Common Mistakes When Growing Tomatoes in Containers

Tomatoes are one of the easiest vegetables to grow in a pot and one of the easiest to grow badly. The plant in June looks magnificent. By August there are four small fruits, some yellowing leaves and a lot of bare stem. Container growing leaves no room for error: the volume of compost is limited, the water supply is entirely your job, and the roots cannot go looking for anything better.

Most disappointing container harvests trace back to a few decisions made at planting time, or to habits repeated every evening with a watering can. These are the ones that cost the most.

A Pot That Fits the Plant, Not the Balcony

Tomato roots want depth and volume, and almost every struggling container plant is living in a pot that is too small. A 10-litre pot on a sunny balcony can dry out in a single July day, and a plant that spends its afternoons wilting will set fewer, smaller fruits.

Aim for at least 20 litres of compost per plant for full-size varieties, and 30 to 40 litres if you have the space. The extra volume buys steadier moisture and a much more forgiving plant. Compact bush types and cherries will manage in 12 to 15 litres, but not less. Depth matters as much as width.

Drainage comes next. One small hole in the base is often not enough, especially if the pot stands on a saucer, so cover it with a shard or a handful of grit to keep it clear. Leave a couple of centimetres of rim free so water has somewhere to sit and soak in rather than running straight off the surface.

Watering: The Problem Is the Swinging, Not the Amount

The classic container tomato fault is not underwatering or overwatering. It is going from bone dry to soaking wet and back again. That swing splits fruit after a heavy watering and sits behind most blossom end rot, which is a calcium transport problem made worse by uneven moisture rather than an actual shortage of calcium in the compost.

Water thoroughly, in the morning where possible, until it runs out of the base, then leave the plant alone until the top two or three centimetres of compost are dry. Check with a finger, not with a glance.

• Mulch the surface with three to five centimetres of straw, dried grass clippings or compost. It slows evaporation and keeps the root zone cooler.
• Skip the daily sprinkle. Small amounts of water wet only the surface and encourage shallow rooting.
• Use saucers with care. Catching runoff is fine; leaving a pot standing in water for days is not.
• Check twice a day in a heat wave. A dark pot in full sun can need water morning and evening.

If you can, shade the pot rather than the plant. Roots suffer in a black container pressed against hot paving, and stressed roots stop taking up water just when the fruit needs it.

Staking: Done Late, Done Loosely

Support usually gets added in July, when the plant is already leaning, which means pushing a cane through the root ball. Put the stake, spiral or cage in at planting time instead.

For an indeterminate cordon variety that will reach two metres, one thin cane and a single tie near the top will not hold it. Tie the stem every 20 to 30 centimetres with soft twine in a loose figure of eight, so the stem has room to thicken, and check those ties as the season goes on. A tie that fitted in June can cut into the stem by August.

On a balcony, wind does more damage than height. A tall plant in a light pot topples, so weight the base with stones or group pots together. Bush varieties need less height but still appreciate a short stake to keep fruit off wet ground.

Feeding: Neither Starve Nor Overfeed

Fresh potting compost feeds a tomato for around six weeks. After that, the plant depends on you, and this is where containers differ most from open ground.

The mistake runs in both directions. Some pots never see a drop of feed, and the plant yellows and stalls with fruit half-grown. Others get a nitrogen-heavy feed every few days and produce a magnificent leafy plant with very little fruit. Nitrogen pushes leaf growth; potassium supports flowering and fruit.

Once the first truss has set, switch to a tomato feed and follow the label, usually once a week or at half strength at every watering. Do not exceed the dose. More feed is not more tomatoes, and excess salts build up quickly in the small volume of a pot.

If leaves yellow between the veins while the veins stay green, a magnesium sulphate foliar feed is the traditional remedy. Spray in the evening so the leaves absorb it rather than drying out in full sun.

Garden Soil and Last Year's Compost

A pot filled with soil dug from the garden is a quiet killer of container tomatoes. Garden soil compacts in a container, drains badly, and can bring weed seeds and soil-borne problems with it. Use a potting mix made for containers and add well-rotted compost for structure and slow nutrition, roughly one part compost to three or four parts mix.

Last year's compost is spent. It can be reused for salads or flowers, but tomatoes need fresh mix and fresh feed. If you do reuse it, expect to start feeding earlier and more often, and never reuse it if the previous crop showed signs of disease.

Crowding, Airflow and Pruning

Two plants in one pot is the simplest way to halve a harvest. They compete for water and nutrients, and the loser shows it. Keep to one plant per 20 to 30 litres.

Air matters as much as space. Set pots so leaves do not touch, and take off the lowest leaves up to the first truss. That keeps foliage away from the soil where spores splash up, and lets air move through the plant, which is the cheapest disease prevention in container growing.

On cordon varieties, pinch out the side shoots that appear in the leaf joints while they are small enough to snap with a thumbnail. Do not do this to bush varieties: their fruit comes on those side shoots, and pruning them removes the crop.

A Five-Minute Weekly Walk

Most container tomato problems are prevented rather than fixed. Once a week, do the same short round:

1. Push a finger into the compost and water only if the top few centimetres are dry.
2. Tie in any new growth and check last week's ties.
3. Pinch out side shoots on cordon varieties.
4. Remove yellowing lower leaves and any that touch the soil.
5. Feed according to the label, then look under the leaves for aphids or whitefly.

That is the whole routine. It takes less time than a phone call and heads off most of what goes wrong.

Photo: Nataliya Vaitkevich / Pexels