good luck, stan

Steph in Michigan

You already know heat gets moved around in various ways, convection (air currents), conduction (touching), and radiation. When you talk about water freezing when the air temperature hits 32F, you’re thinking of convection and conduction, causing the water in the leaves or stalks to freeze because heat is carried away into the air by the first two methods.

But that last method, radiation, also plays a role. Heat will be radiated from a warmer object to a colder one – even in space. You can feel the warmth of a bonfire on your face because of radiation, even from far away.

On a clear night, the sorghum is exposed to the night sky. The sky is very, very cold – basically, the temperature of the upper atmosphere or of space. Ground-level objects will radiate heat to the sky, and so can freeze, even if the local air temperature is above 32F! This is true for windshields, sorghum, and even the pond at times.

If it’s cloudy, it’s less likely to happen – the clouds are warmer than the open sky so the effect of radiation is not as big.

Meanwhile, the ground (which is relatively warm, since it is in contact with the deep earth which has hardly frozen yet in this time of the year) is transferring heat to the lower leaves of plants, and maybe even to the stalks, through all three methods (convection, conduction, and radiation). This may be why some plants you mention (sweet potato and basil, that are low to the ground) did not freeze, since being close to the warm-ish earth was more important than radiating to the night sky, whereas the upper leaves of the soybean plants you observe may have frozen.

So that’s the deal on the sorghum, I think. As for the old-timers’ comments, I don’t know about moon phases, but I can explain dew point (actually, as a farmer, you know this one already). Another way for substances to give up or gain heat is to change phase (state). When a liquid freezes, it gives up heat – that’s why it’s often not as cold out, comparatively, when it’s snowing as when it’s clear. Farmers use this intentionally – in Florida, if there is a risk of frost, citrus farmers will spray their trees with water so that as the water freezes, it gives up heat, and protects the fruit itself from freezing. Since dew point is an expression of how much humidity (water vapor) is in the air, it can affect freezing of plants.

Glad I got to help with sorghum this year, even for only one day! Here’s to many healthy harvests to come.

Does the plant’s roots grow on water or soil?

It seems to me that they should be in contact with the water and not the soil medium.

It’s really impresive what you do. Where is Sandhill?

Joe

Thanks