Rhododendrons are iconic flowering shrubs, prized for their showy spring blooms and attractive evergreen foliage. However, rhododendron health can suddenly take a turn for the worse, leaving bewildered gardeners wondering – why are my rhododendrons dying?
While rhododendrons are relatively low maintenance shrubs once established, they do have some particular cultural requirements. Read on to learn about 8 common reasons rhododendrons fail to thrive, and get tips to revive ailing specimens.
1. Improper Watering
Too much or too little water is the number one killer of rhododendrons. These shallow-rooted plants demand consistently moist, well-drained soil.
Overwatering suffocates roots and leads to fungal diseases. Underwatering causes drought stress and yellowing leaves.
Solution
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Check soil moisture before watering. Water deeply only when the top few inches become dry.
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Improve drainage in heavy clay soils, Add organic matter like compost
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Mulch around rhododendrons to maintain moisture. Keep mulch a couple inches away from stems.
2. Poor Drainage
More rhododendrons are killed by waterlogging and poor drainage than any other cause. If the soil is boggy, heavy clay or compacted, rhododendrons will not survive for long.
Solution:
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Plant in well-draining soil, or amend clay soils generously with compost.
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Create raised planting beds if drainage is very poor.
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Make sure container rhododendrons have adequate drainage holes.
3. Excess Fertilizer
While rhododendrons thrive in nutrient-poor soil, overfertilization can damage or kill them. Their fine root system is easily burned by excess fertilizer.
Solution:
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Avoid excessive fertilizer, especially quick-release types.
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If fertilizing, use slow-release fertilizer made for acid-loving plants, and follow label directions.
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Top-dress with compost instead of chemical fertilizers whenever possible.
4. Fungal Diseases
Root and stem rot diseases like phytophthora are common rhododendron killers. These fungal pathogens thrive in wet conditions and attack root systems.
Solution:
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Improve drainage and avoid overwatering to deter fungal growth.
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Remove and destroy dead or dying stems and leaves.
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Use fungicides as a last resort if disease is severe.
5. Late Spring Frosts
Rhododendron buds and blooms are tender and easily damaged by late frosts after the shrubs have leafed out in spring. Frost-blackened flowers and curled, withered leaves are signs of cold injury.
Solution:
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Choose late-blooming rhododendron varieties to avoid frost damage.
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Plant in protected microclimates away from frost pockets.
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Cover plants with fabric when late frost is predicted.
6. Hot Summer Sun
Rhododendrons perform best with some morning sun and afternoon shade. Their large leaves can easily scorch if exposed to intense midday sun.
Solution:
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Situate rhododendrons where they will receive dappled sunlight, especially in hot climates.
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Keep plants well-watered during heat waves.
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Add shade if sun exposure is excessive.
7. Insufficient Winter Chilling
Some rhododendron species and hybrids require exposure to a certain number of chill hours below 45°F over winter. Without this dormancy period, they struggle to set flower buds and thrive.
Solution:
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Choose rhododendron varieties rated for your USDA zone. Avoid warmer-climate varieties in colder zones.
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In warmer zones, site plants in cooler microclimates. Add shade to reduce heat.
8. Severe Winter Cold
While rhododendrons are considered cold hardy shrubs, extremely frigid temperatures can damage or kill them. Dessicating winter winds also take a toll.
Solution:
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Choose cultivars rated for your coldest winter temperatures.
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Site rhododendrons in protected locations away from prevailing winds.
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Wrap plants in burlap or frost cloth to protect from wind and cold.
How to Save a Dying Rhododendron
If your rhododendron starts declining rapidly, don’t give up on it yet! Here are some tips to revive a struggling plant:
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Rule out disease or pest issues. Treat accordingly.
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Prune out dead wood, but avoid heavy pruning which can stress the plant further.
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Rule out environmental factors like poor drainage or excessive heat/sun. Make adjustments.
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Water thoroughly if drought stressed.
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Apply a balanced fertilizer or compost to support recovery.
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Shelter plants from wind and harsh sun until healthier.
With attentive care tailored to their needs, you can often nurse distressed rhododendrons back to their former glory. But if all else fails, start fresh with a new specimen suited to your climate and growing conditions.
In Summary
Rhododendrons are not the easiest shrubs to grow successfully, but understanding their requirements helps prevent common issues. Provide adequate moisture without waterlogging, filter hot sunlight, choose suitable varieties, and avoid overfertilization. Monitor plants for signs of stress, and act quickly if they start to decline. With proper care, rhododendrons will reward you with years of spectacular bloom.
