Identifying a dormant flower bulb, or geophyte, requires a close look at its physical structure. The term “bulb” is often used loosely in gardening to describe any underground storage organ that helps a plant survive dormancy. Accurately identifying the specific storage organ category is the first step toward successful planting. Knowing the difference between these structures helps determine the correct planting depth, orientation, and timing necessary for the plant to thrive.
Understanding Storage Organ Categories
The plant world utilizes four primary structures for storing energy underground: true bulbs, corms, tubers, and rhizomes. A true bulb, such as a tulip or daffodil, is a highly compressed underground stem consisting of fleshy, layered scales surrounding a central growing point and flower bud. Roots emerge from a flattened, disk-like base called the basal plate, making the structure a miniature, fully formed plant ready to grow.
Corms, exemplified by gladiolus and crocus, are distinct because they are solid masses of stem tissue rather than layered scales. If cut in half, a corm is uniformly dense, unlike the concentric rings of a true bulb. This swollen stem base stores nutrients, and a new corm develops on top of the old one each growing season.
Tubers, including the potato and dahlia, are thickened underground stems or roots that store food. Stem tubers, like the potato, have “eyes,” which are the buds from which new growth emerges. Root tubers, such as dahlia roots, are modified, fleshy roots that lack a basal plate or protective tunic.
Rhizomes are horizontal, subterranean stems that grow along or just beneath the soil surface, like those found on iris or canna. Shoots and roots develop from nodes located along the length of the rhizome, allowing the plant to spread laterally. This horizontal structure sets them apart from the more vertical structure of bulbs and corms.
Key Physical Traits for Dormant Identification
Once the general category is established, closer inspection of physical traits can narrow the identification to a specific flower type. True bulbs are divided into tunicate and non-tunicate types, a significant visual difference. Tunicate bulbs, such as tulips and hyacinths, have a dry, papery outer skin, or tunic, that protects the inner fleshy scales from damage and drying out.
Non-tunicate, or imbricate, bulbs like lilies lack this protective outer layer, leaving their scales exposed and easily bruised. These scales are often loose, making the bulb feel less dense and requiring them to be kept moist even in storage. Size and shape provide further clues: a dormant tulip bulb is typically rounder and more compact than a larger, more teardrop-shaped hyacinth bulb.
Daffodil bulbs are generally larger and more ovoid than tulip bulbs and are frequently found with smaller offset bulbs clustered around the base. Allium bulbs, which are in the onion family, often have a strong, sulfurous odor when handled or cut, immediately distinguishing them from other true bulbs. Corms also show differences in shape and size; a gladiolus corm is generally larger and more flattened than a smaller, rounder crocus corm.
Determining Planting Time Based on Bulb Type
The identity of the underground storage organ directly influences the time of year it must be planted to ensure proper growth. Geophytes are broadly categorized into spring-flowering and summer-flowering types based on their dormancy and growth cycles. Spring-flowering bulbs, including tulips, daffodils, and crocus, are hardy and require a period of cold temperatures, known as vernalization, to stimulate blooming.
These bulbs must be planted in the fall, typically between September and November, allowing them to establish roots before the ground freezes. Summer-flowering bulbs and corms, such as gladiolus, dahlias, and tuberous begonias, are tender and cannot tolerate a hard frost. They are planted in the spring, after the last expected frost date, to ensure they grow and flower during the warmer months.
Assessing the Health of Unidentified Bulbs
Regardless of its species, a dormant bulb must be healthy and viable to produce a flower. The first and most telling check is the “firmness test”: a healthy bulb should feel heavy, solid, and firm when gently squeezed. A soft, spongy, or mushy texture is a sign of internal decay or rot, often caused by excessive moisture during storage.
Visual inspection should also look for signs of mold, which may appear as white, blue, or gray fuzzy patches, especially between the scales of non-tunicate bulbs. Discoloration, such as dark spots or patches, can indicate a fungal or bacterial infection. A bulb that feels extremely light or shriveled is likely desiccated and may not have enough stored energy remaining to sprout successfully. Any bulb exhibiting signs of rot, mold, or an unpleasant sour odor should be discarded to prevent the spread of pathogens to other healthy plants.