Expert advice for watering your garden in a heatwave - and it's less often
Heatwaves put both plants and those who look after them to the test
Gardening: Francis Tophill advises on watering plants without a hose
Watering at the right time can make the difference between a garden that withstands the heat and one that suffers from water stress in a matter of days
Heatwaves put both plants and those who look after them to the test. During this first heatwave of the summer, which, according to the AEMET, will last until Thursday, temperatures are soaring, and the sun is beating down for hours. One of the most common questions among gardening enthusiasts and experts is when is the best time to water plants so that they can make the most of it.
Although it might seem that any time is fine as long as the soil is dry, the reality is that the time of day chosen has a direct impact on the roots’ ability to absorb water, on water evaporation and even on the overall health of the plants. For this reason, experts agree on a recommendation that is particularly important during periods of extreme heat.
The Royal Horticultural Society (RHS) says early morning is the best time to water plants, because there is less evaporation. When the sun has not yet reached its peak intensity, temperatures are milder, the water has more time to soak into the soil before it evaporates, and plants can use the moisture throughout the day
Furthermore, watering in the morning ensures that plants have sufficient water reserves to cope with the hottest part of the day. During the day, leaves lose water through transpiration, a natural process accelerated by high temperatures. If the roots have adequate moisture, the plant is better able to withstand heat stress.
Another benefit is that excess moisture on leaves and stems has time to dry out before nightfall, reducing the risk of fungal infections and other diseases. The next best option is to water in the late afternoon or at dusk, when the heat begins to subside. At this time, evaporation slows down again, and the water remains available in the soil for longer.
However, some experts advise against leaving the leaves wet overnight, particularly in areas with high ambient humidity. They therefore recommend directing the water straight to the base of the plant, where it is actually needed.
What does seem clear is that the middle of the day is the least suitable time for watering. In extreme temperatures, a significant proportion of the water is lost before it reaches the roots, resulting in wasted resources and less effective watering.
The mistake of watering little and often
During a heatwave, many people choose to water with small amounts several times a day. However, experts usually recommend a different strategy: deeper, less frequent watering. When water penetrates several centimetres into the soil, the roots are forced to grow towards deeper, moister areas. This strengthens the plant and makes it more resilient to periods of drought or high temperatures.
The RHS says Shallow watering can leave roots closer to the surface where soil dries out fastest. To find out whether you need to water again, use the ‘finger test’. It involves inserting a finger a few centimetres into the soil. If the soil is still moist, the plant probably doesn’t need any more water just yet. If it is dry, it’s time to water again.
Not all plants react in the same way to heat. Those in pots, planters or hanging baskets tend to suffer more during periods of high temperatures.
The reason is simple: they have less soil volume to store water, and their roots heat up more quickly. In these cases, it may be necessary to water more frequently, always checking the compost's condition first.
It is also helpful to group pots together in partially shaded areas during the hottest part of the day to reduce moisture loss.
As well as choosing the right time to water, there are other measures that help maintain soil moisture for longer. One of the most effective ways is to apply a layer of mulch around the plants. Materials such as pine bark, dry leaves, compost, or plant cuttings act as natural insulators, reducing evaporation and protecting the roots from heat.
The RHS says established trees, shrubs and perennial plants growing in borders often do not need regular watering, even during warm weather. Newly planted specimens, vegetables, containers and hanging baskets should be prioritised instead.
During a heatwave, it is not enough simply to water more; it is also essential to water more effectively. Watering in the early morning, or failing that, in the evening, allows plants to make better use of every litre of water and reduces losses through evaporation. Adopting more efficient watering habits not only helps to keep gardens and terraces in good condition but also contributes to a more responsible use of an increasingly valuable resource.
Heatwave watering checklist
- Water early in the morning where possible
- Water at the base of the plant, not the leaves
- Water deeply rather than little and often
- Check soil moisture before watering again
- Mulch exposed soil to reduce evaporation
- Give extra attention to pots and hanging baskets
- Avoid watering during the hottest part of the day
