Rhododendrons

Rhododendrons are one of the most recognizable plants to gardeners on the West Coast.

There are hundreds of varieties to choose from in a variety of sizes and flower and leaf colours. Not only are they a showy addition in spring but they are also evergreen and shade tolerant. A staple in any West Coast garden.


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Planting Conditions and Techniques

The transplanting of Rhodos can continue almost throughout the entire year, even when in full flower with a little care.  It is always best, however, to do most transplanting in early spring if possible.  When transplanting, make sure to provide the proper soil conditions first and then worry about the proper timing later.  If the soil in which the plants are to be moved is adequate then the timing of the move is not as critical.  When transplanting Rhodos from within ones own garden it is best to dig a wide shallow root ball since the roost system is very fibrous extending down only 8 to 12 inches but extending out to the edge of the drip line sometimes making a root ball up to 5 feet in width.  Rhododendrons must not be planted to deeply since they are a shallow rooting plant.

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Watering and Mulching

Rhodos require adequate quantities of water especially during the late spring when most of the growth is taking place.  This is the most critical time for transplanting, making sure to maintain adequate water during the first season of active growth.  One of the simplest ways of determining dryness is the dropping of leaves, which occurs at the first sign of dryness.  Since Rhodos exhibit this easily without long‐term damage to the plants it is easy to rectify the situation before it can become lethal.  Deep weekly watering is always preferable to a light daily watering.  The most efficient way of keeping the root system moist throughout the year is by regular mulching to help preserve soil moisture.

The fibrous root system of the Rhododendron needs fairly constant moisture in the growing season.  The competition of other plants’ deeper root systems prevents the rhododendron from getting the water they need.  Mulching is an important factor for the health of the plant, keeping the roots moist and insulated in the winter months, using peat or soil rich in organic matter is ideal for Rhododendrons. Adequate materials suitable for mulching can include composted bark, garden compost, compost shredded leaves or peat moss.

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Pruning

Although most gardeners do not prune many of their Rhodos this does not mean that they cannot be trimmed to maintain their shape and flower potential. They can be kept compact and bushier with regular pruning, resulting in a bushier plant with greater flower production.   Large overgrown plants can be brought down in size by hard pruning.  Those Rhodos that have overgrown their usefulness can again retain their usefulness by the simple act of pruning.  When reducing down large plants to a more manageable size it is best to do this procedure over several years.

Most pruning should be done in the early spring months of March and April right after blooming has finished.  Many Rhodos can be deadheaded right after flowering by simply giving them a light pruning.   It is important to note that when large plants are severely pruned that they may either flower lightly or not at all for several seasons as the root system and foliage again start to balance each other.  Normal flowering will return after several seasons. Deadheading Rhodos has now been proven to not affect next year’s flowering. Deadheading will only clean up the look of the plant.

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Pest, Diseases and Disorders

Fortunately there are few serious pests of Rhododendrons.  Aphids, lace bugs and weevils (the most common) are among the few, as well as some infrequent problems with whitefly.  Weevil damage appears as irregular, chewed notches at the leaf edges and can be avoided by cleaning up leaf litter at the base of the shrubs to remove hiding places.  A product such as Tanglefoot can also be helpful in stopping the nocturnal insects from feeding on the leaves

Leaf spot

Brown or black spots on foliage caused by too damp conditions.  Usually harmless, but using a fungicide such as Benomyl should help severe cases.

Powdery Mildew

The first symptoms are light green or yellowish rings on the upper leaf surface.  In a short time these appear on the under side as spots or patches of whitish powder. Spray a fungicide such as Benomyl to help.

Root Rot/Wilt: (Phytophthora)

Poor drainage and excess water are the usual causes of this infection.   This is a lethal disease causing part of or whole plants to wilt and die. There is no known cure other than good drainage in wet areas and fertilizer to maintain healthy plants.

Chlorosis

Yellowing of leaves usually caused by soil problems such as alkaline soil, excess water, and planting to deeply or a lack of fertilizer.  If planted correctly and given good drainage with acidic soil conditions, the cause will most likely be due to a mineral deficiency such as iron or magnesium.

NOTE:  As with any disease problems, when the damaged plant material is removed, do not add it to your compost.  This will harbor harmful spores and diseases that will be spread to wherever the compost is used in the future.

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