Lesson 4
Watering, feeding, and preventing problems
Watering is where most container tomatoes fail. The aim is consistent moisture — never waterlogged, never dust-dry. In high summer, a 30cm pot needs watering once daily, sometimes twice. Check by lifting the pot (heavy means wet, light means dry) or pushing a finger 5cm into the compost. Water until it runs from the base. Irregular watering causes blossom end rot (black sunken patches on the fruit base) and split skins on heirlooms.
Feed once the first truss of flowers appears. Use a tomato feed (high potassium, typically 4-5-8 NPK) at label strength, twice weekly. Paste tomatoes like San Marzano are heavy feeders — if lower leaves yellow while new growth stays green, increase feed frequency. Heirlooms are more forgiving but still need regular potassium for flavour and firm fruit.
Blossom end rot is calcium deficiency triggered by inconsistent watering, not lack of calcium in the compost. The fix is better watering discipline, not adding lime. Split fruit on heirlooms happens when heavy rain or watering follows a dry spell — the fruit swells faster than the skin can stretch. You cannot prevent every split, but consistent watering reduces it. Greenfly and whitefly arrive in July; wash them off with a hose or tolerate them unless infestation is severe.
Feeding and watering rules
- Start feeding when the first flowers open, not before
- Use tomato feed at label rate, twice weekly, every week until harvest ends
- Water at the same time each day in summer — early morning or evening
- In very hot weather, water twice daily rather than once heavily
- Stop feeding two weeks before the first expected frost to harden off remaining fruit
Common mistakes
- Overfeeding early — too much nitrogen before flowering gives leaves, not fruit
- Letting pots dry out completely — roots die back and blossom end rot follows
- Feeding with general-purpose fertiliser — tomatoes need high potassium, not balanced NPK
Stand pots on saucers in a heatwave — the reservoir gives you an extra 12 hours before the plant wilts.