Sun vs. Cold = Sap Flow ????

[dblact38]

With these single digit temps and the strong sun hitting the trees, will it cause a lot of pressure in the tree to make the sap flow, even though day time temps won't rise above freezing ??? Thanks all

[happy thoughts]

I've seen sap flow at temps below freezing if it's sunny enough but the night temps were just a few degrees below freezing as well. With the extreme cold we've had I think it's going to take a lot more than a sunny day alone to get the trees to flow. Give it a week or so and then I think we're going to be swimming in sap. Forecast so far is still looking very good.

[Sandersyrup]

My trees flow pretty good above 30ºF. All are on a southern facing hill with no other trees shading the trunks. I'll get a 1/8 gallon a day from each tap which adds up over the span of a chilly week. Lots of icy buckets though with weird freeze layers so can't toss out the ice. I often have icicles running all the way down into the bucket.

[DrTimPerkins]

With these single digit temps and the strong sun hitting the trees, will it cause a lot of pressure in the tree to make the sap flow, even though day time temps won't rise above freezing ??? Thanks all

No. The important temperature for sap flow is the wood and sap temperature (which are pretty much the same). Sap will not "run" unless the sap temperature is above freezing. Since the pressure comes from the head (the height of the sap column from the taphole to the tops of the tree), then there is no pressure unless the sap is in a liquid state. There would be a gravitational potential, but not a gravitational head pressure.

[CampHamp]

A tree's wood and sap can be warmer in sunlight than the air temps, I assume, so perhaps this is when I've seen taps pop their tubing because the tubing is frozen and blocked tight while there is pressure from the trees to flow.


...the pressure comes from the head (the height of the sap column from the taphole to the tops of the tree)...
I thought that you and a colleague recently proved (with saplings and headless older trees) that sap pressure is caused by upward flow and not just gravitational forces above the taphole. I'm confused now...

[DrTimPerkins]

A tree's wood and sap can be warmer in sunlight than the air temps, I assume, so perhaps this is when I've seen taps pop their tubing because the tubing is frozen and blocked tight while there is pressure from the trees to flow.

Correct. This can happen under certain circumstances.


I thought that you and a colleague recently proved (with saplings and headless older trees) that sap pressure is caused by upward flow and not just gravitational forces above the taphole. I'm confused now...

Not correct. We showed that with a strong vacuum you can induce sap to flow upwards entirely, and that the top of a sapling is thus not necessary for you to collect sap if you are using a strong vacuum, but this does not occur naturally during the flow period (but does during the freeze/uptake phase). The stuff about our stumbling across a headless older tree flowing sap out the top is complete BS that some reporter made up because they didn't bother asking us the full story, but instead wrote their own story based upon what another reporter wrote.

[CampHamp]

So, like a straw, you can suck sap upwards with enough vacuum pressure. Crazy that someone would make up the headless tree story. I read it in Modern Farmer (if you care to push them for a correction). Thanks for the reply!