@Karolina Wrapping the top of trees as shown in the image doesn't help with avoiding cold injury (unless there is a source of heat within the confines of the wrapping). Case in point. If you place a foam cup of water (cup 1) in a freezer set at 18*C and a foam cup of water placed in a quality beverage cooler (cup 2) and
then in the freezer, does the insulation provided by the cooler prevent the temperature of water in cup 2 from freezing or becoming as cold as cup 1 which has no cooler for protection? No. It slows the change in temperature but eventually, both cups of water will be the same temperature. The less effective the insulative value of the cooler, the quicker that occurs.
The wrap you have appears as though it would be effective against desiccation from wind, but it will have little effect on the rate of temperature change and likely no effect insofar as keeping the top of the plant with reaching the same temperature as the ambient at temperature at night. During the day, the day the temperature within the wrap could rise to considerably warmer temps, but that is far more likely to be a problem than a benefit. I'll explain.
To be effective, any insulation used must be used to trap heat from an extraneous source. It could be geothermal heat rising from the ground, or heat passing through perimeter walls of a heated building, or a string of Christmas lights within the confines of the insulation. The roots are the plant organ least resistant to cold if these plants are hardy where you live, so the focus should be on insulating the pot to trap heat rising from the earth. In fact, if there is not ample air circulation within the coverings shown in the image, high humidity levels will probably ensure an incubation period long enough to allow fungal spores to germinate. If they are wrapped in sheets, that might not be an issue.
A good way to insulate your 3 plants would be to leave them on the ground to take advantage of the geothermal heat source, build a wire cage around the pots, and fill the cage with leaves/ straw to a depth at least 6" higher than the top of the soil in the tallest pot. This traps the earths heat within the pots. If the plants weren't as tall, you could place them on a garage floor and cover them with a cardboard box with the bottom open to allow heat into the box.
To prevent a physiological injury called 'sunscald' or 'southwest injury', it's best to over-winter plants outdoors in shade on the north side of a building/ fence/ ..... whenever possible (in the Northern Hemisphere). This is especially true when trees are young or the tree has smooth or thin bark. Sun light striking the bark is turned to heat on contact, which stimulate the rising of sap, which can then freeze at night, causing serious injury to the tree. Trees on the north side of a building don't see wild fluctuations in temperatures due to passive solar gain, so sunscald wouldn't be a worry. Trunks of young, smooth-bark trees in the ground should be wrapped with something opaque to prevent the injury.
Al
