Shallow geothermal energy: a possible solution for anyone growing a palm tree in colder climates.

[macheck]

In 2017 I started to experimentally grow a Rhododendron simsii outdoors. The technique that I will describe should also be useful in palm trees growing in colder climates. According to the American Rhododendron Society the R. simsii is hardy down to -9 C (15 F). I currently live just outside the city of Szczecin, Poland (officially USDA zone 7B). Over the last few years however the winters have gotten more mild and usually the temperature doesn't drop below -11 C (12 F). At the beginning I planted my rhododendron in its' pot in the ground and placed over it a plastic garden cloche. This was done in order to limit the formation of frost on the rhododendron as well as shelter it from the cold winter winds and additionally to capture sunlight during the day. This setup has been used by me from 2017 to 2022. It has generally proved to be effective. The rhododendron was removed from the cloche and placed into an unheated garage during particularly cold winter spells (<-10C). A disadvantage of this method was that during sunny days in winter some of the most exposed foliage suffered burns. This was not a serious problem as new leaves regrew in the spring. However, I decided in September 2022 to improve my technique and  redesigned it according to the principles of passive annual heat storage. I dug a larger hole (40 cm deep) and placed the rhododendron in its' pot inside the hole. The plastic cloche was placed at ground level as earlier. Additionally I made a small temporary wall from some bricks around the cloche at the northern direction. This was to act as a wind break. The principle behind this new setup is that the warmth of the soil is transferred to the plant. Additionally sunlight heats the air within the cloche and that too increases the air temperature. The temporary brick wall on the other hand decreases heat loss from the cloche due to strong northerly winds. The soil contributes the most heat in this setup. Just how much depends on soil depth. In my climate during the winter, soil temperatures at 40 cm depth vary between 6-8 C (43-46 F). During the coldest days the soil temperature at 40 cm drops down to about 1 C (34 F). With palm trees you should need to dig deeper holes. Going deeper will mean achieving higher temperatures. In my climate the soil temperature at 1-2 m depth usually oscillates around 10 C (50 F) which is not bad for a Trachycarpus fortunei for example. So how have I done with my new setup? The first test came on October 21. During the night the air temperature outside dropped rapidly to 0 C (32 F) from 14 C (57 F) the day before. Meanwhile the temperature inside the cloche remained at 10 C (50 F). An interesting thing happened. All plants growing on the garden were covered with frost but not the cloche nor the rhododendron in it. The second attack of winter occurred between 13-19 th December. The daytime and nighttime temperatures were below freezing. Lowest recorded temperature was -10 C (14 F). Expecting for temperatures to drop to that level I took additional measures to protect my rhododendron and covered the cloche with bubble wrap for insulation and then placed snow on top of that. Everything went well. The soil temperature dropped to the expected 1 C (34 F). The rhododendron did not suffer any damage immediately afterwards nor during a two week follow up. This method has proven to be effective as far as I am concerned. I recommend experimenting with this method to anyone growing palm trees in colder climates. Below are photos taken of the old and new setup.