How to protect plants from frost in Arizona

Short answer: when a night near 32°F is forecast, cover frost-tender plants with frost cloth or old cotton sheets that reach the ground, water them a day or two ahead, keep cactus dry, and take the covers off in the morning. Never use plastic, and don't prune frost damage until spring.

When does it frost in Arizona?

In the Phoenix area, the average first frost ranges from about November 21 (Buckeye) to December 12 (central Phoenix), and the average last frost from about February 7 (central Phoenix) to April 3 (Mesa), according to University of Arizona Extension data. Low spots, open desert and outlying areas get colder than the city core. Damage to sensitive plants can start at 32°F.

Plants that need protection

• Most at risk: bougainvillea, lantana, yellow bells, fairy dusters, ficus, and lemon and lime trees.
• Young citrus of any kind, and any plant in a pot.
• Some succulents and tropical plants.
• Usually fine: most native desert trees, shrubs and cacti. A yard full of natives has much less to worry about.

How to cover plants the right way

1. Use the right cover. Frost cloth, old cotton sheets, painter's drop cloths, burlap or blankets all work. Frost cloth protects to about 30°F, and some fabrics to about 20°F.
2. Drape it to the ground, out to at least the edge of the canopy, so it traps warmth rising from the soil. Don't gather it tight around the trunk.
3. Don't crush branches. Use stakes or a simple frame for big shrubs.
4. Never use plastic. Plastic sheets, shower curtains and tablecloths pass the cold straight through and can burn leaves.
5. Take covers off in the morning once it warms up (the Extension suggests when it reaches about 50°F under the cover), so plants get sun.

More ways to protect plants

• Water tender plants a day or two before a freeze. Moist soil holds more heat, and drought-stressed plants take more damage. Keep cactus and succulents dry.
• Move pots against the house, under a patio or into the garage.
• Wrap young tree trunks, especially young citrus, from the ground to the lower limbs.
• Old-style incandescent holiday lights under a cover add a little heat. LED lights don't.
• Skip smudge pots and fires. The Extension does not recommend them.

After a frost

Brown, crispy leaves look bad, but don't prune them yet. The damaged growth shields the rest of the plant from the next cold night, and damage is often less severe than it looks. Wait until spring, after new growth starts, then cut back to live wood. If a grafted citrus dies back below the graft, the regrowth will be rootstock, so the tree may need replacing.

FAQ

Sources

• University of Arizona Extension: Frost Protection
• University of Arizona Extension: Protecting Citrus Trees from Cold
• AMWUA: Protect your plants from the cool desert nights

Frost dates are long-term averages and vary by neighborhood. This is general guidance, not a guarantee against damage.