Rhododendrons, a large genus of popular ornamental shrubs, typically display their spectacular flowers once per year. The answer to whether they bloom twice depends on the specific plant. Some hybridized cultivars are genetically programmed to rebloom, while standard varieties may exhibit an unexpected second bloom due to environmental factors.
The Standard Bloom Cycle
The vast majority of the Rhododendron genus adheres to a specific annual cycle. The main display of flowers occurs in spring, typically spanning from late winter through early summer depending on the cultivar and regional climate. This spring flowering period usually lasts between two and four weeks before the blossoms fade.
Once the current year’s bloom is complete, the plant immediately shifts its energy to setting flower buds for the following year. This crucial bud-setting period generally takes place during the summer months, specifically between June and August. These newly formed buds then enter a dormancy period, requiring specific chilling hours during the winter to ensure they open the next spring.
Cultivars Designed to Rebloom
Horticultural breeding has resulted in specific groups of rhododendrons and azaleas that reliably bloom more than once in a single growing season. The most widely known example of these are the ‘Encore’ Azaleas. These specialized cultivars possess a genetic trait that allows them to bypass the typical single-cycle dormancy requirement.
Instead of setting all next year’s buds at once in mid-summer, these reblooming varieties set new flower buds continuously throughout the spring and summer. This mechanism allows for the initial heavy spring bloom, followed by a second, often less dense, flush of flowers in the late summer or fall. The fall display can be scattered, lasting for three or more weeks. However, the flowers produced in the autumn utilize the energy and buds that would have otherwise overwintered, meaning they will not be available for the subsequent spring show.
Environmental Causes of Unexpected Secondary Bloom
Standard rhododendrons, which are not bred to rebloom, can sometimes flower a second time in the autumn. This unintended secondary bloom is usually triggered by environmental stress that disrupts the plant’s internal dormancy mechanism.
A common trigger is a prolonged period of drought and heat stress in mid-to-late summer, followed by heavy rainfall or consistent watering in the early fall. The sudden availability of moisture can trick the plant into prematurely breaking the dormancy of newly set flower buds, causing them to open as if a new season has begun. Sudden, unseasonal temperature fluctuations can also contribute to this response.
The unexpected fall bloom can also occur if a plant is pruned improperly late in the season. This action removes terminal growth buds and forces dormant lateral flower buds to develop. While the display can be a pleasant surprise, it expends the plant’s resources and can reduce the quantity or vigor of the following spring’s bloom.
Post-Bloom Care for Reblooming Plants
Caring for rhododendrons that bloom late in the season requires focused attention on winter preparation. The primary goal is to conserve the plant’s energy and ensure it hardens off properly before the first freeze.
It is important to avoid fertilizing after mid-July, as this encourages soft new growth highly susceptible to winter damage. Ensure the plant receives adequate moisture through the fall, especially after a dry period. This helps prevent winter desiccation when the ground freezes.
Removing spent flower heads, known as deadheading, is beneficial for reblooming varieties even in the fall. Deadheading prevents the plant from diverting energy into producing seeds, allowing it to focus resources on root establishment and winter survival. Applying a 2- to 4-inch layer of organic mulch, such as pine bark or pine needles, helps to insulate the shallow root system and maintain consistent soil moisture and temperature.