If you have this popular shrub growing in your landscape, you may notice your rhododendron leaves yellowing or dropping from time to time. At certain times of the year, leaf yellowing may be normal, but other times, it’s a sign of bigger troubles. In this article, I’ll share six main reasons for rhododendron leaves turning yellow and fill you in on whether or not you need to take action.
Why are yellow rhododendron leaves a bad thing
Rhododendrons are in a group of plants known as broad-leaved evergreens, along with azaleas, boxwoods, some types of hollies, and various other species. They hold onto their foliage year-round, even in freezing temperatures, and they have broad, flat leaves instead of needles.
Even though broad-leaved evergreens like rhododendrons are green year-round, sometimes their foliage turns yellow or discolored for various reasons. Occasionally this is accompanied by a slower rate of plant growth or wilting, depending on the cause. Yellowing foliage on rhododendrons is typically a sign of a problem, though occasionally it is not. Keep reading to learn how to identify when yellow leaves are indicative of trouble and what might be the cause.
Where are the yellow leaves? Are they old or new?
When you see yellow rhododendron leaves, first make note of where on the plant the discolored leaves are. If they are older leaves further down on the plant stems, it is typically less problematic, especially if the newer growth is still a rich green. If it’s the new leaves at the branch tips that are yellowing, it’s a clue that something bigger could be wrong.
Whole leaf yellowing vs. yellowing between the leaf veins
Next, look at what part of the leaves are turning yellow. Is it a general yellowing over the entire leaf surface; yellowing in spots or splotches; tiny, speckled yellowing across the top of the leaf; yellowing only between the leaf veins; or maybe a mixture of one or more of these? Each of these patterns represents a symptom of one of the six causes of rhododendron leaves yellowing listed below. Read on to learn more about how to use this info to diagnose your issue.
Six causes of rhododendron leaves yellowing
After determining which leaves are yellowing and where the yellowing is occurring, it’s time to dig into the possible cause. Excess yellowing, coupled with leaf dropping, could cause long-term problems for your rhododendron plant so it’s important to figure out what’s going on.
Cause 1: Seasonal yellowing in the autumn
Which leaves are yellowing: The older leaves just beneath the new growth
Type of discoloration: Uniform whole leaf yellowing across the entire leaf followed by leaf drop
Timing: In the autumn, when deciduous trees like maples are changing colors
You may be surprised to hear that broad-leaved evergreens drop some of their foliage in September, October, and early November, just like deciduous trees do (white pines and some other conifers also have fall needle drop). Often the foliage turns yellow or even orange before dropping from the plant. As long as the yellowing is occurring in the autumn and it’s only the older leaves, you can attribute it to seasonal yellowing. No need to fret or fix.
Cause 2: Rhododendron leaves yellowing due to improper soil pH
Which leaves are yellowing: Typically the newest leaves show discoloration first, but older leaves can be affected too
Type of discoloration: Yellowing between the leaf veins (called interveinal chlorosis)
Timing: Can be any time of the year but tends to be worse in late spring and summer, as new growth is expanding
Solution: Test your soil pH; adjust with fertilizers or soil amendments to lower (acidify) the soil’s pH according to the test results
Your soil’s acidity (pH) influences the availability of almost all plant nutrients. If the pH is too high (alkaline) or too low (acidic), certain nutrients are bound up in the soil and are not available for plants for use. Broad-leaved evergreens like rhododendrons use a lot of iron. At high pH levels (alkaline soils), iron is less available to the plants and iron deficiency is often the result. There is likely to be more than enough iron in your soil; it’s just that rhododendrons can’t access it without an acidic soil pH. Interveinal chlorosis is one of the symptoms of this deficiency on broad-leaved evergreens growing in soil where the pH is not acidic enough.
The target soil pH range for rhododendrons is 4.5 to 5.5. Here’s a more extensive article about how to test for and adjust soil pH. An acid-specific fertilizer, such as Holly Tone and elemental sulfur, is recommended to keep the soil pH in the target zone for iron availability. This is especially important to monitor if your plants are growing next to your home’s cement foundation which can create an alkaline soil environment due to leaching of calcium carbonate.
Cause 3: Rhododendron leaves turning yellow from nitrogen deficiency
Which leaves are turning yellow: Older leaves
Type of discoloration: Uniform pale yellow leaves and stunted leaf growth
Timing: Can occur at any time of year but is more typical in late spring and summer
Solution: Amend with compost, bloodmeal, feather meal, or a liquid seaweed spray
Like other plants, rhododendrons need various macronutrients and micronutrients from the soil to survive. There are other nutrient deficiencies that can occur, but iron (as described above) and nitrogen tend to be the most common on rhododendrons. If the older leaves turn uniformly yellow in spring or summer, it’s time for fertilization that will provide the plants with nitrogen they can quickly uptake. The nitrogen in liquid fertilizers, like seaweed and kelp, is available quickly but doesn’t last long. A better fix is amending the soil with a nitrogen-rich organic fertilizer such as bloodmeal, feather meal, or a granular complete fertilizer blend.
