Yesterday (Oct. 13, 2006) was a nice October fall day in Colorado. The high temperature was only 65F (18C), with a gentle breeze around 5 miles per hour. The day was partly cloudy, and partly sunny. I decided to do a container heat test to determine what temperature the growth medium would reach setting in the sun when using a common 5-gallon black nursery container. I had previously done such tests during the heat of July. I used two containers, one 5-gallon black nursery pot with a Clementine Mandarin growing in it, and a 5-gallon black nursery pot that I had painted white to reflect the sun light. The painted white container also had a Clementine Mandarin growing in it. After two hours I used a soil thermometer to determine the temperatures of the growth medium inside each container near the container wall (where a lot of roots grow in containerized trees). The black container was 120F (49C) and the painted white container was 80F (26C). For citrus most all root growth stops at temperatures at or above 95F (35C), and can be killed at temperatures at or above 120F. Therefore, I lost a days growth in the black container, but received a days growth in the white painted container. I was surprised at how high the temperatures could STILL go, at this time of year, even though the outside temperatures was only 65F, it was only PARTLY sunny. This shows the caution required for trees still growing outside, or for indoor containers receiving sunlight from a southern exposure. - Millet

Thanks for your update Millet! I have also been keeping an eye on temps in container pots.. but mostly at night concerned about them getting too cool.. I will also check mine at mid day sun..
I painted all of my pots with white paint this summer.. but I repotted a few and didn't paint the pots as winter was coming on..
I did notice that even my terracotta pots were warm earlier today when I was rearranging my plants in the utility room today.. Plants in that window get full sun most of the day..

Gina *BabyBlue*

Millet, thank you for taking time to do this experiment & for sharing the results! I'm absorbing!

Hello Millet,

I have questions about your experiment...

First, you don't specifically state whether the trees were outside or in the GH.

Second, another arm of the study could include a clay pot. Do you have a clay pot to test?

Third, did you measure the temps in the center of the pot to compare to those around the edge?

Next, let me ask you to speculate on a couple of things...

What effect does your altitude (~5000 ft as I recall) have on the results of the experiment? I suspect I suspect that you have more solar radiation hitting those containers than I do at ~600 ft.

I suspect that the temps are higher near the edge than in the center. What effect does the higher temp near the edge of the container have on root growth? Could the plant "heat-prune" in the same way that a root builder container air-prunes?

Just wondering aloud, here. Thanks for reporting.

Hey Millet,
This probably has no real influence on the outcome. What kind of paint did/do you use? House paint or that special Plastic Paint?
I would also assume you would paint the pots without the plants in it. The fumes would effect the plants? Yes..No?
I think I am going to start the painting of pots for next year and just wanted to have all my ducks in a row.
Thanks!
Darren
P.S. Sadly I will not be able to attend the Citrus Expo this year in VA, Beach. I was looking forward to your presentation. First off, $$ was a major factor, second I will now be MAKING money that day. I have a pretty good, well paying gig.

Another paint question, does anyone know of a type of white wash that could be used to lighten up the colour of terra cotta pots ? I'm looking at trying this, but without removing the Cal from the pot for now. It would have to be a very porous substance obviously. I can't think of anything that won't seal and bubble....except for maybe a very diluted plaster solution painted on.....thoughts?

Cactus, my reading indicates that the type of paint does make a difference. Apparently, the reflectivity and absorption vary between different paints, even among different paints of the same color.

I currently have an experiment where I am comparing two different types of black paints. The first day of results indicates some minor differences.

Robert, the test results that I posted above were done outside. However, I did record the temperature of the growth medium in one black container inside the greenhouse and it was 105F. The greenhouse covering is double poly, which does a good job of defusing the sun's rays. Sorry, but I do not have any clay pots. I did not test the temperatures in the center of the black containers, primarily because in round smooth sided containers almost all the roots are on the far sides of the container circling around and around against the container's side walls and the bottom. Because the root tip inside round containers is never air pruned, but just continues to circle, the root does not develop many of the side roots in the center. My greenhouse is located at an elevation of 5,408 feet above sea level. The solar radiation from the sun is stronger at this elevation, and what difference that would make in temperatures between my area and yours (600-ft) I do not know. Perhaps you could place a black container outside and record the high temperatures reached. I don't think heat pruning would even occur, as temperatures above 120F/125F root death can occur. Because 80/90 percent of the roots are against the side of the container's walls circling, the damage would be severe, in at least 1/2 of the container (the side facing the sun). - Millet

Darren, I used spray paint for plastic & any type of spray paint that was there. I put the pots - with plants in them, on newspaper on grass (paint won't hurt grass, but I didn't want grass to stick to pots). Then, put a piece of cardboard or paper grocery bag inside rim of pot as I sprayed that side. It didn't hurt the plants at all.
And on several, I painted only one side & left other side black. Millet told me about this...so you can turn pot to absorb sun heat or reflect it.
Hi Robert, do you think it's the gloss of paint that makes a diff & not the brand? Something I never thought of. Seems color is more important than brand name (?).

Darren, sorry you won't be able to make the Expo. It would be neat if you would bring your band, and entertain all of us. Tell you wife hello for me. I am going to be in N. Charleston on November 15 and 16, maybe I'll see you then. - Millet

We will miss you Darren