Fruit splitting

What you are looking at

A tomato with a clean split running down from the stalk, opened far enough to show the flesh inside. Or a cherry that was perfect on Friday and has a tear across the tip on Saturday morning. Or a cabbage that has burst from the middle outwards, the head cracked down one side like a log.

In every case the fruit was sound days earlier, the biggest and ripest ones went first, and it happened after rain or after a heavy watering that followed a dry spell.

The crack itself does no immediate harm. What follows it does. Grey mould and brown rot both need a wound to get started, and a split fruit left on the plant gives them one.

Why the skin gives way

A fruit stops making new skin long before it stops filling. From the point it begins to colour, the cuticle can stretch a little and grow hardly at all, while the flesh underneath goes on taking water and sugar. The skin spends the last weeks of ripening under increasing tension, like an overfilled bag.

Add water faster than the skin can accommodate and it tears. A dry fortnight followed by 20 mm of rain does it, and so does a gardener who forgets the growing bags for four days and then floods them.

Variety sets most of what the skin will take, because thickness and elasticity are inherited and vary hugely between cultivars of the same crop. Size accounts for a good deal of the rest, since a large fruit has a larger surface to hold together. And fine cracking of the cuticle, invisible without a lens, weakens the skin over the preceding weeks so that a fruit which has sat through repeated wetting splits at a shower that would not have troubled it in June.

Two different cracks on a tomato

Radial cracks run from the stalk scar down the fruit, straight and open. These are the water ones. They follow a surge of uptake, and they are answered by mulching, steadier watering and picking before rain.

Concentric cracks make fine rings around the shoulder, often shallow and healed over into brown corky lines rather than gaping. These come from swings in temperature and from direct sun falling on the exposed shoulder of a fruit, which is why they show up on trusses that lost their leaf cover to over-enthusiastic defoliation. Ventilation, shading and leaving the foliage alone do more about them than the watering can does.

A plant can produce both at once, and the two need different fixes, which is worth sorting out before changing how you water.

The cherry takes water through its skin

Cherry growers cover their orchards, and the reason is a piece of physiology that surprises most gardeners.

A cherry does not crack mainly because its roots took up water. It cracks because rain sat on the fruit. The flesh of a ripe cherry is a concentrated sugar solution, the skin is permeable to water, and a drop of pure rainwater resting on that skin is drawn straight through it by osmosis. The cells beneath swell within minutes, and the skin over them fails.

That explains why a cherry on a tree in dry soil still splits in a summer shower, and why watering advice on its own disappoints. It also explains why a roof over the tree works so well, and why knocking the water off within the hour saves a large part of the crop.

Grapes crack for a mix of both reasons. An outdoor vine in a wet autumn suffers and one under glass with dry feet does not.

Underground, and in the cabbage patch

Carrots, parsnips and radishes split lengthways after the same sequence of dry weeks and heavy rain. The root has laid down its outer tissue at one growth rate, the core resumes at another, and the two part company.

A cabbage does something similar on a larger scale. The head is a ball of tightly packed leaves that finished expanding some time ago, wrapped around a core that has not. When the plant takes up water quickly the core pushes outwards and the head splits from the inside.

Cutting promptly is the answer to all of these. Where a row is ready at once and you cannot use it, root-pruning the cabbages buys a fortnight, and roots left in the ground through a wet autumn will split whatever you do.

What to do about it

Water little and often from the point the fruit starts to colour, and mulch everything, containers first.

Pick tomatoes, cherries and plums at first colour when heavy rain is forecast, and ripen them indoors.

Roof a cherry from colour break, or shake the tree out within an hour of a shower.

Choose crack-tolerant varieties, and ask about it when buying a cherry.

Feed with potassium rather than pushing nitrogen, and keep the feed regular.

Ventilate and shade a greenhouse to flatten the temperature swings behind concentric cracking, and stop stripping leaves off the trusses.

Cut cabbages the week they feel solid, and root-prune the ones you cannot use yet.

Thin congested bunches of grapes while the berries are small.

Pick off split fruit whenever you harvest, and use it that day.

What it gets mistaken for

Blossom end rot makes one dark sunken leathery patch at the end opposite the stalk, and the skin over it stays whole.

Brown rot spreads soft decay with rings of buff cushions on the surface, and it frequently starts inside a split rather than causing one.

Grey mould coats the wound in fuzzy grey. Both it and brown rot are what happens next rather than what happened first.

Bitter pit puts many small sunken dimples in an apple with brown flecks in the flesh underneath.

Capsid bug damage was there when the fruit was tiny, and it shows as corky brown scars and lopsided growth rather than a fresh tear.

Russeting is a rough brown netting spread over the skin of certain apple and pear varieties, and it is a characteristic of the fruit rather than a fault.