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Transpiration and evaporation

Water is drawn into a plant via its roots. They take up water from the soil, and the most accessible source of water for them is the water held in capillary spaces – the small spaces within and between particles of organic matter.

How much water a soil or compost will hold is known as its water holding (or field) capacity. It is scientifically measured by waterlogging a soil and then allowing it to drain naturally via the force gravity exerts on the water. The amount of water left in the soil is the field capacity. Think of a sponge that has been drenched in water and then left on the side to drain, without squeezing. Eventually water stops leaking out, but the sponge is still wet. The water capacity of a compost depends on what it’s made from – the addition of sand makes a compost more free draining; a higher organic matter content increases water retention.

Plant roots push water upwards by creating root pressure, a small force that is enough to supply water to the whole of a low-growing plant. If you notice water droplets appearing at the tip of plant leaves in the morning (which aren’t dew) those are guttation droplets, a direct result of root pressure.

Larger plants need water to circulate right to the top, and root pressure isn’t enough to power that, but the water gets a lift from transpiration, which we met in Chapter 2. Water loss from the plant’s leaves pulls water up from lower down the plant, and transpiration is strong enough to pull water up to the top of the tallest tree. A plant’s water pumping system is completely silent – a feat of engineering we simply can’t match.

Plants also have to contend with evaporation, which is essentially the vaporization of water from soil or plant surfaces. Evaporation increases with temperature, and so will be higher for containers that are placed in full sun. Giving your plants some shade is one way to decrease the frequency with which you have to water, but isn’t an option for plants that only thrive in full sunlight. Wind also increases evaporation, and even a light breeze can drastically reduce the amount of water available to plants. A wind-break can help, particularly when plants are young and less tolerant of drying out.

How to water containers

Plants growing in containers are almost entirely dependent on us for their water supply. How often you need to water depends on both the plant, and the container you choose.

Short-lived and small plants are happy in shallow troughs and small containers – a patio supply of strawberries and salad needn’t take up much space. The trade-off you make for using small containers is that you need to water them regularly, at least once a day in hot weather, possibly more often. Larger containers hold more water, and so need watering less often provided they are not overloaded with plants.

It can be very helpful to stand containers in trays and saucers in hot weather – they catch any water that drains through the pot, and keep it available to plants as the compost dries out. It means you need to water less often, and also that fewer nutrients are allowed to run-off. Trays and saucers need to be removed in cold or wet weather so that pots are not left standing in water for any length of time.

Home-made self-watering container

Self-watering pots (and you can make your own versions) have a water reservoir at the bottom, then a shelf to retain the potting compost and a drainage hole above that. A tube allows you to water into the bottom of the pot. Your plants send roots downwards to look for water from the reservoir, but the bulk of their roots will be safe from waterlogging, in the growing area of the pot. As the surface of the compost remains dry, water losses from evaporation are much lower.


Another way to reduce evaporation is to mulch the soil surface. A mulch is basically a layer of material that covers the soil surface. It can be something organic (think bark chips, compost or lawn clippings) or inorganic (anything from plastic sheeting through to decorative stones or shells). Mulching also helps with weed control, and keeping your containers weed-free is essential to ensure that the water you pour in goes to your plants.

When to water

When they’re grown in the ground, plants send out roots into the areas of surrounding soil where water is to be found. If the top layers of soil are wet, they send out shallow roots. If the top layers are dryer, plants are encouraged to root deeper. Deeper roots are more able to supply plants with water during periods of drought. To encourage soil-grown plants to grow these deep roots, water is best supplied in big drenches less often. Regular, smaller dribbles of water will encourage shallow roots.

This is less of an issue with containers, where soil depth is small, but it is important that plants have a regular supply of water. How much they need, and when, depends on the plant – think of the difference between salad crops that rapidly wilt without water, and drought-tolerant herbs that hate having wet feet. Seedlings need a more constant supply of water than mature plants, and (as a general rule) perennials are more drought-tolerant than annuals.

In the vegetable garden, water is needed most by the leafy crops and any plants that are cropping or will be shortly. Tomatoes are notorious for their response to uneven watering. A water shortage (when their container dries out) can lead to Blossom End Rot, a ‘disease’ (it’s really a physiological problem) that causes the round bottom of the tomato to begin to rot before it ripens. Blossom End Rot is caused by a calcium deficiency, which in turn is usually caused by a lack of water rather than a simple lack of calcium.

If tomatoes have been short of water and then get a lot, it can cause the fruits to suddenly swell and split. (However, scars on the surface of fruits are usually caused by fluctuating temperatures.) Other vegetable plants also respond to water shortages, and it is best to keep them watered while they’re flowering and fruiting. On the other hand, bulbs like onions are garlic are best not watered once they start to mature, as over-watering can cause them to rot.

Over-watering plants causes them to sit in compost that is too wet for them. To begin with they will simply sulk; then they will start to rot. For outdoor containers it’s important to check that the drainage holes haven’t been blocked, and that pots aren’t sitting in saucers of water for days after heavy rain. To check whether pots need watering, push your finger into the compost – if it’s damp beneath the dry surface then you can wait a little longer.

If compost has dried out completely, it is sometimes hard to re-wet. Water from the top now, and the water simply runs out of the bottom. You’ll need to stand the pot in water for half an hour or more until it takes up water, and then allow it to drain to prevent waterlogging. This is just as big a problem for peat-based composts as it is for peat-free ones, and commercial mixes sometimes contain wetting agents to prevent the problem.

Evaporation losses are lowest during the cooler periods of the day – early mornings and evenings – and so watering is best done then if you can spare the time. The idea that watering during the middle of the day can cause leaves to be scorched by the focusing effects of water droplets has been proven to be largely false, so you can water then if you have to. But make sure that you have mulched the soil surface to reduce evaporation losses, or consider switching to self-watering containers. Watering into a pot or tube sunk into the compost directs water to the plant roots and keeps the soil surface dry – another way to reduce evaporation losses.

Plants that have just been transplanted or re-potted need to be watered in generously (sometimes known as puddling in). Not only have you interrupted their water supply, but it’s likely that disturbing them has broken or damaged at least some of their roots. They need extra water available until they can settle in and grow some new ones.