by George Buehler

Well, August came in like a lion - a really HOT lion. During the last few days of July and the first few weeks in August, a number of weather records bit the dust in Louisville - 23 straight days with 90 plus degree heat, 4 days with 100 + days, a new record high for the area (105 oF, a new record high low (82 degrees one night) and to top it all off, little or no rain. When it did rain, it came down in buckets, and most of it ran off without soaking it. What a month to go through!

Now what did all this do to our bonsai? We may not see any effects for some time, or we may not have any problems. If you did not watch the watering carefully you could be in for big problems later. Due to the intense heat and the time it lasted, the roots of our trees could have been damaged or cooked. I used to think that trees in large pots were less prone to heat related root damage than trees in small pots. It seemed obvious that pots with a large mass would be less affected by heat than small ones simply due to the mass. However, I read a research article recently on growing plants in hot weather. The article was based on Asters growing in ProMix in two different size plastic florist pots, but the results were pretty straight forward. Those plants growing in large pots suffered as much root damage as those in smaller pots. Temperatures were measured in each pot (in the center) and the temperatures were essentially the same in both. Watering regime was the same in each one. This was ProMix potting soil and the pots were plastic, versus our bonsai ceramic pots and the rock soil we use. This may be a major difference. However, I think the indications are the same. Large bonsai pots heat up just as much as the smaller size pots do. And due to the larger mass in big pots, it takes longer for the big pots, once heated, to cool down (all environmental things being equal).

A number of research reports have shown that root growth slows or stops when the temperature is in the high 80's. This is one of the reasons why some of our trees go into semi-dormancy during the hot summer time.

I didn't measure the temperatures in my bonsai, but I did have one occasion when I was turning a tree while the temperature was in the high 90's, and the side facing the sun was almost too hot to touch. This means that the pot facing the sun was at least hotter than 97.8o (body temperature). During the hot temperatures, I was watering twice a day- once at noon (the time I always water) and once at 5 PM. The noon time watering was to water the soil and the 5 PM watering was to help cool the pot. I noticed that the temperatures, always went up in the late afternoon and hit the high for the day between 4 and 5. This was somewhat different than in past years when the temperature generally hit a high for the day around 1 PM.

As I write this, the temperature is predicted to go back to the high 90's again. I decided to run a 'quick and dirty' experiment on a couple of my trees. I selected several trees with different pot shapes and sizes, in several locations and measured the temperature in the pot and around the pot - the results are presented in Table 1 page 9. Of course it is very limited data and I plan on collecting more, but I thought I would report what I have.

Here is what is indicated from this very limited data:

• The soil in small pots prior to watering, was quicker to respond to temperature changes and generally was at least the same temperature as the air temperature and sometime hotter, depending on the location of the small pots.
• Large pots were cooler in the center than on the outside, contrary to the research reports on the plastic pots with Pro-mix.
• Generally, the soil was substantially cooler after watering than prior to watering. However, the temperature rose fairly rapidly after watering.
• Larger pots do seem to be less affected in the center than smaller pots
• The color of the pot may affect the temperature of the soil.
• Pots with the same color and size have higher inner temperatures when situated in the full sun versus one sitting in the partial shade.

A lot more data needs to be collected with different size pots, different colors and in sun or partial shade.

I think I also learned something new about trees during the hot spell. One of our new members, Owen McKinney, brought a Brazilian Raintree to our Bonsai 101 class. The tree, as it should have been, had been sitting out in the full sun and was doing beautifully. It was growing, putting on new leaves and looked very good. When he brought it to the meeting at the library, the meeting room was very cool with the air conditioning blowing out a nice cold air. He set the tree on a table, apparently under an air conditioning vent, where it stayed for the duration of the meeting - about 3 hours. We all noticed that the leaves of the tree were closing, but we thought - or at least I did - that this was due to it being moved indoors with low lighting and a cooler temperature. I had seen several rain trees close their leaves when they were moved to different conditions.

Several days later, I got a call for help from Owen and later several emails with a number of pictures, showing that the leaves at the top of the tree looked like they were dying. I couldn't figure out what was going on. Owen assured me that the tree had been watered like it should have been and that the tree hadn't been exposed to anything that could have done damage to it. Since I was no expert in rain trees, I suggested he take it to Larry Cassidy to get an opinion.

After Larry examined the tree and listened to the explanation of what had gone on, Larry concluded that the rapid change from the high temperatures to the cold air conditioned temperatures may have caused the tree to respond adversely. Larry suggested placing the tree back where it had been sitting and simply wait and see what happens. Several days later, I contacted Owen again, and he reported that the tree was about the same - several of the limbs at the top, about 20% of the tree, were without leaves. That was a week ago from when I am writing this. I recently contacted Owen again, and he reported that there was still no sign of leaves coming back but he was waiting and hoping. He nicked the bark on several of the limbs and found that some were green and some brown.

The reason I am reporting on this is that we often move trees around to work on them, take them to shows, etc. and don't consider the stress that we may be putting on our trees. The next time you take your trees someplace, remember Owen's experience, it may save you a lot of grief. For Owen's sake, let's hope his 'baby' recovers.

We still don't know what caused the dieback - cold air or something else - but we know the top 20% has no leaves. I asked Gene Sewell to look at the tree and perhaps he has an explanation. If he does, I will report on it next month.

Speaking of next month - actually early November, thanks to the efforts of Kenny Popp and Earl Cormney, we have added a club event to our calendar. Set aside Nov. 3 for this special event. There will be complete details in next month's newsletter.