Plants can be made temporarily (several days) resistant to frost damage down to 27 or 28 degrees (maybe lower) by raising the Brix (sugar level) of the sap. The higher the Brix, the lower the freezing temperature.
How to do it -
For Agriculture - mix 1 quart of black strap (horticultural) molasses in 20 to 40 gallons of spray water per acre. Be sure to use a surfactant such as dish washing liquid (Joy, Ivory, etc.). If the mix is allowed to ball up on the leaves due to inadequate or no surfactant, there will be burn-spots and the plants will not take up much of the solution. The amount of water depends upon how high the crop is - as the goal is to wet all of the leaves with the mix. This has been a life-saver when a late frost hits as the winter wheat is in boot stage...
For Gardens - Same instructions as above, but use 2 or 3 tablespoons of molasses per gallon of spray water. It doesn't take much. Way too much will burn the leaves. In fact, a very high concentration of molasses makes an effective (but expensive) herbicide. There are 64 tablespoons in a quart.
Leaves very readily absorb sugars, thereby raising the Brix. Accordingly, the mixture can be applied within a few hours of the anticipated frost. Be sure to wet all of the leaves. Black strap molasses contains a (desirably) high level of sulfur (along with a lot of micronutrients), so it does not attract insects.
If you don't have black strap, you can use other molasses or table sugar at the same rate. Always use a surfactant (spreader-sticker).
How much surfactant is enough? I test the solution by dipping a leaf in it. If I see dry spots, I add more surfactant until the leaf is completely wet, with no dry spots.
