Q. My rhododendrons have brown edges on the leaves. Any idea what causes this?
A. Rhododendrons lavish beauty on Pacific Northwest gardens, with lush bloom continuing this year into late May. But they can addle the gardener's mind with several types of leaf diseases.
For about 11 months of the year, rhododendrons show us only their evergreen leaves. So it's natural to want the bushes to look attractive year-round.
Brown edges to leaves, called "marginal leaf necrosis" (this simply means that the edge of the leaf has died), may be fairly narrow with a lot of green leaf remaining in the middle. Or they may penetrate more deeply into the leaf toward the mid-rib. These can result from a number of different causes. Listing them gives us a good list of what rhododendrons need and don't need.
Planting too deep can lead to browned leaf edges and reduction in bloom. Rhododendrons require air at the root level and have fragile surface-feeding roots. When planting, place the plant exactly where it was in the nursery container (or even slightly higher, with about an inch of rootball above ground level. This will allow for settling). Do not bury the root ball under soil or heavy, deep mulches (two inches of a light mulch such as fine bark chips will be fine).
Bad drainage can also cause brown leaf edges and other disease difficulties. Rhododendrons need moist soils but can't stand sitting around in soggy bogs, particularly in winter. Check the drainage before planting, and adjust irrigation systems to provide even watering without persistent puddles. Standing water can also result in root rots that can eventually kill plants.
Healthy leaves coexist with healthy roots. If the root system has been severely damaged before planting by mechanical injury (such as having part of the roots fall off the plant), you might also see brown leaf edges. Insects called root weevils can, in their larval form, chew the bark off the trunk at ground level, and this damage may also lead to brown leaf edges.
Summer drought can be another culprit. (One of the oddities of plant diagnosis is that the symptoms of wet soils and of too little water resemble each other.) Check plants under eaves where rainfall may not penetrate even on the wettest days. Analyze summer watering systems to be sure plants are getting well soaked down into the roots. A deep watering twice a month in summer will protect rhododendrons better than frequent light sprinkles grazing the top of the roots.
Rhododendrons planted in too much sun can show drought symptoms even if they are watered. A few, such as 'Jean Marie de Montague,' will thrive in full sun, but the general rule is that they need protection from direct hot sun. Late afternoon sun is the most damaging. Filtered light is best, or morning light with afternoon shade.
If too much fertilizer has been applied, and has not washed down into soil, you'll see accumulated salts (a whitish crust) from fertilizer. This too can result in brown leaf edges. Water deeply to reduce salts and avoid overapplication of fertilizer. Rhododendrons grow well if fertilized before they bloom, and immediately after they bloom. A good diagnostic help for all rhododendron and azalea difficulties is a pamphlet from Washington State University, "How to Identify Rhododendron and Azalea Problems," EB1229 (1987). It's well organized by categories of problems and has full-page color photos ($6. To order, go on line to pubs.wsu.edu).
Mary Robson is area horticulture agent for Washington State University/King County Cooperative Extension.