Preparing for a frost - 8 Easy Tips
Some quick suggestions from the San Joaquin County Master Gardeners and Marcy Sousa, San Joaquin UC Master Gardener Program Coordinator.
• Having plants on covered porches
• Cover plants and trees such as citrus, succulents, tender perennials, tropical and subtropical plants.
• Roses and frost -- do they need covering?
• Harvest ripe citrus
• Mulch and frost
• Don't let water stand around plants
• Make sure pipes are wrapped and turn off sprinklers
• Water plants to help protect them from frost damage.
1. Having plants on covered porches
Tender plants with soft wood or that are actively blooming are more susceptible to frost damage. You may want to protect with a covering even if they are under a covering. Better to be safe than sorry.
2. Cover plants and trees such as citrus, succulents, tender perennials, tropical and subtropical plants.
You do not want the sheet or protective covering to touch the plant. Any material that is touching leaves can cause those areas to freeze. Shelters can be built, using 2x4's or stakes that are taller than the plant, covered with a tarp or plastic. For smaller plants a large cardboard box works as well. A breathable material is better if you are going to leave the covering on for a long period of time - so towels and sheets would work. (Burlap for covering can work but is NOT a breathable material.) Be cautious with plastic covering. A long period of plastic around a plant can be damaging, trapping moisture inside. Too much moisture combined with freezing temperatures can be damaging.
NOTE: It is beneficial for the plant to get sunlight as well. If you can get the covering or shelter to reach the ground that is better and will keep everything warmer. Having the bottom open will allow cold air to get in and that defeats the purpose of a cover to keep warmth in. The warmth is coming from the soil radiating heat. A breathable material is ok to leave on during the day, but plastic you would want to remove or open up to allow air circulation. Burlap is not very breathable so treat that like plastic. If we are not in danger of freezing temperatures you want to remove the covering regardless if they are plastic, burlap, cotton sheets, towels, etc. You can "cook" a plant if it gets too warm. Also, keeping a plant too "warm" can cause the plant to break dormancy, begin actively growing and become more susceptible to frost damage.
3. Roses and frost -- do they need covering?
Roses are hardy and do not need to be covered in this area.
4. Harvest ripe citrus
The freezing temperatures can cause the juice vesicles to rupture as ice crystals form inside, causing the fruit to dry out. Wrap the trunks of frost-sensitive trees - you can use cardboard, palm fans, anything that will help insulate. Bare soil is better that mulch, so rake any mulch, weeds, ground cover away. Bare, moist soil radiates heat better (covers reflect the heat). A 100 watt light or old Christmas lights (not the LED) work well strung in a tree/bush to help protect it from frost damage. Wait to do any pruning if it looks like you have frost damage. Exposing sensitive plant cells and tissues can be more damaging than waiting until spring to clean up the yard and garden.
5. Mulch and frost
Rake away mulch to allow soil to warm up during the day and radiate heat into the plant at night.
6. Don't let water stand around plants
Standing water around plants freezes and doesn't let the heat from the soil radiate up into the plant. It's better to give plants enough time to absorb the water before it freezes.
7. Make sure pipes are wrapped and turn off sprinklers
Make sure sprinklers are turned off. Laying garden hoses out straight (versus wrapped up) can help prevent them from cracking if they have any water left inside of them.
8. Water plants to help protect them from frost damage.
Check that plants are well watered because dry plants are more susceptible to damage, and moist soil retains heat better than dry soil . You can water during a frost, keeping the soil moist is the key. Standing water that can freeze will not help radiate heat. A slow, deep water is better. Watering in the evening (before dusk) is best to allow the soil to remain moist at night. (You also want to cover the plant before the sun goes down to trap warmth) The exception: You do not want to water succulents when a frost is predicted.
Before a frost
• Identify plants at risk: citrus, succulents, tender perennials, tropical and subtropical plants.
• Have supplies ready: sheets, blankets or frost cloths, lights, wraps for trunks, thermometers, stakes or framework to hold covers off foliage. Frost cloths come in different weights that can provide 4° to 8° of protection. Because the frost cloth allows some light and air to penetrate, it can stay on plants for a few days at a time. Frost cloth can lie directly on plant foliage.
• Prepare tender plants: avoid fertilizing and pruning after August to minimize tender new growth.
• Rake away mulch to allow soil to warm up during the day and radiate heat into the plant at night.
• Monitor weather forecasts and note how low temperatures will be and for how long.
– Local frost: clear, dry nights, usually warms during day
– Hard freeze: temperature inversion or Arctic front, can last for days or weeks, are very damaging
When a frost is forecast
• Move potted plants to a warmer spot next to the house or under a patio cover, especially on the south side.
• Check that plants are well watered because dry plants are more susceptible to damage, and moist soil retains heat better than dry soil.
• Cover plants before sunset to capture ground heat radiating upward at night. Remove sheets, blankets and other covers daily if it is sunny and above freezing to allow soil to absorb heat.
• Add heat by using outdoor lights: hang 100 watt drop lights or holiday string lights to interior of plant. Use the old C7 or C9 large bulbs, not new LED lights which do not give off heat. Old style holiday lights that give off heat can provide up to 3° of protection. Use lights, extension cords, and multi-outlets or power strips that are rated for outdoor use and grounded (3-prong). Avoid connecting together more than three light strings in a line.
• Wrap trunks of tender trees if a hard freeze is expected, using towels, blankets, rags, or pipe insulation.
• Harvest ripe citrus fruit. Generally both green and ripe fruit are damaged below 30°, but there is some variation by species (refer to chart in UC ANR Publication 8100, Frost Protection for Citrus and Other Subtropicals).
For more information, please visit: San Joaquin County Master Gardeners.