Phoenix is ranked the hottest city in the United States. It is on 33 degree latitude, located in the central part of Arizona, and has an arid, desert climate. It is known as the Valley of the Sun for good reason, it has 325 sunny days a year! From May through September, temperatures routinely exceed 100°F on an average of 100 days during the year, including most days from late May through early September, and highs top 110 °F an average of 18 days during the year. July is the hottest month and January is the coldest month.
Monsoon generally begins in early July and lasts until mid-September, raising humidity levels. Winter months are mild to warm, with daily high temperatures ranging from the mid-60's to low 70', and low temperatures rarely dipping below 40 °F
The most common type of frost occurs on clear and calm nights, when much of the heat is lost by radiation.Temperature inversion occurs, creating a layer of warmer air above the layer of cold air near the ground. The period of lowest temperature is ordinarily very short just before dawn.
The severity of the damage by the frost depends not only on actual temperature and duration of the frost, but on the condition of plants. Young plants are more susceptible to to damage than the old ones, and young plants are more likely to be injured than those that have had time to mature.
We have a few nights of frost in a typical year during winter months. There are different methods you can use to protect your tropical plants. Cover the plants with cloth or paper (not plastic) to insulate. You can use sheets or blankets for minimal protection or use a frost cloth.
I use a 55 gallon plastic barrel filled with water close to the tree. The water heats up from direct sun during the day time. It stores this heat and radiates it at night time when temperature drops. I find this method very helpful in preventing frost damage.
The same trees which are injured by cold in the winter may be injured by heat in summer. Branches, leaves, and fruits exposed to the sun become much hotter than the atmosphere. Heat is generally more harmful when accompanied by low humidity and strong wind.These conditions lead to very rapid transpiration and desiccate the twigs and leaves. During the hot weather in Phoenix these conditions are common.
The ability of plants to withstand such conditions varies greatly with the species. Again young trees are less hardy than old ones.This is a limiting factor in case of certain Subtropical and Temperate fruits.
Most of the subtropical plants need protection from direct afternoon sun in summer months when plants are young. As the plants grow, they create their own microenvironment. I protect my subtropical plants with 50% shade cloth when they are young. They are planted close to each other, so once they grow larger they will create a microclimate and get shade from other trees
Paint the trunk of young temperate and citrus trees with water soluble white paint mixed with 50/50 water to prevent sun burn.