Top tips for newly planted trees in a heatwave

23rd Jul 2021

We all reach for the iced water and cool ourselves down by staying in the shade during these days of extreme (for the UK) heat. Dipping your feet in a paddling pool becomes a real elixir and few risk a day out without suncream. So spare a thought for your newly planted trees! Think of them like thirsty babies you must take care of.

We go on a lot about watering, because really it is key to them thriving. You can't exactly sling a sunhat on a tree, but in the same way you wouldn't let your kids get dehydrated, make sure your darling trees have enough to drink. If you notice any drooping, water the base straight away.

Water in the evening if possible, so they can quench their thirst overnight. Next best is morning, though when the ground starts to warm, the wicking effect can occur. That is, water evaporating from the surface can draw up water from underneath, so tree roots don't get much of a chance to access it before it's turned to vapour.

Watering in the heat of the midday sun is controversial. Of course it's inefficient as water on the surface will quickly evaporate, but what do you do if you have no other option? There are two things to consider: plants have a coping mechanism for hot weather, but watering in the sun (as long as it's plentiful) is better than not watering at all.

How they cope

Plants go into a sort of 'shut down' mode when it's seriously warm. That is, they slow down photosynthesising (turning carbon dioxide and water into energy), close up stomata (the pores that lose water) and the opposite of photosynthesising takes over: photorespiration.

This process takes up oxygen and releases carbon dioxide! Not a lot of people know about that. Without going into too much detail, this process produces something called glycolate, some hydrogen peroxide (!), and then CO2, reducing the overall carbon uptake of plants. Seemingly wasteful, as it lets go of some of very gas (CO2) that plants need to produce sugars, it probably stems from the very different atmospheric conditions in which plants evolved, or it could even be to protect plants from too much internal oxygen. It's a bit of a mystery! Meanwhile, desert plants and tropical grasses like corn and sugarcane have evolved a way around this problem in the form of something called Kranz leaf anatomy and succulents only 'consume' carbon dioxide at night when they won't lose so much water.

Anyway, we digress! The point is, your tree will probably be able to wait until a cooler part of the day for its delicious drink. But, especially if it's in a pot, it's crucial not to let the roots completely dry out. This will be the death knell. So do drench your tree, even in the sunshine, if that's the only option and it's urgent. DON'T sprinkle a little on the top - this is detrimental. Water must really penetrate, so we're talking at least a couple of bucketfuls.

How do you tell if it's worth it?

  • If it's in the ground and not wilting, this signifies it has access to water. You can breathe a sigh of relief and wait till evening to water.
  • If it's wilting, it's started the shut-down processes. It may perk up later on anyway, and if you can water at a cooler time, do it then. If there's a serious droop going on, though, it will probably thank you for a good soaking there and then.
  • If it's going crispy, water ASAP. A lot!

See our watering guide and aftercare guide for more info.

Photo by Jarosław Kwoczała on Unsplash


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