Signs of rhododendron distress can usually be traced to incorrect culture or siting. Determining the underlying problem is necessary to restore the rhododendron's health. Leaves are the best indicators of plant problems. Wilting, browning at the edges, brown spots in the centre of the leaf or very dark green veins are all signs of different problems.
Troubleshoot your rhododendron by taking a small cutting to a local Master Gardener for a diagnosis. Reviving a distressed rhododendron may require a combination of remedies if there is more than one stress factor involved. Some simple diagnoses and remedies can be tried by the homeowner and will often cure the problem.
Check the area where the plant is sited first. Heavy shade causes lack of blooms. Consult a rhododendron book for the sun recommendations for your variety. Cut back overhanging plants as needed. Transplant the rhododendron if necessary.
Cut back on high-nitrogen fertiliser because too much causes shoot growth, not bloom.
Deadhead rhododendrons continuously during the spring to prevent seed set. Producing seeds lowers the plant's production of flower buds.
Protect your rhododendron from freeze-thaw cycles in the winter. According to Bruce Zimmerman in his 2003 article for Hort-Pro, "Rhododendrons the King of the Garden", you should create a burlap screen for the rhododendron in extreme climates. For milder climates maintain a good layer of mulch and make sure not to plant it in openly windy areas or low spots.
Pruning improperly will prevent flowering and can cause dieback to bare wood in some varieties. Check a good gardening book to find the difference between flower buds and terminal leaf buds and prune according to a pruning guide for your variety.
Diagnose by checking the leaves, which will be very pale or yellow with dark green veins. Determine the cause by analysing your soil and your culture practices.
Lifting the rhododendron and resetting it higher up in its planting bed may help. Planting too deeply prevents the roots from functioning to take up nutrients.
Destroying shallow roots with cultivation may be the cause. According to the American Rhododendron Society in their 2009 bulletin, "Subsequent Care", "little or no cultivation should be done around rhododendrons" due to their shallow roots.
Stop over watering and over fertilising your rhododendron. Check a good gardening manual for your variety and follow the care recommendations.
Test your soil for lack of iron or magnesium. Treat lack of iron with a foliage spray of chelated iron according to the manufacturer's directions. Spray your rhododendron with 2 tablespoons of Epsom salts in 1 gallon of water if it is suffering from a lack of magnesium.
Finding round brown patches near the leaf centre or along the edges can be a sign of sunburn. Water your rhododendron for a longer time and more often. Shade the plant with a reed screen or change its location.
Browned edges on new growth usually means windburn. Protect your rhododendron with a reed screen until you can move it to a less exposed location. Plant a windbreak of taller shrubs like thuja pyramidalis around your rhododendron.
Browning of tips and brittle browning of margins indicates salts injury. Test your soil for alkalinity and drainage or have it tested by your local university extension office. Lift the rhododendron and enlarge the planting hole to three times the size and depth of the root ball. Add soil amendments recommended to improve drainage and correct the pH as needed.
Distorting and killing of leaves is caused by frost. Protect tender plants by erecting a burlap screen and filling it loosely with straw after the ground has frozen. If your rhododendron is injured every year move it to a new location in early March.
Splitting bark is a dangerous problem and rhododendrons distressed this way may die back. Paint splits with a grafting compound or tree wound sealer.
Correct drooping leaves by spraying foliage with water in the early evening.
Check your soil drainage and if water runoff is too fast, add soil amendments and water longer and more deeply.
Mulch the root zone out to the drip line to maintain moisture.
- Young rhododendrons have to form a framework of branches before they flower. Some species may require five years to bloom.
- Consult a local Master Gardener whenever you encounter plant problems. Bring a section of leaves with you, and a soil sample if possible.
- Avoid planting rhododendrons in low spots and windy exposed sites. This predisposes them to several types of injury.
- Test your soil and amend as necessary before planting rhododendrons. Prevention is the best cure for soil problems.