flower properly.
Watering Bulbs
Early spring-blooming bulbs rarely require supplemental watering. If you live in a climate that has a snowy or rainy winter, the bulbs will put down roots well before growth is seen in the spring. Once flowering has finished, the plant will begin to go dormant for the summer and will die back fairly quickly.
Summer-flowering bulbs need watering during dry periods once the shoots emerge. Some species require consistently moist soil during their growing period. In the winter, no supplemental watering is required, and too much dampness can cause rot while the bulb is dormant.
Bulbs growing in pots will need regular watering when the top 1-2 inches of soil have dried. Start watering when shoots appear above the soil line. While the bulbs are dormant, the containers should be placed in a sheltered location where they can be allowed to dry.
Fertilizing Bulbs
Bulbs typically appreciate a light application of fertilizer at the time of planting. Bone meal is often sprinkled in the planting hole to strengthen the bulbs and encourage good flowering. Alternative fertilizers without animal by-products include alfalfa meal, cottonseed meal, and fine kelp meal. To continue supporting healthy growth, a layer of organic compost can be applied each spring. Apply 1-2 inches of compost around the bulb as growth begins.
Pruning Bulbs
Deadheading typically does not encourage bulbs to bloom more, but it does help the plants store more energy instead of producing seed. Dahlias and some daylilies are the only exceptions and will continue flowering as long as they are deadheaded. After flowering stops, allow the plant’s foliage to turn yellow or brown and die back naturally. Removing the foliage too early can reduce the vigor of the bulb during the next growing season.
Caring For Plant Bulbs in Pots
Bulbs are ideal for growing in containers, especially if the garden soil is not suitable. True bulbs like tulips, daffodils, or crocus can be planted quite close together to create a full container display. Dahlias and other larger rhizomes or tubers will grow best on their own in a container. Select a pot that gives the roots at least 1-2 inches of space all the way around.
To care for potted bulbs, water the soil regularly while the bulb is actively growing. When the weather turns chilly, store potted summer bulbs in a frost-free, dark spot to protect them from wet, freezing weather. Potted spring bulbs can remain outdoors, but make sure they are not sitting in water or directly exposed to heavy winter winds and storms. Plants in pots are more vulnerable to cold temperatures and wet conditions.
Winter Care For Bulbs
Spring bulbs are typically very cold hardy and require winter chilling for proper blooming. In the fall, mulch the planting area with 2-3 inches of shredded leaves or arborist chips to provide insulation and mark the planting site. Pull back the mulch in the spring to give the shoots room to grow.
In cold climates, tender summer bulbs and rhizomes should be removed and stored in a frost-free spot for the winter. Allow the first frost in fall to kill off the top growth of dahlias and gladiolus before digging them up. Clean the soil off the tuber or rhizome and pack it in sawdust, newspaper, or shredded paper to keep it dry over the winter.
Sources:
“Bulbs.” Illinois Extension, University of Illinois. extension.illinois.edu
“Plant Bulbs Now for Spring Color.” Wisconsin Horticulture, Division of Extension. hort.extension.wisc.edu
“Storing Tender “Bulbs” for Winter.” Wisconsin Horticulture, Division of Extension. hort.extension.wisc.edu