Summer Heat, keeping plants cool
Today is the first day of triple digits for me. This is the first day of many weeks and an endurance race to make sure the plants stay hydrated during this California drought. While the Governor has said the grass must die, container plants can be hand watered as well as watering to keep trees alive. My trees are in containers so I think I’m double protected. There is no doubt that summer heat can play havoc on any plant in a container. Since most bonsai container tend to be dark in color, they attract heat and can send soil temps into the stratosphere. Soil has a good ability at buffering away heat. Add water to that and soil temps even in triple digit heat can stay well below 80 degrees. Soil temps above 90 degrees, roots essentially stop growing. Prolonged contact with soil at this temp will eventually die. It is important to keep soil temps well below or at least 80 degrees or lower. The temperature band for optimal growth is narrow. 60 degrees seems to be the temp at which most roots wake up from Winter slumber, and 85 degrees seems to be the limit for summer soil temps without root damage. Anything above 90 degrees will shut the plant down and above 100 degrees for prolonged periods will show sign of stress like drooping leaves and shoot die back. Leaves turning Herman Munster grey green and dry and crisp is borderline death. The stems may still remain green and emergency watering may save the plant. When the leaves are brown and crispy and the bark on smooth skinned species starts to get wrinkly, death is sudden and without saving. It’s too late.
How can we protect our plants in the summer time to keep them from death?
There are many way to keep plants cool thru the summer. One of the easiest ways is to keep them watered (hydrated). For some people this may mean watering several times a day. If one has an automatic watering system this is simple to do. If one does not have a water system and one works, then auxiliary methods must be employed. There are many soil amendments that can help retain water in hot and dry conditions. The addition of clay as in the Japanese red soil, akadama is one way. This clay holds lots of moisture and gives a sort of time release to the soil container providing water vapor for most of the day.
Adding a wood component to the soil mix will also add an extra measure of moisture holding properties to the soil mix. Orchid seedling mix is a good additive to the soil mix and this will also add water vapor thru the day.
Transpiration not only happen on our leaves, it happens on our soil. During the hot day water vapor is released to the air until there is no more to give up. In an effort to keep this soil transpiration to a minimum, the soil surface can be covered with many water holding mediums. Wood bark is a good choice since it has many bacteria fighting properties due to its tannic acid. This keeps the formation of molds and fungus to a minimum. A good layer of fir bark will add moisture for the day. Sphagnum moss is another good medium for keeping added moisture at the top layer of the soil. It also aids in keeping fine surface roots cool and not burning off when the temps soar. I have seen people use wet towels to cover their pots and keep down transpiration. This will work but only if the towel is full contact with the soil. If air pockets between the towel and soil are there, the air in the pocket can rise to unsuitable temperatures.
What about mechanical contraptions for keeping a collection cool?
Shade cloth is a simple solution for many to help shade plants from the sun during the hot part of the day. Optimum height for shade cloth is 12 feet from the plants. Most people do not want nor have the room to build a structure so tall in their yard. It will provide shade for the plant but just know that the percentage of shade stated by the manufacturer is for the 12 foot height. If you have 50 percent shade cloth 4 feet above your plants it will probably be around 75 percent shade. There will not be enough peripheral light to give the 50 percent value. Yard trees make an excellent shade medium. make sure to set up benches, if possible, on the east side of the yard with a good tree on the west end of the yard. The hot sun in the latter part of the day will be shaded by the trees and lend the needed shade to help keep pot temps down. A misting system on a time clock may help keep plants cool in the hot part of the day. Misting is a good way to cool down a pot but some go overboard with the misting and start fungal problem with the overly moist conditions. In most climates the hottest part of the day seems to be around 3 to 5 PM in the afternoon. A one time a day misting in that time frame may be all that’s needed to help keep the pots cool. If your misting, which I do not do, spray with fungicides….Justin Case