Hydrangeas are popular garden plants often mistaken for flowering bulbs because they reliably return each spring. However, hydrangeas are not bulbs. They are classified botanically as the genus Hydrangea and grow as deciduous, woody shrubs. Like true bulbs, hydrangeas are perennials that survive the winter underground and reappear year after year.
Defining the Difference: Woody Shrub Versus True Bulb
The distinction between a woody shrub and a true bulb lies in the plant’s core survival structure. A true bulb, such as a tulip or an onion, is a modified underground stem that acts as a storage organ, holding all the plant’s food and embryonic flower parts. This fleshy, subterranean structure is the plant’s complete perennial unit, allowing it to survive periods of dormancy below the soil line.
In contrast, a hydrangea is a woody plant that develops a persistent, above-ground framework of stems. While both are perennial, the hydrangea’s structure relies on these durable stems and an established root crown, not a single, self-contained storage organ. This growth habit categorizes it as a shrub, which loses its leaves but retains its woody stems during cold months.
How Hydrangeas Start and Grow
A hydrangea begins its life as a seedling or a rooted cutting. The young shrub develops a fibrous root system that is shallow and spreads widely across the soil. This structure anchors the plant and efficiently draws in moisture and nutrients from the upper soil layer.
Growth occurs from the established crown, where the stems meet the roots, and from buds along the existing woody stems. This mechanism is characteristic of shrubs, unlike bulbs, which sprout from a singular, fleshy base. Nursery-grown hydrangeas are usually propagated by rooting stem cuttings in soil.
Winter Survival Strategies for Hydrangeas
Hydrangeas survive the cold through dormancy, relying on the resilience of their root crown and existing stems. The root crown, located at or just beneath the soil surface, is hardy and can tolerate low temperatures. However, the woody stems above ground can be susceptible to damage from extreme cold or drying winter winds.
Bloom survival depends on whether the species flowers on “old wood” or “new wood.” Varieties blooming on old wood, such as Hydrangea macrophylla (Bigleaf), form flower buds the previous summer, requiring them to survive winter on exposed stems. Protecting these stems with a thick layer of mulch or wrapping the plant in burlap can insulate the delicate flower buds from severe temperature drops. Smooth hydrangeas (Hydrangea arborescens) bloom on new wood formed in spring and are more resilient. They can lose their entire top growth and still produce flowers from the hardy root crown.