Raychis

Disorder

Tipburn

Also called Internal tipburn, Marginal leaf necrosis, Blackheart of celery, Internal browning, Black tip

Young inner leaves with scorched brown edges, often hidden until the head is cut open. Calcium reaches the leaves that transpire, and the leaves buried in the heart of a head do not transpire at all.

A row of green lettuce with brown papery scorching along the margins of the inner leaves, the outer leaves still sound
The margins go and the rest of the leaf stays green, on the youngest leaves rather than the oldest. The photographer recorded this after a frost, which fits: any check that interrupts the flow of calcium to the fastest-growing tissue produces the same edge.Photo by Dwight Sipler from Stow, MA, USA, CC BY 2.0. Resized from the original.

At a glance

Type
Disorder
Cause
Feeding or nutrients
Severity
Destroys the crop or display
Affects
Herbs and edibles
Active
May to September

What you are seeing

  • Brown or black scorched margins on young leaves, with older outer leaves clean
  • Nothing visible from outside until a lettuce or cabbage is cut in half
  • A brown crescent along the edge of every leaf through the middle of a cut head
  • The growing point of celery blackened and dead in the centre of the stalks
  • Damage appearing over a few days during fast growth in hot bright weather
  • Soft rot and slime following in the dead tissue, especially in humid weather
  • Whole beds affected at once, with some varieties clean beside badly hit ones

Before you treat anything

The browning itself is harmless to eat. Discard any head where the dead tissue has gone soft, slimy or smelly, since bacterial soft rot follows tipburn into the same leaves and that is not something to wash off.

What you are looking at

Cut a lettuce in half from top to bottom. Through the middle of the head, every leaf carries a brown or black crescent along its edge, dried out and papery, sometimes only a millimetre wide and sometimes a centimetre. The outer leaves that wrapped around them are perfect.

That is the shape of tipburn and it is why so many people meet it in the kitchen rather than in the garden. A crisphead lettuce, a Chinese cabbage or a white cabbage can be ruined inside and flawless from the path.

Celery shows a version of the same thing with its own name. Blackheart kills the growing point in the centre of the stalks, and the plant carries on looking respectable while the middle turns black.

Where humidity is high the damaged margins go soft and slimy within a couple of days, because bacterial soft rot moves into dead tissue readily.

The leaves that get missed

Calcium goes where the water goes. It is carried up from the roots in the transpiration stream, pulled along by evaporation from the leaf surface, and once it has arrived somewhere the plant cannot move it again. Every part of a plant therefore gets calcium in proportion to how much water it evaporates.

Now think about a leaf in the centre of a lettuce heart. It is enclosed on all sides by other leaves, it sits in still saturated air, it gets no light and no wind, and it evaporates almost nothing. It is also the fastest-growing tissue on the plant, expanding daily and building new cell walls that need calcium to hold together.

Those two facts are in direct conflict, and the margin of the leaf is the point furthest from the vein supply, so the margin is where the shortage bites first. The cells there fail to build proper walls, collapse, and go brown.

The soil almost never comes into it. A bed with abundant calcium produces tipburn in exactly the same way, because the problem is delivery rather than supply.

Why humid nights make it worse

Growers of glasshouse lettuce run fans overnight, and the reasoning follows straight from the mechanism above.

A still, humid glasshouse at two in the morning has no evaporation happening anywhere on the plant. The water stream slows to nothing, the calcium stops moving with it, and the inner leaves spend the night expanding on whatever they already have. Move air across the crop and evaporation restarts, the stream picks up, and the heart gets its share.

That is also why tipburn is worst in hot bright weather following rain. The plant grows fastest, the humidity is highest, and the two effects compound.

Outdoors you cannot run a fan over the bed, so the controls are the ones that slow the growth rate down rather than speed the delivery up. Steady watering, restrained feeding, sensible spacing and a prompt harvest are all versions of the same instruction.

The third page in a family of three

Anyone who has grown tomatoes or stored apples has met this before under different names.

Blossom end rot is the same failure in a tomato, a pepper or a courgette. The end of the fruit furthest from the stalk is the last tissue the water stream reaches, it goes short of calcium while the fruit is filling fastest, and it collapses into a leathery brown patch.

Bitter pit is the same failure in an apple, showing weeks later in store as many small sunken pits with corky brown flecks beneath them.

