How often should I water and feed tomatoes in a growbag?

Why a growbag is the hardest place to grow a tomato, the watering rhythm that prevents splitting and blossom end rot, and when to switch feeds.

Difficulty: beginner
Time: 10 min
Read: 5 min

Short answer

Check twice a day in summer and water whenever the top few centimetres are dry — often 1–2 litres per plant per day at peak, split into two goes. Feed with a high-potash tomato feed at every watering or every second watering from the moment the first truss sets, and never let the bag dry out completely.

A standard growbag holds around thirty litres of compost and is usually planted with three tomatoes, which by August are each transpiring more than a litre a day. It is a small reservoir supporting a very large demand, and almost every growbag tomato problem — splitting, blossom end rot, bitter fruit, sudden wilting, a plant that stalls in July — traces back to the same swing between wet and dry.

Step by step

Plant two per bag rather than three.The printed planting positions assume commercial watering. Two plants in a domestic bag get half as much again of root run each, and the difference in reliability across a hot summer is large.
Increase the compost volume if you possibly can.Stand bottomless pots or ring culture pots on top of the bag and plant into those, so the roots have depth as well as the bag's width. It roughly doubles the buffer and makes watering far more forgiving.
Cut drainage slits low in the sides, not holes in the base.Slits about 2cm up from the bottom leave a shallow reservoir the roots can draw on while still letting excess out. Holes in the base drain it dry within minutes of watering.
Check moisture with a finger, twice a day in summer.Push a finger in to the second knuckle. Damp, leave it; dry at that depth, water. The surface tells you almost nothing, especially with a peat-free compost that dries pale on top.
Water little and often rather than a flood every other day.In a heatwave, half a litre per plant morning and evening beats two litres once. Steady moisture is what prevents splitting and blossom end rot — both are caused by the swing, not the total.
Start feeding when the first truss sets fruit, not before.Growbag compost carries enough nutrition for about four to six weeks. Feeding earlier than that pushes leafy growth at the expense of flowers.
Use a high-potash tomato feed, at the stated dilution.Potash drives flowering and fruiting; nitrogen drives leaf. Feed at every watering at half strength, or every second watering at full strength — either works, and the first is gentler on the plant.
Flush the bag with plain water every couple of weeks.Constant liquid feeding leaves salts behind in a closed bag. A good plain soak, letting water run out of the slits, washes them through and prevents the scorched leaf margins that show up in August.
Mulch the surface of the bag.A layer of compost, bark or even a strip of cardboard over the open slit cuts evaporation noticeably and keeps the compost temperature steadier in full sun.

Tips

A simple gravity drip kit on a timer is the best £20 a growbag tomato grower can spend, because it removes the human variability that causes most of the damage.
Peat-free composts hold water differently from the old peat bags — they look dry on top while still damp beneath, so trust the finger test rather than the colour.
If a bag has dried out completely and water runs straight through, stand the whole bag in a shallow tray of water for an hour to re-wet it from below.
Water at the base, never over the foliage. Wet leaves in a greenhouse at night are how botrytis and blight get established.
Warm the watering can in the greenhouse rather than using cold mains water straight onto roots on a hot day.

Common mistakes

Following a schedule instead of checking the compost — Demand changes by a factor of five between a cool cloudy week and a heatwave. A fixed 'twice a week' either drowns the plant in June or starves it in July.
Starting tomato feed at planting — Fresh growbag compost already contains a base fertiliser. Adding more nitrogen-heavy feed on top produces a huge leafy plant that flowers late.
Standing the bag in a permanent tray of water — Roots sitting in water go anaerobic and rot. A tray is useful to catch run-off and for occasional bottom-watering, not as a reservoir the bag sits in all summer.

If it doesn't work

Wilting at midday, recovering by evening

Cause: Transpiration outpacing the small root volume, not necessarily dry compost — Fix: Check moisture first. If damp, provide shading and ventilation rather than more water — watering a wet bag makes it worse.

Black sunken patches on the bottom of the fruit

Cause: Blossom end rot from interrupted water supply during fruit formation — Fix: Even out watering to twice daily, mulch the bag, and increase the compost volume. Fruit already affected will not recover.

Leaf margins scorched brown and dry in late summer

Cause: Salt build-up from continuous liquid feeding — Fix: Flush with plain water until it runs freely from the slits, then resume feeding at a lower concentration.

Water runs straight through and out of the slits

Cause: Compost has dried past the point where it will re-wet — Fix: Bottom-water by standing the bag in a shallow tray for an hour, then keep it from drying to that point again.

Questions people ask

How much water does a growbag tomato need per day?

At peak in mid-summer, roughly one to two litres per plant per day, less in cool or cloudy weather and very little in spring. That is a guide, not a rule — the finger test in the compost is what should decide it on any given day.

Can I reuse a growbag next year?

Not for tomatoes in the same bag. The nutrition is exhausted and soil-borne problems build up. Empty it onto a bed or a border as a soil improver and start with fresh compost for the next crop.

Is tomato feed different from ordinary plant food?

Yes, in the ratio. Tomato feed is high in potassium relative to nitrogen, which is what pushes a plant towards flowers and fruit rather than leaves. Any high-potash feed will do the same job on any fruiting crop — peppers, chillies, courgettes, aubergines.