warm roots colder air?

hairmetal4ever(Z7 MD)January 29, 2010

I've heard many sub-tropicals and even tropicals can tolerate lower air temps than they normally would, if the soil is kept warmer.

When I grew citrus under lights in my basement, I was able to get better results with 55 degree air and 65-70 degree soil than I did when it was the other way around.

However, aside from pads and cables, which only seem to have enough power to warm a couple inches of soil, is there a reasonable way to warm a large pot of soil for a larger plant, like a citrus, guava, or banana, in this situation?

I suppose you could just wire up a soil cable for each pot, and maybe intertwine it w.roots but that would be a nightmare come repotting time!!

Any experience here?

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eric_wa(San Juan, z8 WA)

No experience.

How about this. Wrap the soil cable around the exterior of the pot that the plant is in. Then set that pot inside a larger pot with sand. Not a lot larger, maybe a few inches. You may also have to insulate the exterior of the larger pot.

My crazy thoughts.


    Bookmark   January 29, 2010 at 1:40PM
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hairmetal4ever(Z7 MD)

Would the heat distribute more or less evenly through the center, or would we have a super-hot outer portion and super-cold inside?

    Bookmark   January 29, 2010 at 1:52PM
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Dan Staley

Conduction proceeds inward depending upon surface temp and soil moisture and root density, in addition to R-value of pot. It wouldn't be super-hot outside as soil cables are not super hot. As long as the energy continues to be applied, after time the soil would heat in the center. As it would with any solution: forced air, hot water at perimeter, dark pot absorbing sunlight, etc. IIRC you are looking at ~$1/ft for good quality cable and 1w/ft consumption, not 100% sure tho.


    Bookmark   January 29, 2010 at 2:40PM
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eric_wa(San Juan, z8 WA)

Here is another experiment.

Build a insulated plywood box. Cut holes in the top to match your pots. Most pots have a lip around the top. Drop them in. Bottom of the box you have a heat source. Christmas lights, ceramic sockets and incandescent bulbs. etc... Maybe a couple adjustable vents to control heat.

Some of this topic was covered in "Cheap Bottom Heat" post.


    Bookmark   January 29, 2010 at 6:15PM
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stressbaby(z6 MO)

Lots of folks have wrapped Christmas lights around their citrus pots.

    Bookmark   January 29, 2010 at 7:08PM
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Dan Staley

Let me just add my experience to Eric's idea: the box should be well-built and painted a color pleasing to the spousal unit.



    Bookmark   January 29, 2010 at 8:13PM
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hairmetal4ever(Z7 MD)

Eric - like the idea...just not sure how to handle drainage from the pot. Water could either short out the light or react with the hot bulb and crack it!

    Bookmark   February 1, 2010 at 1:23PM
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eric_wa(San Juan, z8 WA)


How about metal flashing. Set at a slight slop. Drains out one end into a container or another potted plant. The V openning could also be your heat control. Set it up like a gate valve.


    Bookmark   February 1, 2010 at 3:10PM
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curlygirl(5-6 Massachusetts)

I currently heat my citrus in pots using soil warming cables. It is indeed a pain! First I laid it out on the gardeners cloth wire mesh (as were the instructions that came with the cables) so that they would not overlap (which is important because you can melt the wires -plus you want even distribution of heat to the soil). But it was so difficult getting the grid curled up inside the conical pot that I had to keep redoing the wiring to avoid it overlapping. Even after all that effort, the mesh stuck up above the soil line and looked pretty awful. -I did not care because it was an experiment. Then when I repotted it, it was a nightmare. The roots had grown between the mesh which is what I was expecting but I had not thought it through how difficult repotting would be. So, now I have the cables wrapped around wire V-shaped stakes that I made. I have them wrapped and taped so that they do not overlap and it works really well. It is still challenging because you have to loop it around the whole plant first but other than that, repotting has been no problem at all. However, since my experiment a product has come out called a thermo planter (http://www.logees.com/prodinfo.asp?number=DS1200) which is $50 but it is pretty small for what I want to do with citrus. I am still going to order a couple in the next month or two for some of my smaller plants because after what I went through, the $50 is so worth it! I do hope that this company comes out with more sizes.

Here is a link that might be useful: Thermo Planter

    Bookmark   February 8, 2010 at 6:34PM
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