Readying your rhododendrons for dry summer
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Q: How can I care for rhododendrons with a projected summer drought coming up?
A: Your question makes sense as we consider the rainfall over this winter, only half of the normal amount since October 2000.
If your rhododendrons have been in the landscape longer than 10 years, they will have some drought tolerance and will require less irrigation than newly-installed plants. The decades-old rhododendrons in Seattle's Washington Park Arboretum thrive despite limited summer irrigation. Many of the larger, tree-like plants have lived through many dry summer months.
With younger, and more recently installed landscape rhododendrons, be sure they are planted in well-amended soil with organic material at the root level.
Rhododendrons have shallow, pancake-like root systems and need the air spaces in the soil provided by organic matter. They won't live on heavy clay soils. Organic material also helps to retain moisture in the soil, another summer benefit.
Mulch carefully around the rhododendron. Don't pile a heavy stack of mulch over their shallow root systems. Keep the mulch at 2 to 3 inches and don't wedge it against the trunk.
Group plants with similar water requirements and surround them with soaker hoses or other efficient irrigation systems. Less water will mean less lush growth and shorter growth, but the plants can survive well if these steps are followed.
Be sure to site rhododendrons where they receive shade from the hottest part of the day, such as bright afternoon western sun.
Unprotected plants may show signs of scorching or stress even when they receive adequate water. If they do, move them to a more protected location when fall rains return.
Certain rhododendrons will endure heat, such as `Jean Marie de Montague' (bright red) and `Blue Peter' (blue). Check with a nursery about these and others. Don't neglect the soil conditioning before planting. Fertilize moderately. One fertilization before bloom, with an acid-based fertilizer formulated for rhododendrons, will be adequate for this coming summer.
Gardeners are often advised to fertilize several times for rhododendrons, but in the interests of saving water it's desirable to avoid a big flush of soft new growth that will require extra irrigation.
Spring care for rhododendrons includes pruning during or after bloom. Pruning during this period protects next year's flower show. Pruning too late, such as in fall or winter, removes newly set flower buds and reduces the quantity left for 2002 spring bloom.
Should you remove dead flowers when pruning? Gardeners often ask if faded blossoms reduce next year's bloom.
The answer depends more on aesthetics than on botany. Rhododendrons can set bloom for the next year even if the old blooms are allowed to go to seed on the plant.
Taking off the blooms before they seed improves the overall appearance, especially if it's in a prominent landscape position.
If you have to choose, it's more important to dead-head the larger-leaved, large-flowered types. Cultivars such as 'Anna Rose Whitney,' 'Blue Peter,' 'Bow Bells,' 'Gomer Waterer,' 'Jean Marie de Montague,' all benefit from dead-heading. These cultivars are classified as elepidote, one botanical description for rhododendrons with large leaves, lacking scales.
The other broad classification, lepidote types, such as Rhododendron augustinii, 'Blue Diamond' and 'PJM' have smaller leaves and are scaly. In general, the lepidote types don't need dead-heading while the elepidote sorts do. In my garden, Rhododendron augustinii seems to drop its petals without much need for dead-heading. You can judge by the size of the flower truss.
Look carefully at the plants when snapping off large, old flower trusses. The new growth emerges quite close to the old bloom stalk. Snap the old flower head off above the new leaves so that you don't break off new growth, too. Spring shaping helps to keep rhododendron branches from becoming floppy and lanky.
Prune back to a whorl of leaves or to an obvious growth point. Wobbly, lanky branches may indicate that the area has become too shady. Rhododendrons appreciate filtered shade and will generally not set bloom properly in dark shade.
When shortening a branch, don't chop it into a stub at a spot where there is no green-growth bud. The buds far down on the stems are small and easy to miss; they are rounded and small, slightly larger than a heavy pencil point. If the stem is pruned back to one of the growth buds, it will put out a new whorl of leaves.
You can often feel potential growth buds by running your hand up a bare rhododendron branch and sensing the little nubs.
Mary Robson is area horticulture agent for Washington State University/King County Cooperative Extension.