Do any of you use heatcable during winter? I want to get my soil temp u quite a bit and I thought of heat cables but these are supposed to be layed in soil while I was thinking of wrapping them around the pots (since I have no soil to lay them in).
What do you guys think of that?

edit: found cables that can actually be used around containers. Comes in handy .

After reading through the winter guidelines again a question arises in my head. Now I have a room temperature of about 15-16°C whilst the containers are about 13°C. How high can the pot temperature go before I get intro trouble with WLD or other negative symptoms?

High root zone temperature is not a concern with WLD. It is low root zone temperatures that causes the problem. Keeping the root system at 70F or higher, almost always completely eliminates any chance of WLD. - Millet 3042

I am not questionning what you are saying Millet but I would like to quote this from the winter guidelines:

"if you can't provide it with some extra light then simply don't go for higher temps. your tree will be happier dormant, with lower temps and light level, than with higher temps and insufficient light. which usually results in leaf drop sometimes even followed by twig dieback. a citrus tree won't grow unless all of the three, temperature, light and humidity thresholds are met."

This is why I was doubting that heating the roots was a good idea.

First of all citrus are not high light trees. They are only capable of using 650-PAR (about 1/3 of full sunshine). They originally developed as under story trees, growing beneath the taller vegetation. You can grow your tree however you wish, its your tree. Root zone temperature around 70F and lower light levels, as from a south facing window, has grown thousand of trees very successfully. Even trees in north facing windows. - Millet

Thanks for the explanation! I will keep it in mind

Seven, here is a copy of the paper explaining the findings of the research done on the causes of WLD. Written by the people that carried out the experimentation. This should help you understand what is involved. - Regards Millet

http://www.steffenreichel.homepage.t-online.de/Citrus/WLD.html#English

Sven, what is " the winter guidelines"?
Where is it?

I've printed out the article and red it. That is really usefull information!

One think that I find confusing is this:
they mention soil temperatures should not go lower than 12.5°C but just a few lines lower there is a statement 'Even on cold days the sunlight can heat up the leaf surface quite quickly to levels beyond the critical temperature of 12.5°C'. They only mentioned 12.5°C as a minimum for soil and suddenly they use it as a maximum for leaf temperature which doesn't make sense to me at all.

Thank you