So just in case you have not noticed yet, the heat of summer has finally arrived as a long-term trend. Plants, like humans, are looking for ways to beat the heat, and we can help them along. Plants, unlike humans, cannot migrate inside to central air conditioning or shed more layers or go sit by the pool (just a little biology review to get everyone up to speed). Plants are stuck in their muck and must deal with conditions the best way they know how.
Believe it or not, temperature spikes are less concerning to me then other environmental conditions such as relative humidity and wind speed. A hot, humid day is going to cause less stress on most plant species, than hot, dry weather conditions. It all has to do with what affects water movement and water loss from the plant. Water loss occurs greatest from leaves. The tiny holes in the underside of leaves, stomata, open to allow gas and water vapor exchange with the surrounding atmosphere. The process of water moving from soil to roots to stem to leaves to air is called transpiration, and the rate of this process is controlled by a number of factors including relative humidity. When the humidity is low, water loss from the leaf is greater, regardless of temperature. When you combine low relative humidity, with warm weather AND the all important sunny sky (cloudy, warm, dry days are less stressful than sunny days), you have a condition called photorespiration (i.e.- Wow, my kids are eating food out of the frig faster than I can buy groceries).
In short, the problem facing the plant and gardener alike is how to raise the relative humidity and lower the internal temperature of the leaf. The answer, syringing. Yes, go grab a turkey baster and go spritz your plants! No, that is not it. If you are a golfer, you may have noticed greenskeepers out hosing greens. They are cooling off the poor stressed bentgrass by raising the relative humidity around the leaf and providing evaporative cooling. Most plant species’ natural response to increasing temperatures, like mammals, is “sweat” off the heat. It takes 570 calories to heat one gram of water, thus that amount of heat is removed with each gram of water released through the stomata. Lightly wetting leaf surfaces in the heat of the day slows down water loss and cools the plant down. Most people are afraid to do this citing concerns of increased opportunity for disease or scorch the leaf surface. However, hot dry weather is going to dry leaf surfaces quickly and the idea of water droplets magnifying the sun enough to burn leaves is myth.
So this summer when you notice some droopy, poopy plants give them some natural AC with a little syringing action.
Josh Steffen, Horticulture and Facilities Manager
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Humidity is a limiting factor in the chaparral climate around San Jose, and the desert climate around Los Angeles. Outsiders do not take that into consideration. They think that the weather in San Jose is too warm for Japanese Maples. The weather is actually not all that warm, and really not as warm as regions in the Southeast, where Japanese maples do quite well. The difficulty is the lack of humidity. The arid air that is so comfortable for us is what roast the foliage of Japanese maple when the weather gets warm.