Leaf scorch

What you are looking at

Damage that appeared all at once and has not moved since.

Sun scorch shows as pale, bleached or straw-coloured patches in the middle of the leaf blade, on the side of the plant facing the light, with a sharp edge and a thin papery texture. Wind and salt scorch show as brown, dry, crisp margins and tips, worst on the exposed side, with the boundary between brown and green running roughly parallel to the leaf edge.

Both share one character that separates them from every disease on this site. They do not spread. A fungal infection appears on a few leaves and is on twenty by next week. Scorch is the record of something that happened on a particular afternoon, and it is the same size a fortnight later.

Look also at where it is on the plant. Damage confined to the south or west side, or to the side facing an open field, is a physical cause. Damage scattered through the canopy without regard to aspect is not.

Three different problems under one name

Sun scorch happens when a leaf receives more light than it can use, and the excess energy destroys the pigments and cooks the tissue. Leaves grown in shade have thin cuticles and few protective compounds, so they burn at light levels a sun-grown leaf handles without difficulty. That is why the plant that suffers is almost always one that has just been moved.

Wind scorch is drought at leaf level. Moving air strips away the humid layer that sits against a leaf surface, so water is lost faster than the roots can replace it, and the tissue at the margins dries and dies. On evergreens in February and March the soil is often cold or frozen, so the roots can supply almost nothing while the wind keeps taking, and a laurel or a photinia can be badly burnt by weather that never dropped below freezing.

Salt scorch produces the same margin burn from fertiliser build-up in a pot, from de-icing salt thrown up beside a road, or from coastal spray. Look for a white crust on the compost or the pot.

The water droplet myth

Watering in sunshine does not scorch leaves. The idea is that droplets act as lenses and focus the light, and it has been tested directly. On smooth flat leaves the droplet sits too close to the surface for its focal point to fall on the tissue, and the water evaporates before any damage occurs.

One documented exception explains where the belief came from. On leaves with dense water-repellent hairs, a droplet can be held up above the surface far enough for the focus to land on the leaf. Almost nothing in a British garden has that kind of hairy surface.

So water when it suits you. Early or late is still better, and the reason is evaporation rather than burning.

Why the plant that gets scorched is usually the one that just moved

The pattern is consistent enough to be a rule. A greenhouse tomato hardened off over two weeks is fine, and one carried straight out on a bright May morning is bleached by evening. A monstera that has lived beside a north window for a year burns in a south one. Bedding plants bought from a covered garden centre display and planted the same afternoon are scorched by the weekend.

Leaves cannot be re-engineered once they are formed. What a plant needs is time to produce new leaves suited to the new position, and that takes two to three weeks. Every hardening-off instruction is a way of buying that interval.

What to do about it

Nothing brings a scorched leaf back. The tissue is dead and the plant will replace it rather than repair it, so the work is all in prevention and in getting the plant growing again.

Shade it while it recovers, keep it watered, and leave the damaged leaves on unless they look bad enough to bother you. A plant that has lost half its leaf area to scorch needs the rest, however poor it looks.

Then fix the cause. Move it back and reintroduce it slowly. Shade the greenhouse. Put a mesh screen on the windward side of the new hedge. Flush the pot if there is a salt crust on it. Water thoroughly ahead of the next hot spell, because a plant that is already dry closes its pores and loses the only cooling mechanism it has.

What it gets mistaken for

Drought produces brown crisp leaf edges too, and the two often arrive together. Drought affects the whole plant and shows the compost dry; scorch is confined to the exposed side.

Fungal leaf spots have defined round or angular marks, often with a yellow halo or a darker border, and they appear on more leaves each week. Scorch is a single event.

Frost damage blackens and turns young growth limp rather than papery, and it affects the tips and the youngest tissue rather than the sunny side.

Spray drift and weedkiller damage twist and distort new growth as it expands, which no amount of sun or wind will do.

Red spider mite dulls and stipples a leaf with a fine even speckling, and there is often webbing at the shoot tips. Scorch is a solid patch rather than a scatter of points.

Hard water marks and foliar feed residue leave white deposits that wipe off with a cloth. Scorched tissue is part of the leaf.