Rhododendron FAQs

What kinds of rhododendrons are there?

There are four major divisions of rhododendron species. Two of these are azaleas, which are now classified as rhododendrons.

There are about 1000 species of rhododendrons and more than 25 000 registered hybrids. Sub-tropical varieties (vireas) are becoming more popular but will be pot plants and need winter protection in our climate.

How big do rhododendrons get?

They vary in size from alpine varieties a few centimeters high to lower elevation varieties that are trees that can be in excess of 25 metres high. Books and labels at nurseries give the expected height after 10 years, so are minimum values to expect.

How do I plant a rhododendron and when?

Tease out the roots and plant it shallow. Make sure the crown is above ground level to allow for settling. Potted plants are often root bound. Tease out or cut the rootball into sectors so the roots can be spread out. Root balls on a well grown plant should be shield-shaped and shallow.

What kind of soil is best?

Ideal soil is loam which is rich in organic matter and has a pH near 5.5. Other soil types are acceptable, but good drainage is critical.

How much sun exposure do rhododendrons need and tolerate?

Generally 50:50 sun and shade and protection from the late afternoon sun are best but azalea varieties and small leaved rhododendrons can tolerate more sun (full sun for some alpines and most azaleas). Large leaved rhododendrons do best with more shade but need dappled light to bloom. Too much shade, especially for azaleas and small leaved rhododendrons can be a problem.

Can rhododendrons be pruned? If so, when?

Yes – forms of pruning include deadheading, bud pruning, pinching and woody pruning. Remember, rhododendrons flower on the prior year’s wood.

Single bud or growth terminal pruning encourages two or more branches to develop (to make plants bushier). Do not remove flower buds. To be certain, remove the single growth terminal just as new growth starts.
Woody pruning, for shaping and size control. Light woody pruning is done after flowering; severe pruning is best done in the spring. Remove interior dead and weak or crossing limbs as needed.
Evergreen azaleas can be sheared as a hedge or border plant.

The new leaves of my plant have a white or brown film on them – what is wrong?

Indumented plants (those with hairs on the undersides of the leaves), like yakushimanum, sometimes have white or brown hairs (tomentum) on the surface of the new leaves. This is normal and can be very attractive.
A white coating on azalea leaves late in the season is a kind of mildew. I ignore it, but you can try spraying with dilute skim milk if it bothers you. Rake up and discard the affected leaves in the fall when the leaves drop.

Leaves on my rhododendron are turning yellow, what is wrong?

Sun can bleach exposed leaves – add N to green them up again
natural for older leaves – leaves are replaced in 1 to 3 years
If the area along the veins remains dark green, it is chlorosis, which is caused by Mg, N or Fe deficiency. Before anything else, check the pH of the soil. If it is too high, the plant may not be able to take up the trace metals that it needs. If pH is close to 5.5, try adding Epsom salts (for Mg) first, then if that does not work, a nitrogen source, and finally chelated iron.

Leaves on part of my plant suddenly wilted and started to droop, why?

It probably has root rot caused by the phytophora organism. Too much water (poor drainage, clay soil, etc.) or too deep planting are often causes. Remove the infected limbs and improve drainage to see if you can salvage the plant.

Fertilizing – how and when?

At Glendale Gardens we use slow release 10-8-6 with micronutrients but an organic mix or just compost tea can be effective. Fertilizer is applied sparingly during the winter to offset leaching (December) and spring (March). Fish fertilizer is used after the rhododendrons have finished blooming (Canada Day). Alternatively an organic fertilizer mix is applied once a year (warning: it is a slow process, it takes a year to work. After that, once a year will do the job).

When and how much do I have to water?

Summer watering is needed for most rhododendrons. Local nurserymen suggest an inch a week.

What caused the leaf edges on my rhododendron to turn brown?

Leaf necrosis has many potential causes but desiccation by dry, cold winds is a common one. Other causes include the plant being too wet, too dry, or under- or over-fertilized,

What do I do for azalea leaf gall (hard green lumps)?

How do I recognize and what do I do about powdery mildew?

Ensure adequate sun exposure and free flow of air through the plant. You can spray with fungicides but I either move the plant to a more favorable location or treat it with compost tea to improve plant health so it can combat the mildew itself. Diluted Skim Milk may also be effective. I avoid using chemicals if possible.

Recognition of pests

Weevils -narrow trails chewed in from leaf edges. Feed at night, so can be seen then with a flashlight.
Caterpillars and leaf rollers – areas of leaf edges eaten or everything but the veins at times. Look for rolled up leaves, the grub will be inside – squeeze firmly.
Slugs – slug trails, leaf damage similar to caterpillars. The can be seen with a flashlight at night (for the dedicated!).
Aphids and white fly – prefer new growth; Safer’s Soap can discourage them.

What is compost tea?

A product made by “brewing” a mixture of water (chlorine- and pesticide-free) and high quality compost or manure to make a solution rich in microbe populations. The brew must be aerated. This solution is sprayed on new growth and the ground around the plant to augment natural microbe populations and purportedly improve conditions and health of the plants. Commercial compost tea is available locally.

What is the key to having healthy rhododendrons?

WASH: Water in the summer; have Acidic soil; about half a day of Sun each day and protection during the late afternoon; a source of Humus (bark mulch is good).
The LADS: Loam soil; Acidic conditions; good Drainage; Shallow planting

Podcast Link: For the Love of Rhododendrons

The Jim Barlup Legacy Garden