Yellow new leaves at the growing tip
Yellowing on the newest growth is a different problem from yellowing at the bottom. It usually means iron, or a pH that is locking it away.
What it usually is
1 Iron shortage, usually locked up by alkaline water
| How to tell | The newest leaves are pale yellow with the veins still distinctly green, a pattern called interveinal chlorosis. Older leaves lower down are normal. It is commonest on gardenias, citrus, hydrangeas and other lime hating plants in a hard water area. |
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| The fix | Water with rainwater rather than tap water, and feed with a sequestered iron product or an ericaceous feed through the growing season. Repot into ericaceous compost if the plant is one that needs acid conditions. |
2 Roots that cannot take anything up
| How to tell | Pale new growth with wet compost and older leaves also declining. The plant is not short of nutrients in the pot, it is short of working roots to collect them. |
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| The fix | Check the roots. Firm and pale means look at the feed; brown and soft means treat it as root rot and rebuild the plant before feeding it anything. |
3 Overfeeding, which burns the fine roots
| How to tell | Pale or scorched new growth on a plant fed at more than the label strength or fed every watering, often with a crust on the compost surface. |
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| The fix | Flush the pot through with several times its volume of water, stop feeding for two months, then feed at the bottle's strength and only in the growing season. |
Through the year. Worst in spring, when a plant is putting out new growth quickly and any shortage shows in it first. A plant that greens up as summer goes on has usually solved it through sheer growth.
Where the yellowing sits tells you what kind of shortage it is, and this is one of the most useful things in plant diagnosis.
Some nutrients move around inside a plant. Nitrogen and magnesium can be pulled out of an old leaf and sent to a new one, so when a plant is short of them the old leaves go first while the growing tip stays green. That is the pattern described in the entry on yellow lower leaves.
Other nutrients cannot move once they are placed. Iron is the important one indoors. A plant short of iron cannot rob its old leaves to supply the new, so the newest growth comes out pale while everything below stays green. When the veins stay green and the tissue between them goes yellow, on the youngest leaves only, that is iron chlorosis and it is almost diagnostic.
The interesting part is that the pot usually contains plenty of iron. What it lacks is iron the plant can take up, and that depends on pH. Above about pH 7 iron binds into compounds roots cannot use, so in a hard water area a lime hating plant is starved of iron within a year of being watered from the tap, however well it is fed. This is why gardenias sulk in some houses and thrive in others a mile away.
The remedy follows from the cause. Water with rainwater where you can. Use a sequestered or chelated iron product, which holds the iron in a form roots can take even at higher pH. Use an ericaceous feed and ericaceous compost for the species that need them. Ordinary all purpose feed will not fix this, because the problem is availability rather than absence.
Two other things cause pale new growth and should be ruled out. Roots that have rotted cannot take up anything, so a plant in wet compost with pale new leaves needs its roots looked at before its feeding is changed. And too much fertiliser burns the fine root hairs that do the collecting, which produces the same pale growth as too little. If in doubt, flush the pot and wait rather than adding more.
Entries that name it
What the journal says about this symptom on each plant.
| Cherry laurel | Chlorosis on a shallow, chalky soil, where the plant cannot take up enough iron even though it otherwise tolerates alkaline ground. Work organic matter into the planting area to buffer the pH, or feed with a chelated iron product made for alkaline soil. This is a soil problem, not a pest or a disease, and will not respond to either. |
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| King protea | Phosphate poisoning from an ordinary fertiliser or a rich compost. Stop feeding entirely and flush the root area with plenty of clean water. Use only a low phosphorus feed made for fynbos, or nothing at all. |
| Lemon | Iron chlorosis, from alkaline tap water locking up the iron. Water with rainwater and feed with a citrus feed containing chelated iron. Yellow between green veins on the newest leaves is the sign. |
| Raspberry | Iron shortage on limy soil, showing as yellow between green veins. Mulch with composted bark or leaf mould to acidify the surface and water with rainwater. Raspberries are among the first plants to complain about lime. |
Questions from the margin
- Why are my plant's new leaves yellow?
- Where the yellowing sits tells you what it is. Nutrients that can move inside a plant are taken from old leaves first, so a shortage of those shows at the bottom. Iron cannot move, so a shortage of it shows in the newest growth, with the veins staying green.
- How do I treat iron chlorosis?
- The pot usually holds plenty of iron; what it lacks is iron the plant can take up, because above about pH 7 it binds into compounds roots cannot use. Water with rainwater, and feed with a sequestered iron product or an ericaceous feed through the growing season.
Checked against RHS, Chlorosis; Clemson Cooperative Extension, Iron chlorosis in plants.