How Often to Water and Feed Tomato Plants

Start feeding when the first fruit is golf ball size

The first feed should go on when the fruit on the lowest truss has swelled to about the size of a golf ball. Feed any earlier and the plant runs to leaf and stem, the flowers drop, and no fruit sets at all. Whatever you worked into the bed at planting is enough to carry the plant to that point.

After that, feed every two to three weeks. Scatter a handful of a balanced granular fertilizer around the plant and scratch it into the surface. Keep it about eight inches (20 cm) away from the stem, because fertilizer against the stem burns the roots that matter most.

1. Judge it by fruit size, not by the calendar While the lowest fruit is smaller than a golf ball, wait. Once it starts to swell, apply the first feed in a ring around the plant.
2. Keep it eight inches from the stem The feeding roots are at the edge of the leaf canopy, not at the stem. Apply it out there and scratch it in lightly.
3. Repeat every two to three weeks Write the date down, or log it in a growing app. Feeding by memory is how plants end up either starved or overfed.

Look at the plant each time before you feed. If the leaves are very dark and the tips are curling inwards, skip that feed. That is the plant telling you it has had too much nitrogen.

The leaves tell you whether you have the feeding right

A tomato shows you its nutrient status in the shape of its leaves. Leaves at the tip curling tightly inwards, with a thick stem and short gaps between leaf joints, means too much fertilizer. Leaves at the tip held open and looking pale, with a stem getting thinner, means it is running out.

The trusses tell you as well. If a truss produces a leafy shoot at its end instead of flowers, that is overfeeding. Small, pale flowers mean underfeeding. Read the plant rather than counting weeks.

Water deeply, but only once the soil has dried

A tomato in open ground needs very little watering once its roots are down. Under a plastic mulch, rainfall alone will usually carry it through early summer. Tomatoes are drought tolerant, and overwatering gives you a watery flavor and a shallow root system that then suffers badly in the heat.

The exception is while the fruit is swelling. A sudden flood of water at that stage splits the fruit. In a dry spell, rather than one heavy soak, water a smaller amount every few days so the moisture level in the soil stays steady.

1. Push a finger into the soil The surface can look dry while the soil below is damp. If it is moist at a depth of two inches (five cm), do not water. If it is dry, water the base thoroughly.
2. Water in the morning Water in the evening and the plant sits damp all night, which invites disease. Water early, while it is still cool.
3. Keep water off the leaves Wet foliage spreads blight and leaf spot. Hold the can low and put the water on the soil at the base of the plant.

Containers need water every day, twice in high summer

There is only so much compost in a pot, so container tomatoes need watering daily, unlike plants in the ground. Water until it runs from the drainage holes, and tip away anything left standing in the saucer. In the hottest weeks that may mean morning and evening.

All that watering washes the nutrients out, so containers need feeding differently: either a smaller granular dose every week, or a liquid tomato feed once a week. Little and often beats one large application.

Blossom end rot is about water before it is about calcium

A black, sunken patch on the bottom of the fruit is blossom end rot. It is not a disease. It happens when calcium does not reach the end of the fruit, and while a genuinely calcium-poor soil can cause it, far more often the calcium is there and the plant cannot take it up because the soil keeps drying out or because it has had too much nitrogen. Correct the watering and it usually stops with the next truss.

Fruit that already has it will not recover. Pick those off so the plant stops spending energy on them, and concentrate on keeping the soil evenly moist. Working lime into the bed before planting takes the soil side of the problem off the table.

1. Pick off affected fruit A tomato with a blackened end will not size up. Remove it so the plant can put that energy into the rest of the truss.
2. Stop the soil drying out Mulch with straw or plastic to slow evaporation, and water the base when it dries. The bigger the swings in soil moisture, the worse it gets.
3. Skip a feed Excess nitrogen blocks calcium uptake. If the leaves are curling, miss out the next feed.

How to water to stop fruit splitting

When heavy rain or a big watering arrives just as fruit is coloring, the inside swells faster than the skin can stretch and the fruit splits. You cannot control the weather, but a simple rain cover over the row cuts splitting dramatically. Even a sheet of clear plastic over the tops of the stakes helps.

Without a cover, pick fruit slightly early once it starts to color and let it finish indoors. A tomato ripened on the windowsill is far better than one that split and rotted on the plant.

1. Pick early when rain is forecast Any fruit that is half colored can come off ahead of a downpour. It will finish coloring indoors in two or three days.
2. Hold off watering after rain For a few days after heavy rain, do not water at all. Adding more to already wet soil brings on both splitting and blossom end rot.

Working out what went wrong

Curling leaves, dropping flowers, small fruit and yellow lower leaves nearly always come down to water or feed. Change one thing at a time, otherwise you will never know which of them was the problem.

1. Flowers drop without setting Too much nitrogen, or heat above about 90F (32C). Stop feeding and tap the stake gently on a sunny morning to shake the pollen loose.
2. Fruit stops growing while small Either hunger or drought. Check the soil: dry means water, damp means a weak liquid feed.
3. Lower leaves yellow first Struggling roots or a shortage of nitrogen. Suspect overwatering first, let the soil dry a little, then feed.
4. Leaf edges go brown and crisp Fertilizer burn or a soil that has dried out hard. Water thoroughly to dilute the salts and delay the next feed.