JARS v63n1 - Winter Season Care for Rhododendrons

Winter Season Care for Rhododendrons
Reprinted from the North Island Chapter (Vancouver Island, BC) newsletter, November 2008.
The article is based on comments by Peter Cox in Amateur Gardener.

Freezing weather
Hard frost produces rather similar reactions in rhodos as hot sunny weather does. In both instances, the plants have a tendency to curl up their leaves, and in extreme cases they become tightly rolled like little cigars. This is a natural mechanism to save moisture in times of stress.

Sun and hard frost create conditions where a plant is unable to take up enough moisture to keep the leaves fully turgid. A curled leaf has a much smaller surface area. The leaves may remain curled up over a long period, but return to their natural shape as soon as conditions return to normal.

Soil frozen to some depth can mean moisture becomes completely unavailable and as frost deep into the ground takes some time to thaw out, so the plants may look distressed for days after the weather warms up. Many of the hardiest rhodos curl their leaves the tightest.

Winter sun combined with frozen roots can do severe damage to some evergreen plants. In continental climates, sun hitting the frozen sap in leaf cells can cause the cells to split, resulting in dead tissue and brown leaves. Luckily this happens rarely in our area. You can reduce frost penetration with a 4-inch mulch of bark or wood chips to insulate the plants.

Fertilizing
The literature of three experts, Peter Cox, Ken Cox, and the late Ed Reiley was consulted for advice on fertilizing. All three agree that rhododendrons are not greedy feeders, and many varieties in naturally fertile soil will need no fertilizer at all. Peter Cox in The Cultivation of Rhododendrons says that he believes in applying fertilizer when and where he reckons it will stimulate growth and flower bud set and improve the general appearance of the plants. He thinks the further the growing conditions are from those found in nature, the more artificial help including fertilizer is needed.

Ken Cox in Rhododendrons, A Care Manual, directs gardeners to look at their plants: vigor, good leaf retention and deep green foliage are signs of healthy plants that need no added help, while short growth, sparse appearance, pale or yellowish leaves indicate a plant that may need fertilizer - though drought or poor drainage may cause similar symptoms. He also recommends "little and often" doses of fertilizer rather than one big dose.

Ed Reiley in Success with Rhododendrons and Azaleas wrote that if rhododendrons in the landscape do need fertilizer, application should be made after a killing frost in the fall or in spring after early azaleas flower. To prevent leaching from the soil, nitrogen fertilizer should be applied in organic forms such as soybean meal or synthetic organic urea. If chemical forms are used, ammonium sulfate is the best choice.