People sweat to cool off, and plants do something similar. We call it transpiration.

Thank you Freepik !

As summer heats up, some of the water drawn up through the roots exits the plant through pores – or stomata— which are in its leaves.  As it evaporates, heat is removed, providing a cooling effect.  However, if there isn’t enough water available or if the relative humidity gets too high, the stomata close. If this happens, the plant heats up, ceases to grow, and can eventually die.

Stomata on a leaf surface.  Thank you Nature !

Evaporation of through the leaves doesn’t just cool the plant, it runs the whole system. The vacuum created as vapor escapes pulls water up through the plant tissues, and the water pressure keeps the green parts of plants firm and upright. Because water is also necessary for photosynthesis, in high temperatures there is competition for it, as more is used for cooling, there is less for making sugar.

At 86*F and above most plant cells start to suffer damage. If the air temperature stays over that point for a long period — especially if it doesn’t drop at night – the plants wont get a break and suffer lasting harm. Above 95*F most plants are in cooling mode with little photosynthesis.  Some actually shut down altogether a process called aestivation or summer dormancy.

What can you do

Containers dry out quickly, and may need to be watered every day in 90*F heat — perhaps more often if they are small or in sunny locations. And any container that is porous like clay will dry out even faster. It is possible to move small ones into the shade on the hottest sunny days, but for most larger ones this is impractical. Try using non porous plastic, resin or metal planters instead.

Small Terra cotta pot will dry out fast in the heat and sun!  Thank you !

In hot weather, sunnier parts of your garden will dry out faster. Place the heat tolerant ones there.

Laying mulch over the plants’ roots insulates the soil and greatly reduces evaporation.

Mulch those pots ! Thank you Nature and Garden Nature and Garden !

Some very cool loving temperate-zone plants are easily scorched or wilted. Try using plants that better tolerates a strong sun and summer heat.  Japanese maples for example evolved in cool moist forests – so they are less likely to tolerate a hot sunny rooftop.

For summer color, try to use those native to tropical rain forests, they shrug off stifling heat and humidity. Hibiscus, lantana, and many other annuals are high heat tolerant.

Lantana – and so many varieties !  Thank you Home Stratosphere !

Location, matters also.  Plants next to a concrete wall or an ac unit exhaust that radiates heat may be hit with temperatures much higher than the thermometer says.

Young plants or recently transplanted ones, with scanty root systems, have fewer defenses and suffer a lot.  Plant during cooler weather.

HVAC exhaust damage! Thank you !

Heat Stress – what to watch out for

Plants can go into wilt and recovery cycles – too many are not good

Heat wilt !  Thank you !

If the wilt is persistent they can become plasmolized (previous blog) and get damaged

Leaves lose their green color and become yellowish, pale, or grayish

Leaves may have scorch marks on the blade.

Leaf scorch ! Thank you Missouri Botanic Garden !

Leaves may curl – bamboo will roll up like a cigar!

Way too hot and dry.  Bamboo leaf curl.  Thank you the Garden of Eaden !

Flowers dry up and some cool loving plants stop blooming altogether – Try getting a flower out of a Osteospermum in July !!!

The best strategy

Call Plant specialists ! 

We have the people, experience and knowledge of which plants to choose to take our extremes of cold, heat and humidity. 

So even in hot spells, you can worry less.