Why are my tomatoes splitting and cracking?

Almost everyone who asks this question is watering plenty. That is the confusing part. A tomato does not split because it had too much water; it splits because the supply changed. A plant in a growbag or a pot goes through that change every hot week, which is why container tomatoes split far more than plants in the ground.

Step by step

1. Look at the crack pattern before you change anything.Cracks radiating out from the stalk scar down the shoulder are deep and let rot in — use those fruit at once. Fine rings circling the stalk are concentric cracking, usually shallow and corky, and those fruit keep and eat normally.
2. Accept that the fruit already split will not heal.Pick anything cracked as soon as you see it. Left on the plant in a warm damp spell it moulds within a day or two, and the mould spreads to the fruit touching it.
3. Change the rhythm of watering, not the volume.The aim is that the compost never swings between bone dry and saturated. In hot weather that usually means a smaller amount twice a day rather than a soaking every other day.
4. Check the pot or bag first thing and last thing in a heatwave.A growbag holds very little compost for the size of plant standing in it. On a hot day it can go from damp to dry between breakfast and lunch, and that is exactly the swing that splits fruit.
5. Mulch the surface of pots and beds.A couple of centimetres of compost, bark or even a sheet of cardboard on the surface slows evaporation and flattens the peaks and troughs. In the ground it makes a bigger difference than any change to how much you pour on.
6. Give the roots more compost if you can.Two plants in a standard growbag is a lot of plant on very little root run. Standing bottomless pots on the bag, or planting into a 30-litre pot instead, roughly doubles the buffer against drying out.
7. Shade the pots rather than the plant in extreme heat.Sun on the side of a black pot or bag drives the compost temperature up and the water out. Grouping containers, or standing a board on the sunny side, steadies the root zone without cutting the light the fruit needs.
8. Pick fruit slightly under-ripe during a wet spell.Heavy rain after a dry fortnight is the classic split trigger outdoors. Fruit showing full colour will finish ripening indoors in a day or two, which takes it out of the firing line.

Tips

• Cherry and small-fruited varieties split more readily than large beefsteak types because the skin-to-flesh ratio is higher — if you split every year, changing variety is a legitimate fix.
• Splitting and blossom end rot have the same root cause. If you are getting both, the watering rhythm is the thing to fix, not the feed.
• A drip or wick system on a timer is the single most effective answer for anyone growing in bags who is out at work all day.
• Split fruit is perfectly good for sauce or soup — cut out the crack and use it straight away rather than composting it.

Common mistakes

• Responding to splitting by watering less — Less water makes the dry spells deeper, so the next drink produces a bigger swing and more splitting. Consistency is the fix, not restriction.
• Adding calcium or a specialist feed — Splitting is mechanical, not nutritional. Feed changes nothing, and over-feeding a plant that is already water-stressed makes the compost harder to re-wet.
• Letting a growbag dry out completely once — Peat-free compost that has dried right out repels water — it runs down the sides and out of the slits, so the plant stays dry despite the can being emptied on it.

If it doesn't work

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