Why container tomatoes run out of food so quickly

Tomatoes are hungry plants, and a container is a tiny, enclosed larder. In open ground, roots roam in search of nutrients and moisture. In a pot, every root is competing for whatever you put in that compost back in spring, and there is nowhere else to go. Most multi-purpose composts hold enough nutrients for around four to six weeks of active growth, which takes a young plant neatly to the point where it starts flowering — and then the cupboard is bare just as the plant begins the most demanding job of its life.

That timing is why so many gardeners end up with tall, leafy plants and a disappointing handful of small, tough fruit. The good news is that feeding container tomatoes is not complicated. Get the timing and the type of feed right, keep the watering steady, and you will be picking sweet, heavy crops from a pot on a patio.

Start feeding when the first truss has set

The single most useful rule is this: do not start liquid feeding until the first truss has set. Look for the lowest cluster of flowers on the main stem and wait until you can see tiny green fruitlets forming, roughly the size of a pea or a small marble. Until that point, the plant is building stem and leaf, and the residual nutrients in the compost are usually plenty.

Start too early and you push soft leafy growth at the expense of flowers, which means fewer trusses and a later, lighter crop. Once the first truss has set, the plant's priorities change completely. It now needs potassium to swell fruit, and it will draw on your liquid feed for the rest of the season.

• First truss set: begin feeding.
• Second and third trusses setting: move to full-strength feeds.
• Plants in small pots (under 10 litres): start a week or so earlier, as the compost runs out faster.

Switch to a high-potassium tomato fertiliser

Once fruiting begins, a general-purpose feed is the wrong tool. It is too high in nitrogen, which encourages leaf at the expense of fruit. Instead, choose a dedicated tomato fertiliser with a high potassium (potash) content — look for a formula where the potassium figure is roughly double the nitrogen, something in the region of 4-6-12 or similar. That balance supports flower retention, even ripening and properly flavoured fruit.

If you are growing in a greenhouse or a sunny south-facing spot, the plants will be working hard and will appreciate a feed at every watering during peak summer, mixed at the dilution on the bottle. Outdoors, in a duller or cooler spell, weekly feeding at full strength is usually enough. Never be tempted to double the dose to make up for a missed week; over-strong feed can scorch roots and lock up other nutrients.

Feed little and often, and watch the leaves

Tomatoes in containers respond best to small, regular meals rather than occasional feasts. A weak feed with every watering keeps nutrient levels steady in the root zone and avoids the boom-and-bust cycle that causes split fruit and uneven ripening.

The leaves will tell you how you are doing:

• Yellowing between the veins of older leaves: often a magnesium shortage. A foliar spray of diluted Epsom salts once a fortnight through summer usually corrects it.
• Deep green, curled leaves with few flowers: too much nitrogen. Ease back on general feeds and stick to tomato fertiliser.
• White crust on the compost surface or pot rim: a build-up of salts. Flush the pot with plain water, letting it drain freely, then resume feeding.
• Pale, tired-looking plants with slow fruit: check you have not simply missed a fortnight of feeding.

Water consistently to prevent blossom end rot

Blossom end rot is the heartbreak of container tomatoes: a sunken, dark, leathery patch at the base of the fruit, just as it starts to ripen. It is not a disease and, in most cases, it is not a shortage of calcium in your compost. It is a transport problem. Calcium moves through the plant in water, so if the compost dries out and then floods, the developing fruit at the end of the chain is starved of it and the tissue collapses.

The cure is consistency, not chemistry:

• Water at the base of the plant, in the morning, so the compost is moist before the heat of the day.
• Aim for evenly moist, never soggy and never bone dry. Push a finger two centimetres into the compost to check before watering.
• A 10-litre pot may need watering daily in July, and twice a day during a heatwave. Bigger pots are far more forgiving.
• Mulch the surface with compost, bark or grass clippings to slow evaporation.
• Group pots together and shade the pot itself, not just the leaves, on scorching days.
• Empty saucers half an hour after watering; standing roots in water invites rot.

If a fruit is already affected, remove it. Keep watering evenly and the later trusses should come through perfectly clean.

Keep the routine going to the end of the season

Feeding and watering are not a one-off job but a rhythm that carries you from midsummer into September. As the days shorten and ripening slows, ease the feed back slightly, and remove the growing tip of each plant about four or five trusses up so the plant's energy goes into finishing the fruit it already has. Pick regularly, too — a plant left loaded with over-ripe fruit will slow down and stop setting new ones.

Do those few things well, and a modest pot on a UK patio will reward you with a crop that tastes far better than anything you can buy. There is real satisfaction in a bowl of home-grown tomatoes picked warm from the vine, and it all comes down to feeding at the right moment and never letting the compost swing from dust to swamp.