Hardy only in U.S. Department of Agriculture plant hardiness zones 8 through 10, guava trees (Psidium guajava) thrive in most of the same areas where citrus trees grow best. Cultural requirements for the two species are similar -- like citrus, frost-tender guavas need protection when winter’s freezing temperatures threaten. While a light, short-lived frost isn’t likely to seriously injure a mature specimen, you’re better off preparing your guava tree for the worst-case scenario. Consider taking the words of Patrick Young, of global investment and trading firm Liquidnet, to heart: “The trouble with weather forecasting is that it's right too often for us to ignore it and wrong too often for us to rely on it.”
Weed the area around the guava tree from the trunk to the dripline. Remove all grass, if possible, and rake the mulch out. This allows the exposed soil to emanate heat and help keep the tree warm.
Water your guava tree from the trunk to the dripline immediately when frost is in the forecast. Water deeply enough to evenly moisten the soil surface. The damp soil will protect the tree’s roots from freeze damage.
Wrap the guava tree’s trunk from ground level to the lowest limbs with several layers of heavy corrugated cardboard. Secure it in place with tape. Cover the cardboard with plastic garbage bags and tape them securely. Leave the trunk covering in place until the freezing weather passes.
Set up a portable oscillating fan under the canopy near the dripline and direct the airflow at the guava tree’s foliage. The active air circulation helps protect the tree from frost damage down to 20 degrees Fahrenheit.
Place a concrete block on the ground near the tree’s trunk. Put a 100-watt lamp meant for use outside on the block. Turn it on at sundown and off at dawn. This will provide enough warmth to help minimize frost damage.
String outdoor holiday lights through the guava tree’s limbs to add a little more heat to the microclimate you’re creating. Switch the lights on at dusk and turn them off in the morning.
Sink four 6-foot wood stakes into the ground around the guava tree to construct a frame to support a covering. This will keep the covering from coming into contact with as much of the plant as possible, preventing cold damage to the tender foliage. Space the stakes evenly apart about 12 inches from the dripline.
Cut a 1-inch slit in a tennis ball with a sharp knife. Stuff one end of a 12-foot bamboo pole firmly into the slit. Hold the pole upright with the tennis ball at the top and lean it against the guava tree’s trunk. Thread the pole carefully upward through branches to position the tennis ball about 6 to 12 inches above the canopy. The tennis ball will keep the pole from poking a hole in the covering. Tie the pole to the guava tree’s trunk with several lengths of lightweight rope.
Toss old sheets over the frame to cover the guava tree completely from the top of the canopy to the ground. Cover the sheets with a plastic tarp if the weather is wet. Remove the coverings when the temperature warms to above 32 F.
Postpone hard pruning until spring after your guava tree resumes active growth. Don’t trim cold-damaged foliage until after the last predicted frost for your region. The spring flush will reveal frost damage more readily, making it easier for you to assess what needs to be trimmed.