Yes it is. I'm going to build my trench widely and deeply. I think it wiil be have the same insulating effect as this current trench. The insulating effect depend on some factors such as deep, ground characteristics, insulation material, cover, usda zone, deep of snow etc.

My citrus after winter.

You must be the happiest citrus grower havings this type of results with your tranch

Hello! I have one trouble. It is a "leaves dropping". Some leaves (i would say many) drop down. This trouble is caused by the difference beetween the soil temp and the air temp in the greenhouse. Despite my citrus in the pots that on the top of the soil, there is the difference beetween temps. I don't know what to do.

To resolve in quick way is to uncover for a little time during day time so that leaves can cool down to the pot temperature.

It is much better than to heat up the container.

But also it is usual that lemon trees discad some of their leaves prior to new flush in spring

one of the methods i read about about use a few hundred years ago ,was the use of horse manure and straw,to keep the temps up might work for you as you already have raised the pots,mybe a bit smelly for most....artie ps As the pots are aready of the ground i was thinking more of trying to fill into that void not round the pots,just to keep air temp.up..

I don't think so artie , he grows them so that they receive some cold in the winter , and breaking bud dormancy can cause some leaf drop but in his case I think it is better to cool the leaves than heat the pots in case that WLD occurs

Serj,

Is your photo posted "Sun Jan 23, 2011 9:58 am" representative one
for winter period ?
Is your photo posted "Tue Mar 15, 2011 8:05 pm" representative one
for current situation?

If so,
I guess that trees didn't have time for proper acclimation i.e.
transition period from WinterSetup (low light, low temp) toward current (much more light, higher ambient temp, cold soil) was perhaps too abrupt (too fast).

What to do now?
Lower ambient temp (as danero2004 have said)
and also reduce light.
Wait a few weeks, don't overwater.

Totally agree with Ivica

I'm sorry for not answering for a long time. I was bussy. I'm agree with Daniel and Ivica. The best way is to hold my citrus tree at lower temp and low light. But my current weather help me - we have some cold snap from Cyberia. It was +8 C today. My citrus lost a half leaves - meer lemon, calamondin. My satsuma, orange looks better than previous. The best citrus is ponderosa lemon and grapefruit. I wiil post new pics on my off day. Thanks for help, guys!

P.S. There is the another way to improve this situation. It is possible to accumulate sun heat with tubes in the ground with fans. In this way i will reduce ambient temp and increase soil temp.

Venting your greenhouse is the right solution for you , also some bottles of water arround the edges could cool down the greenhouse during day and warm up during the night

My citrus start to grow!!! Olny qumquat and lemon lisbon don't grow yet.

serj as always , please show some pictures

OK! I wil share them later, when the leaves become larger size!

I build a greenhouse also , not in the ground but above it. Hopefully will be good enough to keep my citrus trees on winter months