A Very Late Frost...Maybe

Our weather is wobbling all over the place this spring (except without any rain), with potentially freezing temperatures tonight and 90 and humid by Memorial Day.  Tonight's potential frost is what concerns me, as the plants are well-prepared for 90 degree weather and won't mind it a bit.

You can keep off a light frost by irrigating the gardens before or at sunset, deeply watering.  Moist soil holds much more heat than cold soil, and brings warmth up from the lower levels very, very well.  Studies show a 5° F difference in surface air temperatures even at 6 AM the following morning when the sun is already rising.

I also run the grass' irrigation system around sunset, and it's currently running now.  Twenty to thirty minutes will dampen the soil and deposit plenty of moisture on the grass blades, which can slowly evaporate (and carry off heat from the grass) all night long.  I'll probably touch it up around or just after midnight to make certain that the air over the grass is moist and warmer.

That doesn't benefit the grass, which won't be harmed by frost, but helps in the adjacent gardens.  Like carbon dioxide, water is a greenhouse gas--but a very strong greenhouse gas.  Infrared radiation trying to escape the soil into space has a better chance of being captured by moist air than by dry air.

The generally more humid air will hold and reflect back more heat.  Additionally, if temperatures drop below the local saturation point, water will condense back on the lawn and plants, releasing copious amounts of heat as it does so.  Essentially, I'm creating a light fog and using it to my advantage.

Here, the goal is to keep the dew point well above 32 degrees, as well as to attempt to keep the air temperature far enough over 32 that frost isn't possible.

I'd estimate I have a 95% chance of making it through tonight with minimal to no damage, and a 5% chance that the lowest-lying plants in the garden will be damaged but not killed.  I can recover damage if I have to, but I'd rather not.