Shade their roots:
Just as a nice shady spot out of the sun can be a nice respite for humans on a hot summer day, so can a little shade work wonders for plants. Don’t shade at the canopy level (although you may want to do this with young starts – see below), but at the roots, where plants perform some of their most vital functions. For best results, use a white sheet – the light surface will deflect most of the sun’s rays. Prop the sheet a few inches above the soil or hydro layer to create a buffer of air in between. With this protective layer between your plants’ roots and the hot outside air, they will be much better equipped to handle the hot summer sun on their canopy.
Diffuse light for young plants:
While you generally won’t need to shade mature plants at the canopy level – thus stealing the main source of their energy! – young starts need some partial shade to avoid frying in the hot sun. The goal is to simulate nature – in the wild, young plants tend to sprout below a higher canopy created by more mature plants, so they’ve evolved to grow best under partial shade conditions. To simulate this scenario, use a diffusing cloth like thin canvas or cheesecloth – some commercially available clothes have been designed for use by nurseries for exactly this purpose, as well.
Covering bare soil with a layer of mulch has many varied benefits, of which keeping plants cool is only one. Thus, if you’re growing in soil you should always add a healthy layer of mulch, even if you’re already shading at the root level. If you’re not shading at the root level, airy and fibrous mulches like pine can be the next best way to create the shady buffer of air so critical to keeping plants cool. This step is especially critical if you’re growing organically – not only your plants’ roots but earthworms, mycelium and many other organisms which symbiotically benefit their roots will thank you for the respite.
Heatwave 911 – Ice:
If all other methods prove inadequate in the face of blistering heat, you can step up the assault on high temperatures with some well-placed ice cubes during the hottest part of the day. Here, care must be taken – dropping ice cubes directly on the roots may drop the plant’s temperature below its ideal range or even shock the plant. But if dropped on top of a buffering layer (say, a layer of pine mulch), the melting runoff from the ice cubes can warm to just the right temperature to be drawn up through the roots and through the stalk to the stomata at the canopy – thus cooling plants from the inside out. Here again, a well-placed white sheet will redound to great benefit because the shade under it will encourage the cubes to melt at a steady rate as well as preventing the cubes’ moisture from escaping into the air above.
In conclusion, these techniques work best when used together – using a diffusing cloth to shade young plants before putting them out, root-shaded and mulched, in direct summer sunlight. If used in tandem, these techniques can reduce water requirements and avoid having to use more expensive interventions, like ice cubes, at all.
Article by Jeremy Daw.