Frost Protection for Sensitive Plants

We are fortunate to live in a mild climate that allows us to grow certain fruits and vegetables during the winter. However, one of the biggest worries a gardener may have is the threat of freezing weather and frosts that can harm or kill plants and damage crops. When the local weatherman warns of “a chance of frost,” it’s important to take precautions to protect your frost sensitive plants. The timing of the first frost varies from year to year, typically it is mid-November or sometime around there. Frost injures plants by causing ice crystals to form in plant cells and on the leaf surface. This process makes water inaccessible to plant tissues and interrupts the movement of fluids.

It is helpful to know the difference between a frost advisory and a freeze warning, as well as terminology used in weather predictions. Case in point: covering plants before the sun sets can help retain heat near the plants when a frost advisory is issued. The same plant protection may have less impact following a freeze warning. NOAA’s definitions:

  • Frost: The deposition of ice crystals directly on the surface of exposed objects. In the right conditions (clear skies, winds less than 6 mph) frost can occur when observed air temperatures are several degrees above freezing.
  • Freeze: When observed air temperatures fall to 32 F or lower.
  • Killing Freeze: When observed air temperatures fall to 30 F or lower for at least two consecutive hours.
  • Frost Advisory: Issued when frost is forecast to occur at 3 or more weather observation sites.
  • Freeze Warning: Issued when a freeze is expected to occur at 3 or more observation sites.

Here are some tips that will help you be prepared and give your plants a better chance at surviving our winter weather.

Before a frost

  • Identify cold spots in landscape by monitoring with thermometers.
  • 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 lay 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 temperature warms during the day
    Hard freeze: temperature inversion or Arctic front, can last for days or weeks, are very damaging
  • Move potted plants to a warmer spot next to the house or under a patio cover, especially on the south side.
  • Water the soil thoroughly (except around succulents). Wet soil holds heat better than dry soil, protecting roots and warming air near the 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 are 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).

When a frost is forecast

After a frost

Plants can be remarkably resilient. If you see signs of frost damage, do not prune off the affected parts or dig up the plant immediately. Wait until the weather warms up in late March or early April to see whether new leaves sprout. You may see healthy new growth at the base of the plant, at which point you can prune out the damaged parts. If no regrowth is noted, you may want to remove the dead specimen and replace it with a more cold-tolerant species.

For more information or if you have gardening questions, please call the Master Gardener Helpline at 209-953-6112.

 

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  • Blog Author

    Marcy Sousa

    Marcy Sousa is the San Joaquin County UC Master Gardener Program Coordinator. She is a Stockton native and enjoys teaching others about gardening. She has her bachelors from Stanislaus State in Permaculture. She has been with the program since 2007. Read Full
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