The temperatures cool and winds become crisp. Fall has overtaken the woodland garden and a few seasonal tasks regarding rhododendrons begin to fill the weekend "to-do" list. The pointed flower buds on rhododendron branch tips are mature now, and the concern focuses on getting those shrubs through the upcoming winter in good health. A shrub that is not stressed entering winter will produce a nicer, more vibrant flowering display next spring.
The warm soil but cooler temperatures, and often more abundant rainfall, that follows in winter and early spring makes fall planting of rhododendrons an opportune time. According to the American Rhododendron Society, fall is the best time to plant these shrubs unless you garden in a very cold winter region, such as U.S. Department of Agriculture hardiness zones 4 or 5. In these areas, spring planting is best.
Pay particular attention to the soil moisture in the root zones of rhododendron plants if the growing season was especially hot and dry. Dry soil in fall leads to plants that are weakened and perhaps not as equipped to remain hydrated across the windy, cold winter months. Therefore, irrigation around rhododendron shrubs from fall's start until subfreezing temperatures will ensure that the soil is moist. Regulate your irrigation to supplement any natural rainfall so that soils never become soggy.
Promptly end any fertilization regime at the end of summer. This will give the shrub ample time to absorb any potassium to help mature and "harden off" its young growth before the cold weather invades. While any liquid feeding of rhododendrons must halt in fall, any leftover granular fertilizers on the soil can remain across the fall and winter months. These slow-release pellets will still emit trace minerals into the soil over winter and early spring with rain or snow melts, but will not instigate untimely new growth from branch tips on plants.
Benefits of Mulch
Once you are satisfied with the soil moisture around the rhododendrons, late fall is the time to tackle replenishing mulch layers in the beds, according to the University of Virginia Extension service. Utilize fallen tree leaves as a component of the ground mulch, and add any organic matter to a depth of 2 to 3 inches over the root zone. Keep mulch at least 2 inches away from the rhododendron trunks, or any branches that lay on the ground, to discourage fungal growth. Scattering the mulch in the fall is much easier and timely than delaying it until spring, when soils may be wetter and other pressing garden tasks occur.
Site-Specific Winterizing Tasks
Fall is also the time to install any features that act to improve the overwintering capabilities of your rhododendrons. Depending on your climate or gardening situation, certain rhododendrons may benefit from a windbreak fence over winter, or you may wrap trunks to prevent rodents from eating bark. Fences may also limit the extent to which browsing deer may eat flower buds across the winter and early spring. Place burlap guards and wraps around young or vulnerable plants to limit the amount of cold, drying winds that will hit plants and cause leaf dessication in exposed spots in the landscape. Hold off on any pruning–even removal of dead branches–until the spring.