Digging In - Advice on Small Rhododendrons and a Cherry Tree Fungus

By Scott Aker
Special to The Washington Post
Thursday, November 27, 2008

Q I noticed rhododendrons blooming in my neighborhood this year, about eight feet tall and with trusses of pink and white flowers. I've been looking for a late-season rhododendron that would do well in Washington in white or pale pink. A slightly smaller variety would be perfect. Any suggestions?

A A variety named Nova Zembla, which is pink, and another called Roseum Elegans, which is purple, are the most commonly encountered rhododendrons in our area, where the plant blooms in the spring. Both were derived from Rhododendron catawbiense, a shrub native to the southern Appalachian Mountains. The blooms you saw are much lighter, however. You might have seen a variety named Album or another common native rhododendron, R. maximum. Maximum here refers to the shrub size, up to 30 feet, not the flowers, which are relatively small. This is not the shrub for you.

You may have spotted a native Japanese rhododendron called R. yakushimanum. It grows well in our climate, given the proper soil conditions. Mardi Gras is a blush pink hybrid and is said to be heat-tolerant. Since you would like a more compact plant, I suggest Yaku Princess, which is a compact form of this species with shell-pink flowers.

Even with heat-tolerant varieties, you must take steps to ensure they thrive. They do best where they get sunlight in the morning and shade in the afternoon. They require a soil that is acidic, high in organic matter and perfectly drained but moist most of the time. They will not do well in heavy clay soil, nor do they compete well with trees with aggressive root systems such as maples.

I have seen them growing well in soil that is mostly small alluvial stones and gravel, along with some sand, silt and organic matter. The organic matter tends to hold on to soil moisture, while the gravel and sand ensures that excess moisture drains rapidly.

I have a Japanese cherry tree that began to lose its leaves early in the season. The leaves turned yellow and then developed holes ringed by green spots before falling off. What's the remedy?

The problem is caused by a fungus, Blumeriella jaapii. It is not a serious problem every year, and its appearance this year is related to the unusually wet spring. The holes in the leaves are formed when the rapidly growing leaf is infected by the fungus. The tissue around the infection is killed, and as the leaf continues to grow, the dead spot of tissue does not expand, so a hole develops.

The spores for this disease overwinter on dead leaves near the tree. Get your rake or your blower and collect them this fall to prevent a repeat problem next year. If you can remove every leaf from the site, you will have eliminated much of the spores available to cause infections next year.

If you want to be sure of no further problems, you can use a fungicide. Treat the foliage when it first appears, continuing at seven-day intervals for best results.

Scott Aker is a horticulturist at the U.S. National Arboretum.

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