Deciduous Alpine Aesthetics - 4/20/21

Very good steam about styling and horticulture of potentilla. At about 45 minutes Ryan discusses why he feels that potentilla, larch, and other deciduous alpine species need to be kept in partial shade once the temperatures exceed 85-90F (29-32C). I think he got the thermodynamics wrong.

Ryan said that we water with cool to cold water which heats up in the pot fairly quickly and then cannot cool the foliage since it is close to the temperature of the foliage. This is different from their native environment where the roots are buried in the mountain and therefore stay fairly cool, particularly when you consider that the night time lows often approach freezing even on the hottest days.

There are two cooling mechanisms in play here: 1) the heating of the water from whatever the roots take up and 2) evaporation which is also known as transpiration when dealing with plants. The problem is that it takes very little energy to heat the water while it takes a large amount of energy to evaporate the water. It takes about 4 J to heat 1 gram of water by 1 degree C in the temperature range of interest to plants. So heating 1 gram of cool water 13C (55F) to 32C (90F) will require 76 J of energy - energy that will be absorbed from the foliage to keep it from overheating. However, it requires about 2200 J to evaporate 1 gram of water which is 29 times as much as the energy to raise the temperature. As a result the cooling of foliage depends far less on the water temperature in the pot than the ability of the tree to transport water to the foliage.

I am proposing that the problem is that the roots of these species shut down and can longer absorb water when the temperature in the pot rises since they are evolved to have cool roots and warm, sunny foliage. This will prevent transport of water to the foliage and eliminate transpiration and cooling. Putting the tree in partial shade prevents the pot from getting hot enough for the roots to shut down. This concept is fairly well documented for many species, but generally at higher temperatures where the roots are actually killed.

My observation about larches is that the Japanese larch (L. kaempferi) will grow well all season in in nearly full sun in my climate (fairly dry, highs of 30+C) in a bonsai pot that is probably a bit oversized aesthetically. However, Western larch (L. occidentalis) which is native to my area does not. The trees in forests will die off starting on the sunny side of the pot.


I agree with you (though I’m not intelligent enough to calculate the energy aspect of it…). The olden day gardeners used to say warm water makes the roots sick closely followed by sick plants. This is why my water butts get as much shade as I can give them to keep them cool.