A、Hanging a bulb on the leaves. B、Wrapping the tree with plastic film. C、Cutting the branches of the trees. D、Spraying water on

Sudden cold can be the biggest threat for plants, especially after a warm period. Plants have not had a chance to harden their defenses. Those that are actively growing or flowering are at high risk. Most frost damage takes place at night.[30]Ice crystals form on the leaf surface. They pull moisture from the leaves and keep plant tissues from getting water. Cold weather is most likely to damage or kill plants that do not have enough moisture. So keep the garden watered. Moist soil absorbs more heat than loose, dry soil covered with something like vegetation.
    University of Arizona extension experts say covering plants and small trees with cloth or paper can help prevent frost damage. A one-hundred watt light bulb designed for outdoor use can also provide warmth. Some people place Christmas lights on young trees for warmth. The bulbs should hang below the leaves to let the heat rise into the tree. In Florida, as temperatures fell to record lows,[31]the growers of orange trees sprayed water on their trees to help prevent freeze damage. It is explained that as the water freezes, it produces heat, and the ice forms a protective blanket a-round the tree. If you spray a tree, keep the water away from nearby power lines. Heavy ice can form and break them. You can also wrap a tree in palm tree frond leaves, cornstalks or fiberglass. Adding plastic film works well in rain and snow. But experts say plastic alone does not help much.
29. What kind of plants are most susceptible to the effects of sudden cold?
30. What is the main reason for the death of plants in cold weather?
31. According to the speaker, what is an effective way to protect trees from frost damage?