In each of them the plant has plenty of calcium available and the growing tissue gets none of it, and in each the trigger is irregular watering, hard feeding and a plant growing flat out. Adding calcium to the soil makes no difference to any of the three, which absorbs a great deal of gardening effort every summer to no effect at all.

Foliar calcium sprays do something for the apple, because the fruit is out in the open where a spray can land on it. They do very little for tipburn, since the leaves that need it are sealed inside the head where nothing sprayed can reach.

What to do about it

Buy varieties listed as tipburn tolerant, and grow loose-leaf lettuce rather than crispheads in a hot summer.

Feed lightly and skip the late nitrogen.

Water regularly enough that the bed never dries out and never gets flooded.

Cut heads as soon as they firm up rather than growing them on for size.

Space at the recommended distance rather than wider.

Run a fan or open the vents overnight under glass, and shade the crop through the hottest hours.

Test the pH before liming, and expect the lime to make no difference to this.

Strip the browned leaves and use the rest of the head the same day.

What it gets mistaken for

Fertiliser scorch burns the oldest outer leaves first and works inwards, which is the reverse of the pattern here.

Sun scorch bleaches and browns whatever the light falls on, and the inside of a cabbage is the one place it cannot reach.

Downy mildew makes pale angular patches bounded by the leaf veins, with a grey bloom on the underside in damp weather.

Slug damage removes tissue and leaves slime. Tipburn kills the margin in place without taking a bite out of it.

Bolting stretches the plant upwards and turns the leaves bitter, and a bolting lettuce and a tipburned one are frequently the same plant, since hot weather causes both.

What to do, in order

  1. Start here

    Prevention, and the changes that stop the problem returning. Most of these pages should end at this tier.

    Choose varieties bred for resistance
    Susceptibility varies more between cultivars than between growing methods, and it is the strongest lever available. Iceberg and other crisphead lettuces suffer worst, cos types less, and loose-leaf types that never form a closed heart barely at all. Seed catalogues list tipburn tolerance for lettuce and Chinese cabbage, and it is worth reading before ordering.
    Do not push the crop with nitrogen
    A plant growing flat out expands leaf tissue faster than the water stream can supply calcium to it, and a high-nitrogen feed is the usual way of producing that. Feed a leafy crop enough to keep it moving and stop there, and leave the late top-dressing off altogether.
    Water steadily and never let the bed swing between wet and dry
    Calcium travels in the water stream and stops when the stream does. A dry spell followed by a soaking gives a burst of growth on tissue that was starved of calcium while it was forming, which is when the margins die.
    Cut as soon as the head is ready
    The damage builds up during the last stage of filling, so a lettuce left an extra week to gain weight is a lettuce more likely to be brown inside. Cut on readiness rather than on size.
    Space for airflow rather than for maximum size
    Wide spacing produces bigger heads, and a bigger head has more enclosed inner leaf and a longer distance to move calcium. Standard spacing and a prompt cut beat show-sized plants in a bad year.
    Keep the salt level in the soil down
    A high concentration of dissolved salts, whether from heavy feeding, manure applied fresh or irrigation water carrying dissolved minerals, makes it harder for the roots to take up water and calcium with it. Growing bags and glasshouse borders cropped year after year are the usual offenders.
    Leave the soil calcium alone unless a test says otherwise
    Almost every garden soil already holds far more calcium than the crop needs. Liming a bed that is not acid will not prevent tipburn and it will lock up other nutrients. Test the pH first, correct it if it is low, and expect no improvement in the tipburn.

  2. Then reach for a barrier

    Netting, hand removal and water. Nothing here harms anything except the target.

    Move air over the crop at night, under glass or fleece
    Calcium arrives only where water is evaporating from the leaf, and a still, humid glasshouse at night stops evaporation completely, which is when the inner leaves go short. A fan running overnight is a standard commercial control for lettuce tipburn and it works in a polytunnel too. Venting to drop the humidity does the same job.
    Shade through the hottest part of the day
    Heat drives the growth surge that outruns the calcium supply. Shade netting over a summer lettuce crop slows it down and produces a cleaner heart, alongside its more obvious benefit of stopping the crop bolting.
    Cut the damage out and use the rest
    A lettuce with a brown margin on the inner leaves is unappetising rather than unsafe. Strip the affected leaves, wash the rest and use it the same day, since the dead tissue is where the soft rots start.

Plants this turns up on

Guides that name it among the problems to watch for. The list is the plants it is characteristic of rather than everything it will feed on.

Sources