JARS v64n3 - New Journal Column: 'Ask the Experts'

New Journal Column: "Ask the Experts"
Glen Jamieson
JARS Editor

If there is interest, I propose to dedicate space in JARS to give answers to questions about any aspect of rhododendron biology, culture, taxonomy, hybridization, etc., that readers have, and particularly those questions that might not normally be answered in published articles or notes. In particular, I welcome enquiries about everyday phenomena that readers are curious about. Answers to selected published questions are solicited from experts. Questions and answers should be concise. Please submit questions and answers to ars.editor@gmail.com.
To get the ball rolling, I have shame-facedly borrowed a Question and Answer from the December 2006 Cowichan Valley Chapter newsletter. For a period of time, that newsletter had a column called "The Question Box," in which Norm Todd answered questions posed by members.

Question: "What causes the leaves on my rhododendron to have brown tips?"

The questioner should not feel especially concerned, as this is a common complaint; it is not often a life-threatening condition.

There are four main causes for plants having brown tips to their leaves: (1) lack of water at the roots, (2) sun burn, (3) fertilizer burn, and (4) frozen tissue. The first is the most likely. Rhododendrons need 25 mm [1"] of water per week in spring, summer, and fall. To check whether a plant has adequate root moisture, dig down just outside the drip line to a depth of 25 cm [10"]; the soil should be moist for the complete depth of the hole. When using automated irrigation, it is a good idea to place tin cans in the irrigated area and measure the amount of water being dispersed. It should accumulate to about 25 mm [1"] over a week. Some allowance should be made for evaporation from the container.

Other rhododendrons show their thirst by curling their leaves. They do this to reduce the area of the exposed underside of the leaf where the transpiration of water takes place. Others do not. I think it is fair to say that the ones that do not curl their leaves come from very moist summer climates where they have no need for this defensive strategy.

Some rhododendrons are just unable to pump enough water to keep their leaves turgid. The old saw that says "the bigger the leaf, the more the shade" is something that every grower should keep in mind. After all, the reason a plant has big leaves is so that it can catch every photon of light and keep up its productivity.

My experience is that many yellow-flowered rhododendrons do not like the hot afternoon summer sun. Paul Wurz [from Campell River, BC, Canada] mentioned that he found several of the yellow-flowered plants did well when given a lot of light. Further, if that situation had really good air movement, the plant was not as liable to become infected with powdery mildew. The lesson we can take from this is that rhododendrons should be sited to receive as much light as possible and we should not be slow about moving them around - they are very portable. More light also gives a heavier bud set.

Rhododendrons are not begonias; they are not gross feeders; however, they do like a constant supply of nourishment. Slow release fertilizers are safest. The general fertilizer that I use most frequently is 10–8–6. This is coated with a resin so it releases its nutrients slowly. The resin dissolves more rapidly with warmer soil temperatures, but this is a good arrangement as the plants' need for food doubles with every 10°C (18° F) rise in temperature. I did hear of a very unhappy outcome when using a 10–8–6 fertilizer that was coated with an inferior resin that had no lasting effect at all. The plants OD'd and their leaves went brown at the edges.

The amount of antifreeze that a rhododendron has in its system varies with the variety. Leaf tissue can suffer cell damage from becoming frozen. Many of us [in BC, Canada] remember February 1989. January had been fairly mild and the sap was running in many plants. At our place (near Elk Lake outside Victoria, BC) the temperature went down to about –9° C [16 °F] with a strong northeast wind. Some foliage was desiccated and froze. The big leaf species such as Rhododendron macabeanum had lots of brown edges. Some of the even more tender ones just gave up the ghost. The moral in all of this is just don't be too blue about the odd brown leaf. With a little thought and effort, most browning can be prevented.

Norm Todd, Victoria, BC, Canada