Frost—Avoidance and Dealing with Damage

Garden Help > How To

While first and last frost dates are important for gardeners to be aware of, it's also good to know what you can do to prepare for frost and the deal with frost damage after it happens.

First and Last Frost Dates

The approximate first and last frost dates for Santa Clara County, depending on your location, are November 15 and March 15. These are important dates for gardeners to remember.

First frost date—this is the earliest date you should expect frost to occur. If you have plants that need to be brought in for the winter, or crops you need to pick before frost, this date will be important to you.

Last frost date—after this date, you wouldn't expect any more frosts. It's generally used as a milestone when pruning frost-sensitive plants (such as citrus where you don't want to stimulate delicate new growth until the danger of frost is past).

For our climate, the last frost date is not a reliable metric for when to transplant summer vegetables. Daytime temperatures and soil temperatures are more important. See entries for individual vegetables for specific recommendations.

Preparing for Frost

When there's a threat of frost, make sure your frost-tender plants are well-watered. Soil that is damp can hold more heat than soil that is dry. According to the UC publication on the Principles of Frost Protection, ”when the soil is wet ... more heat is stored during daylight for release during the night.”

Frosts or a hard freeze can kill tender plants and can damage citrus, especially young plants so protection is a good idea on those cold, clear winter nights. Place stakes around tender plants and cover with clear plastic or fabric such as a sheet or old drapery. Don't let the material touch the foliage.

Wrap larger plants with strings of Christmas tree lights (incandescent bulbs not LEDs) or position a 100–150 watt light in the center of the tree and cover the plant with a sheet. Turn the lights on at night.

If plants are potted, move them to a sheltered area such as a porch, under the eaves on the south side of the house or even under a tree. Be sure to uncover them during the day. Moving them indoors to a cool room would be good if possible.

Frost protection sprays (known as anti-transpirants) are marketed for their ability to minimize water loss and reduce frost damage, however, results have been mixed and they're not recommended (sources: Placer & Nevada County, Ventura County)

Ice covered fruit trees at Emma Prusch Park in San Jose, caused by burst water pipe during a hard frost in January 2007, by Allen Buchinski
Ice covered fruit trees at Emma Prusch Park in San Jose, caused by burst water pipe during a hard frost in January 2007, by Allen Buchinski

Dealing with Frost Damage

According to a UC article on frost:

  • Frost injury to plants depends upon a number of factors, including species, age, health, soil moisture, and location.
  • Frost injures plants by causing ice crystals to form in plant cells, making water unavailable to plant tissues and disrupting the movement of fluids.
  • Frost-damaged leaves appear water-soaked, wither, and turn dark brown or black.

As much as you might want to remove frost-damaged branches right away, it's important to wait until spring. Early pruning can lead to further dieback along stems and branches because the dead material helps protect the plant from further damage. In addition, spring growth helps you assess the amount of damage, making the extent of damage clear to see.

Additional Information

Source URL: https://class.ucanr.edu/site/uc-master-gardeners-santa-clara-county/frost-avoidance-and-dealing-damage