Excessive application of fertilizer
Overfertilization can result in damaged roots and leaves, and some rhododendrons can be killed with fertilizer. Plants like these have evolved to do well in places with few nutrients, and they don’t need many extra nutrients in good garden soil. If soils are poor, top-dressing with compost can do the trick. There are a few things to think about before using granular or other broadcast fertilizer to fix very poor soil. Acid lovers like rhodies do best in acidic soils. If you planted your rhodie in neutral to slightly acidic soil, you can use a special fertilizer made for plants that like acidic soil. At a low pH, these fertilizers break down, letting plants get the nutrients they need to be healthy. These fertilizers are not necessary if your garden soil is already acidic.
Applying nitrogen fertilizer in late fall is not a good idea in New England or other colder places because it will cause new, soft growth that won’t be able to harden off before the cold and wind of winter. Most of the time, the best time to use balanced fertilizer is in early spring, when flower buds start to swell. Do this on an as-needed basis—every other year or so.
Phythopthera is a cosmopolitan, soil-dwelling group of fungi that afflict thousands of plant species—including rhododendrons. In their search for food, these microbe pathogens thrive in conditions that are always wet and attack plants’ roots. When a plant gets an infection, the underground vascular system that moves water and nutrients between the roots and the plant parts above ground goes away. As the infection gets worse and the plant can’t get water, the leaves suddenly and permanently wilt (top photo). Some plant breeders are using species from warm, wet places where these fungi thrive in their plant development programs, even though most rhododendrons and azaleas are susceptible. In doing so, they hope to add resistance to fungal infection.
Rhododendrons do not have thick, deep taproots; their roots are fine and grow very superficially. Therefore, they may need reliable moisture to avoid damage from drying out. That said, once established, most rhodies are relatively drought tolerant (and slow growing). Be sure to site them appropriately and provide regular watering. Soils should be well draining, and mulching is highly beneficial. Over time, drought stress can be seen when a plant can’t keep up the leaves it had last year, making it grow in a more open and scraggly way. Not all cultivars get dense and full with age. If a cultivar has a bad habit, it could be because of things in its environment, like not getting enough water.
When you plant a rhododendron, putting its roots too deep or covering them with too much mulch can do a lot of damage or even kill it. Roots should be no more than 1 to 3 inches deep, close to the surface, so they can keep taking in nutrients and getting rid of gas. When you plant new plants, you should first find the spot where the trunk widens and roots grow out from the plant. Often some excavation is required. These should be set at or just above the soil line, assuming some settling will occur. In addition, a light mulching will help maintain moisture and insulation.
With over a thousand species, rhodies can vary wildly in their capacity to withstand changes in temperature. Do some research on how hardy the cultivars you choose are, and know how much cold and wind your garden can handle in the winter. Know your zone! Rhodies’ leaves may curl and droop in the winter. This is how the plants try to protect their leaves by reducing their exposure and surface area (photo). If they are not overly damaged by winter conditions, these leaves rebound and perk up in spring. If they don’t, you can assume they are toast. Most of the time, it’s not the cold itself that causes this stress response and kills these beautiful shrubs, but the dry winter winds. Siting plants in protected locations is important to helping them maintain their foliage for more than one season.
A lot of rhododendrons do better with some morning sun, but some species and cultivars don’t mind any sun at all. Most rhodies do not fare well if exposed to long periods of hot, midday sun. The large, dark green leaves do not have the capacity to deflect or mitigate excessive sun and heat. You may notice leaves turning yellow and dropping off, or getting brown margins. In hot, dry weather, keeping the soil moist can help plants stay cooler and hydrated. However, this can sometimes cause root rot in plants that are already stressed out by the heat. For best results, keep rhododendrons out of direct sunlight and give them a more even glow.
Daniel Robarts is a horticulturist and plant propagator/breeder at Coastal Maine Botanical Gardens in Boothbay.
Here are some common issues gardeners face when growing rhododendrons
Rhododendrons have a predilection for moist, acidic soils. Their roots are fine and shallow. Sometimes they can handle a lot and live a long time, but there are some very simple ways to kill these hardworking shrubs.
Overwatering and planting in heavy, poorly draining soils are primary causes of death for newly planted rhododendrons. Most gardeners know to dig a hole twice as wide and twice as deep as the root ball they are planting. The plant is then placed in the hole and backfilled with a mixture of compost and soil. If you see rhododendrons in the wild, you’ll know that they like to grow in soil that is shallow and drains well. The typical crater for perennial planting can form a basin that holds water, especially if the soil around it is heavy (clay), packed down, or doesn’t drain well in other ways. The continual presence of water around the root ball will inhibit gas exchange and stress your new rhodie. Also, soils that are too wet are better for fungi, and a plant that is already under a lot of stress may be more likely to get hurt by microbes. The best way to keep your valuable rhododendrons from getting damaged by too much water is to keep the soil in and around the garden where they are planted in good shape.
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FAQ
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