Dahlia bulbs are a subterranean root system that stores reservoirs of nutrients, allowing the dahlia plant to mature year after year without benefit of seed or spores. Dahlias do not technically have bulbs, the way that tulips or hyacinth do. They have non-hardy, or tender, tubers; they do not survive winter frosts and must be protected from freezing if they are to regenerate the following spring. However, their underground root system may be commonly referred to as a bulb made up of smaller tubers. After autumn, the aboveground blooms and foliage shrivel and die, leaving the bulb to rest until spring, when it will grow and bloom again.
Dahlias are from the flower family Compositeae. Dahlia bulbs are non-hardy, like those of Gladiolus. True bulbs, like daffodils, have fleshy leaves underground that protect the central embryo. However, a tuber combines the stem and the root, but has swollen to accommodate more moisture and nutrients. The dahlia bulb is composed of many distinct tubers, each a separate lump. To sprout the next season, each tuber must have one eye, the whitish shoot that will grow vertically. The shoot breaks through the soil and turns into stems, leaves, and blossoms. A bulb may have a healthy tuber without any eyes, healthy tubers with some eyes, unhealthy or shriveled tubers, and the "mother" tuber that produced this past year's dahlia plant.
To harvest their own dahlia bulbs, gardeners must dig up tubers during autumn, after the first light frost kills foliage, but before hard-frost freezes the ground and kills the bulb. They must keep part of the dried stem, but dead blooms and smaller roots can be trimmed away. After identifying the different kinds of tubers, the gardener selects firm, large, shiny ones with eyes, and pares them off from the rest of the bulb. Tubers must be properly stored, away from sunlight, with the right amount of moisture to remain viable. A cool temperature of 35-45° F (2-7° C) is recommended. They might be wrapped in sawdust, vermiculite, or shredded paper to absorb extra moisture without letting them dry out completely. For some climates, a garage or basement might be appropriate; for others, the vegetable drawer of a refrigerator will be fine.
The dahlia bulbs can be planted in the ground, with the eye facing up, during May or June. Some varieties of dahlias are hybridized to be "forced" indoors during winter. This means they can be planted in containers inside in the dead of winter and the ambient room temperature will fool them into growing and flowering for Christmas or Valentine's Day.