Hamish Brown
Mon, 17 Aug 2015 03:13:30 PDT
I have also been curious about how best to keep my alpines cool.  Are friend told me double potting was the way to go but to make it work properly the requirements are quite specific.

*         The outside pot has to be terracotta

*         The inside pot should fit snugly (using the same size terracotta works best)

*         The outside pot has to be sitting in a basin of water kept as full as possible without saturating the bottom of the inner pot

Not being satisfied with unsubstantiated recommendations I ran an experiment in my driveway where I had 4 replicates of a range of different pot treatments

*         Green plastic with a dark stone mulch

*         Green plastic with a white stone mulch

*         Plastic covered in aluminium foil with a dark stone mulch

*         Plastic covered in aluminium foil with a white stone mulch

*         Terracotta

*         Terracotta plunged in sand

*         Double terracotta

I place a thermopile in each pot and logged temperature at 15 min intervals over a one month period.  I can't remember all the results off the top of my head (I will write them up properly soon) but the interesting bits where this:

*         On a cloudy day all pots had a temperature similar to air temperature

*         On a sunny day all pots where hotter than the air

o   The green plastic with dark stone mulch was up to 10 degrees hotter

o   Adding a white stone mulch cooled the pots by 2 degrees

o   Adding aluminium foil to the outside of pots (reflecting the suns radiation) made pots a further 2 degrees cooler

o   Double terracotta pots were by far the best system for keeping pots cool.  Provide the outer pot was kept moist they got no hotter that air temperature.
Double terracotta is also good because the inner pot stays clean so it is ready to go if you want to take it to a show.  The down side of double terracotta is the cost.  Wrapping pots in foil and giving them a light coloured surface mulch is a cheaper alternative to reduce heating of the roots

Hope this information is useful

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