Hydrangea leaves turning yellow
There are 8 common causes, and they need completely different responses — so working out which one you have matters more than treating quickly.
3 of them can kill the plant. Rule those out first.
Check these two things first
- Push a finger into the soil, two knuckles deep. Wet soil plus a wilting plant means too much water, not too little — and it is the commonest reason a plant is dying. Dry soil plus crisp leaves means the opposite. The leaves look nearly identical either way, so the soil is what tells you.
- Turn a damaged leaf over. Half of all diagnoses live on the underside: fuzz means downy mildew, fine webbing means spider mites, clusters of insects mean aphids. Nothing on the top of a leaf is as informative.
Most garden problems are cosmetic and resolve without treatment. Reach for a spray last, not first.
The 8 causes, and how to tell them apart
1.Overwatering
Act nowGrowing conditions
Yellowing lower leaves that feel soft rather than crisp, often falling while still yellow. The plant wilts despite obviously damp soil. Stems may go soft and translucent at the base, and the soil surface may develop a green or white film.
How to know it is this one
Wilting while the soil is wet. This combination is the whole diagnosis.
2.Root Rot
Act nowFungal disease
Above ground, yellowing and wilting that looks like drought but does not respond to water. Below ground, roots that are brown or black, soft, and smell sour — healthy roots are firm and pale, and the outer sheath of a rotted root slides off leaving a thread.
How to know it is this one
Wilting that does not improve after watering, and often gets worse.
3.Spider Mites
Act nowPest
Fine pale stippling across the leaf, as though it had been sprayed with a very fine dust of pinpricks. Leaves go bronze or grey, and fine webbing appears between leaves and along stems in bad infestations. The mites themselves are just visible as moving specks.
How to know it is this one
Fine even stippling rather than discrete spots or holes.
4.Aphids
TreatablePest
Clusters of small soft-bodied insects, usually green but also black, grey, pink or white, packed along new shoots and the undersides of young leaves. Leaves curl and pucker, and everything below becomes sticky with honeydew.
How to know it is this one
Dense clusters on new shoots and flower buds rather than scattered individuals.
5.Iron Chlorosis
TreatableNutrient problem
Leaves yellow between the veins while the veins themselves stay distinctly green, producing a fine green net on a yellow background. Newest leaves are affected first and worst; in severe cases they come out almost white with brown scorched edges.
How to know it is this one
Green veins on a yellow leaf — the net pattern is diagnostic.
6.Nitrogen Deficiency
TreatableNutrient problem
Uniform pale yellowing of the whole leaf, including the veins, starting with the oldest lowest leaves and working upward. Growth is slow and the plant looks generally thin and pale.
How to know it is this one
Whole leaf yellows evenly, veins included.
7.Underwatering
TreatableGrowing conditions
Drooping leaves that feel dry and papery, crisp brown edges and tips, and soil that has shrunk away from the sides of the pot. The whole plant looks deflated, and lower leaves go crisp and yellow before dropping.
How to know it is this one
Leaves dry and crisp rather than soft and limp.
8.Powdery Mildew
Mostly cosmeticFungal disease
White or pale grey powder on the upper leaf surface, looking exactly as though the plant has been dusted with flour. It starts as small round patches that merge. Affected leaves yellow, then go brown and papery.
How to know it is this one
White powder on the top of the leaf that rubs off with a finger.
Still not sure?
The diagnostic key adds two more questions — which part is affected and where the trouble started — and those usually settle it.
Run the diagnostic key