Cause 4: Rhododendron leaves yellowing due to pests
Which leaves are turning yellow: Old and new
Type of discoloration: Tiny pale yellow to cream spots on the upper leaf surface that appear as stippling (lace bugs); notches in leaf margins (black vine weevil)
Timing: Spring and summer into fall
Solution: Horticultural oil sprayed on upper and lower leaf surfaces for lace bugs, 2 applications 14 days apart; beneficial nematodes for black vine weevils
There are two common pests of rhododendrons, one that is responsible for stippled leaf discoloration and one that causes notching in the edges of the leaves.
A rhododendron pest known as the lace bug (Stephanitis pyrioides) causes pale stippling (see photo below) on upper surface of leaves. Flip an affected leaf over and you’ll see tiny insects (1/8” long) with clear wings that leave speckled droplets of black, tar-like excrement behind. Both the nymphs and adults are easily smothered by sprays of horticultural oil. Lace bugs are very common on rhododendrons that are growing in full sun. Keep this understory plant in shade to partial shade and lace bugs likely won’t prove troublesome.
Another rhododendron pest is the black vine weevil. Though the adult weevils leave notches in the leaf margins, they do not cause the leaves to turn yellow. Their larvae, however, can cause rhododendron leaves to yellow as they feed on the roots below ground. If you notice a combination of many notched leaves, wilting, and leaf yellowing and drop, this pest may be to blame. Applications of beneficial nematodes to the soil have been shown to be effective. The best species of nematodes to use include Heterorhabditis bacteriophora and Steinernema carpocapase. Read more about how to use nematodes here.
Cause 5: Rhododendron disease issues
Which leaves are turning yellow: Any
Type of discoloration: Yellow with drooping; leaves can also have brown or black spots
Timing: Any time of year
Solution: Depends on the pathogen
There are several fungal disease issues that can cause rhododendron leaves yellowing. Most fungal infections are followed by branch or whole-plant wilting and/or leaf or stem death. Aesthetic diseases like powdery mildew can be worsened by poor air circulation, but more serious fungal diseases, such as dieback, root and crown rot, and cercospora leaf spot, are far more problematic.
Root and crown rot caused by the Phytophthora fungus is the biggest troublemaker and it thrives in poorly drained soils. Overall wilting of the plant, followed by leaves that curl, yellow, and drop, is a common symptom. It’s particularly problematic if the plant is set too deeply in the soil or if the soil is poorly drained. Once symptoms appear, there is little you can do. Fungicide products applied to the soil can suppress the disease but won’t cure it. Once present in the soil, the fungus can easily survive. So, if you lose a rhododendron to this disease, don’t replace it with another rhododendron. Choose a different plant species.
For cercospora leaf spot, thankfully, this mostly affects the plant’s looks and not its overall health. Splotches on yellowing older leaves are the biggest symptom. Remove fallen leaves to reduce spores. Copper-based fungicides are helpful, especially during humid weather.
Cause 6: Plant stress can cause yellow rhododendron leaves
Which leaves are turning yellow: Any
Type of discoloration: Uniform yellowing across the entire leaf; often occurs with overall plant wilting
Timing: Can be any time of year
Solution: Care for your rhododendron properly
Stressed rhododendron plants can also exhibit yellow leaves. Here are some good tips for overcoming stressors that can cause leaf yellowing and even leaf drop.
- Good drainage is ESSENTIAL for rhododendrons. If they are planted in an area of poor drainage, the roots will be waterlogged and starved of oxygen which can lead to whole-plant wilting and yellowing leaves. Transplant to a site with better soil aeration and drainage.
- During droughts, rhododendrons are prone to drought stress which can cause leaves to yellow and drop. This is true whether your plants are new or old. Water deeply during times of drought by setting the hose on a trickle at the base of the root ball and allowing it to run for several hours once every week to 10 days. A layer of pine needle mulch is helpful for maintaining soil moisture levels.
- Rhododendrons are understory plants. They should NOT be growing in full sun. Shade or partial shade is best. Rhododendron growing in full sun will be stressed out and may show signs of that stress in the form of yellowing leaves. Move the plant to a shadier site.
- If your rhododendron are growing in pots, be sure to up-pot them into a larger pot every year or two. Keep them properly watered and use a high-quality potting soil mixed 50/50 with compost. Fertilize regularly with an acid-specific fertilizer.
Now that you know the most common reasons for rhododendron leaves yellowing, you can do some investigating and find out what’s wrong with your plant and quickly remedy the situation. With this info and a little extra TLC, you’ll be able to enjoy their beautiful blooms for many seasons to come.
For more on growing and managing flowering shrubs, please visit the following articles:
- Flowering shrubs for the shade
- Planting hydrangeas
- Hydrangea fall care
- Low-maintenance shrubs
- Early spring flowering beauties
Pin this article to your Landscaping board for future reference.