Why Grow Bags Dry Out So Quickly

A grow bag looks generous when you first buy it — 30 or 40 litres of compost sounds like plenty for two or three tomato plants. The trouble is the depth. Most bags are only 20 to 25 centimetres deep, so the roots have a shallow, wide layer to work with, and every surface of that layer is exposed to the air. The sun warms the black plastic, the wind skims across the top, and moisture evaporates from above and below at once.

Add a mature tomato plant and the demand becomes startling. In the heat of a July afternoon, a well-grown plant can lose three or four litres of water a day through its leaves. Replacing that through a shallow root run is a real challenge, which is why grow bags need more of your attention than a border or a raised bed ever will.

The reward for getting it right is straightforward: steady moisture means smooth, unblemished fruit, and it keeps the plant feeding evenly right through to the end of the season.

Check the Compost Every Single Day

Once plants are flowering, a daily check stops being optional. It takes ten seconds and it will save you a lot of disappointment later in the summer.

• The finger test. Push a finger into the compost up to the second knuckle. If it feels dry at that depth, water. If it is merely damp at the surface but dry underneath, water. Tomatoes want consistently moist, not sopping.
• Water in the morning. Early watering gives plants a full reservoir to draw on through the hottest hours, and any splashes on the leaves dry off quickly rather than sitting overnight.
• Check again on hot evenings. In a heatwave, a morning soak may not carry the plant through to dusk. A second, lighter top-up is often needed.
• Lift the bag. Weight is a surprisingly good guide. A well-watered bag feels solid and heavy; a dry one is light and hollow-sounding.
• Do not trust rainfall. Grow bags have only a few small drainage holes, so most rain runs straight off the plastic and down the sides. A wet day does not mean your plants have been watered.

Water Slowly, and Keep Going Until It Runs Out

The single most common mistake is emptying a full can over the bag in a few seconds. Compost that has dried out becomes water-repellent, and a fast deluge simply channels down the inside of the plastic and out of the drainage holes, leaving the root ball as dry as it was.

Water gently instead, using a can with a rose or a hose on a very low flow, and aim at the compost rather than the foliage. Work along the bag slowly, letting each area absorb before you move on. Keep going until you see water running from the base — that is your signal that the whole depth of compost is saturated, not just the top centimetre.

If the bag has gone really dry, split the job in two. Give it a good soak, wait ten minutes for the compost to take it up, then soak again. A useful trick is to sink a bottomless flowerpot or a short length of pipe beside the stem and pour into that, delivering water directly to the root zone rather than across the surface.

What the Plants Are Telling You

Tomatoes are communicative, and most watering problems show up in the fruit long before the leaves look troubled.

• Split or cracked fruit is the classic sign of irregular watering — a dry spell followed by a heavy soak. The inside grows faster than the skin can stretch.
• Blossom end rot, the sunken dark patch on the base of the fruit, is not a disease. It is a calcium shortage caused by the plant being unable to move calcium properly when moisture swings about.
• Curled, rolled leaves and small, dull fruit usually mean the plant is short of water, especially in strong sun.
• Yellowing lower leaves, algae on the compost surface and a sour smell point the other way: you are overwatering, and the roots are sitting in airless conditions.

Simple Ways to Hold Moisture in the Bag

A few small adjustments make a grow bag far more forgiving.

• Mulch the surface with a couple of centimetres of grass clippings, homemade compost or gravel to slow evaporation.
• Stand the bag on a saucer or tray. Runoff collects there and the compost wicks some of it back up during the day.
• Turn up the sides of the bag to create a shallow trough, so water soaks in rather than running off the edge.
• Shade the plastic. A plant in front of the bag, or a sheet of card leaning against the sunny side, keeps the compost cooler.
• Consider a self-watering tray or a drip line on a timer if you have several bags. It is the closest thing to a guaranteed even supply.

Hot Spells, Holidays and Late Summer

During a heatwave, water morning and evening and check the bags at midday. Move what you can into light shade for the hottest hours. If you are going away, ask a neighbour to check daily rather than relying on a single enormous soak before you leave — and if that is not possible, mulch heavily, group the bags together in a shaded spot and stand them in deep trays of water.

As the fruit ripens, keep the routine steady. Many growers ease off slightly to concentrate flavour, but an abrupt change is exactly what causes splitting. Reduce gradually, keep testing the compost with your finger, and your plants will reward you with fruit that ripens evenly right through to the first frosts.