One of the parts of your garden that can get very hot is one that you’ve probably given little thought too. It’s the soil. The soil in your garden and potting mix inside plant pots heats up on hot days and it can be fatal for plants.
Even when air temperatures are mild, pots standing in full sun become hot. The temperature of potting mix inside pots can be 10 degrees or more above the ambient air temperature.
One grower told me that during the recent heatwave he measured the soil temperature inside an exposed pot. He was shocked to see the thermometer shoot up to 70 degrees C.
Needless to say, the roots in the pot couldn’t cope with these extreme temperatures and were dead. Indeed, root death and damage begins to occur at much lower temperatures than this extreme recording. Prolonged soil temperatures even in the mid-30s can be damaging for some plants. Every care should be taken to keep pots and soil cool.
The first line of defence to keep soil and potting mix cool is shade. This can be from shading or insulating material or from surrounding plants. Long lawns, for instance, keep the soil beneath them cooler than closely mown grass.
Groundcover plants and mulch over soil in garden beds also keep temperatures down below damaging levels. Having a mulch layer over soil can help plants survive hot days just by keeping roots cool, even where there’s little moisture about.
Mulch laid on the top of the potting mix in a pot also helps keep the temperature of the potting mix down.
If you have pots in hot spots around the garden, consider moving them to a more shaded location on a hot day. If a pot is too heavy to lift, use a trolley to move them out of the sun.
Providing some sort of shade is important for pots that are getting sun after midday or that are standing on or near surfaces that become very hot, such as masonry paths, paving or walls.
Plastic pots, particularly black plastic pots, heat up even more quickly than pots made of other types of materials such as terracotta, stone or concrete. Thick or glazed ceramic and concrete pots keep plant roots cooler for longer than other materials.
Many of the plants we buy at the nursery are grown in black plastic pots and so are vulnerable to overheating. If you can’t transfer a plant out of its plastic pot into the ground to protect it from heat damage, a quick way to keep the pot cool is to stand it inside an empty ceramic pot. The outer pot is called a cover pot.
Watering a pot or the garden can help to cool it down, but if the roots have already been damaged by the time the water arrives, the plant may not be able to absorb moisture through its roots to survive.
More on potted plants ideas:
Want more? Check out: How to create potted plant gifts.
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