Why are my rhododendron leaves turning yellow?
Yellow leaves with green veins mean the soil is not acid enough.
When a rhododendron's youngest leaves turn yellow between dark green veins, the soil is too alkaline: above about pH 6 it cannot take up enough iron. This is lime-induced chlorosis. Lowering the pH with sulphur fixes the cause. Old leaves yellowing, or a whole plant going pale and limp, is a different problem.
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- Young leaves yellow, veins green: the pH is too high. Sulphur.
- A few old, inner leaves yellow and drop: normal, especially after flowering.
- The whole plant yellow and wilting: look at the roots, not the pH.
Which yellow is it?
| What you see | Likely cause | What to do |
|---|---|---|
| Newest leaves yellow, veins stay green | pH too high, so iron is locked up | Test the pH. Sulphur at up to 25 g/m², early spring and late autumn |
| Oldest leaves yellow and drop | Normal: evergreen rhododendrons shed their oldest leaves each year | Nothing |
| Whole plant pale and slow | Planted too deep, or hungry | Top of the rootball level with the soil; feed in spring |
| Yellow and wilting in wet ground | Waterlogging or root rot | Replant higher, on a mound or in a raised bed |
| Yellow beside a wall or path | Lime leaching from mortar or concrete | Test that spot; sulphur, or move the plant |
Rhododendrons and azaleas want pH 4.5 to 6.0 (RHS; Cornell). Rates as on our product page: University of Georgia and Cornell (established plants); Ohio State, Purdue and University of California (beds); RHS (pots). Never more than 73 g/m² in one dose on beds, or 112 g/m² a year on planted ground.
Work out your exact amount. Enter your area, your test reading and your soil type, and the calculator gives the dose, the timing and the bags to buy.
Fixing high pH
- Test first. A cheap kit is enough. If the soil fizzes with vinegar there is free lime, and acidifying is not realistic: grow in a raised bed or a large pot of ericaceous compost.
- Sulphur for the long term. Up to 25 g/m² over the root area, out to the edge of the leaves. Work in lightly, water, and retest after three to six months.
- Chelated iron for this season. A sequestered iron tonic regreens leaves within weeks but does not change the pH, so without sulphur the yellowing comes back.
- Rainwater, not tap water. In hard-water areas, tap water pushes the pH back up every time you water.
Common questions
Will yellow rhododendron leaves turn green again?
Mildly yellow leaves often regreen once iron is available again. Badly yellowed or scorched leaves usually do not, but new growth comes through green.
Can I use sulphur on a rhododendron in a pot?
No. The RHS advises against sulphur in potting media. Repot into ericaceous compost and water with rainwater.
How quickly does sulphur work?
Soil bacteria turn it into acid, so it works from spring to autumn when the soil is warm, and slowly in winter. Allow a full season.
Why is the rhododendron next to my wall going yellow?
Mortar and concrete leach lime and raise the pH of the soil beside them. Test that spot. Sulphur helps, but moving the plant away from the wall may be simpler.
More: Sulphur Soil Acidifier · soil pH calculator · camellia leaves turning yellow · azalea leaves turning yellow · blueberry leaves turning yellow · iron sulphate vs sulphur · how to make soil more acidic
Fix the soil, and the green comes back
Sulphur-bentonite pastilles that spread without dust and break apart when wet. Up to 25 g/m² over the root area, spring and late autumn.
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Free UK delivery over £40. Buy now goes straight to checkout.
Free UK delivery over £40. Dispatched in 1 to 2 working days. Packed and posted from Stockport.