The last three weeks of June and the first week of July have been perfect examples of “Global Wierding”. Everything I thought I knew about the weather, and I learned a lot being out in the woods for twenty years and growing bonsai for many more, is all for naught. One day it is 105 with a nighttime low of 92 and the next day (I mean the very next day) it is 80 with a nighttime low of 60. These wild swings of temperature do bad things to our trees. Most frequently with deciduous material the leaf margin will turn brown or yellow, or the leaves will be deformed, or the tree may simply drop all its leaves. If your trees are in the sun a lot of this damage will occur because the pot gets very hot.
For Junipers and Pines it is different. They like the heat and sun but they often suffer when temperatures fluctuate over about 25 degrees from one day to the next. With conifers and junipers damage takes much longer to be manifest. It can often take a couple of months for your pine tree’s needles to turn yellow after it has been burned.
Remember all of our new roots and tender young shoots are growing right next to the inside wall of the pot. If it is a ceramic container the temperatures where the roots are trying to grow can get over 140 degrees. Guess what happens… the roots in that zone die. The tips of your pine needles will start to turn brown and the needles will die back down from the tip. Junipers will start to drop weak limbs. All the foliage will turn yellow or brown and the branch will die.
This is becoming more of a problem in the last 10-15 years than it ever was before. During my visits to Asia, I find that bonsai nursery persons over there are having to be more careful with their trees. Wrapping the pots with burlap, swamp cooler pads, or wet rags will really help. If possible move the trees into full shade or a space with morning sun only. Don’t be afraid to water two or three times a day. If your bonsai are in good soil there will not be a problem. You can spray the leaves with Cloud Cover or Anti Stress 2000. Both of these are a harmless latex coating that will reduce water loss through evapo-transpiration.
If your tree looks ugly and it is good and strong, move it into a shady spot and cut off all the leaves. It will revive in about a months and look good. If it is weak move it into the coolest spot you can find and let it rest. As the leaves die, you should be able to see tiny little buds emerging right at the base of each leaf. On pine there should be new buds right below where the old needles are dying. For Junipers look into the base of the dying branch or on a strong branch nearby and you should see new lime colored growth.
Remember, Mother Nature does not even know we’re here and she can be a wild and crazy girl.