We are again in the midst of excessive heat events in many parts of the United States. Records were broken for the highest temperatures ever recorded just a few days ago. This is also a time when the days are at their very longest, so high temperatures have large impacts on plants in landscapes.
High temperature can have immediate (acute) and continuing impacts (chronic) on plants. When temperatures get much over 90F photosynthesis becomes less efficient and in some plants may stop all together. As temperatures increase beyond 90F photosynthesis shuts down and transpiration may also stop to avoid breaking the chain of water molecules that plants must have to move water. When this happens heat builds up in the foliage leading to cell death and eventually symptoms (acute response). These may initially show as wilting, loss of color in the leaf and rapidly within days show as yellowing and then necrosis. This is usually seen in the center of the leaf first as the edges of leaves dissipate heat faster and more efficiently than around the mid vein area of leaves.
Chronic effects of heat are related to the poor efficiency of photosynthesis at high temperatures. When plants are hot and the photo systems that capture sunlight energy are impaired, or not working, the plant must still use energy in all its cells for respiration. Stored carbohydrates are not available for growth as cell maintenance (respiration) is the first demand for energy. When temperatures are high for long periods, stored carbohydrates in roots and stems are depleted. Since energy for growth is not available, slowed or stopped growth is the biggest chronic effect of hot days on most plants. This is why even hydrated plants just seem to stop growing in hot weather.
What can be done to mitigate high temperatures? First, never let plants dry out during high heat events. Evenly moist soil (but not saturated) will allow plants to absorb water and cool themselves as much as their physiology will allow. If soils are dry the damage of high heat events is “magnified” many fold and foliar damage will increase. Irrigate late in the day or early to avoid evaporation of applied water. Get your plants ready for high heat by irrigating before it hits. We usually have good weather prediction a few days ahead of high heat events.
Another way to mitigate high heat is to avoid plantings in “high albedo” environments. Albedo is the reflection of sunlight. Low albedo surroundings abosorb sunlight energy, high albedo environments reflect it. Plants exposed to reflected sunlight will be more readily damaged by sunlight during high heat events because they can not transpire enough water to cool their leaves. Reflective soils like decomposed granite, or some kinds of rock will damage young trees during heat events. Cover the soil with arborist wood chips which have a relatively low albedo. Young plantings can be protected by placing shade cloth over their canopies until the high heat subsides. If you don’t have shade cloth, a white sheet will do fine as it will reflect heat away from the canopy.
Ensure that the mulch or soil is moist before the heat of the day starts so humidity increases during the day. This will reduce the demand on transpiration and and the possibility of cavitation (the disruption of water chains in the plant and introduction of air which stops water movement), thus preventing a catastrophic heat death event.
A final word of precaution- Never fertilize during high heat events. Even when watered this changes the osmotic potential of water in soil making it harder for plants to pull water in. Adding fertilizer is like adding salt and this is a big NO during high heat events. Try to ensure that plants have all the mineral elements they need before heat becomes an issue.
You might think that during heat events its a wise idea to prune. This is not the case! Avoid pruning, especially thinning, as the removal of leaves will increase the impact of heat on the remaining canopy. Pruning and removing leaves will decrease the humidity around a plant and the remaining leaves will have to transpire more to cool the plant. This can be a disaster during a high heat event.