by Mel Kendall
Frost does sneak up, that is a given. You can prepare ahead of time for frost and avoid much of that "kicking yourself" part. The first and easiest way to avoid this is to keep a regular watch on the weather forecasts. It's the weather, after all, and it changes daily.
Another way is to plan your garden accordingly. Avoid planting frost tender plants in low areas of the landscape. These areas are where cool air will pool and create cold microclimate areas. Frost tender plants, like citrus, in a protected area. If planted next to a south or west facing wall these frost tender plants will benefit from the residual heat radiating off that wall at night. Choose plants native to our area as they are prepared for our region's temperatures. Some cold hardy plants are crocus, pansy, tulip, calendula, sweet alyssum and snapdragons. Cold hardy veggies, that actually taste better when touched by frost are carrots, potatoes, beets, parsnips, turnips, onions, garlic, radish, rutabaga, broccoli, cauliflower, kale, brussels sprouts, cabbage, bok choy and collard greens. Along with those veggies that benefit from frost some fruit tree varieties actually need a number of chill hours to produce a good crop.
Keeping our plants hydrated will also fend off the effects of cold weather. If you water during the heat of the day the moist soil will insulate the plant's roots and the rising heat of the water evaporating will help to warm the foliage at night. Succulents are an exception as their leaves are normally full of liquid and are more susceptible to freezes even without additional water
Cover any frost sensitive plants in the evening and uncover after the frost has passed will provide further protection for plants. Cover smaller plants individually with bell shaped covers made of plastic or glass. Plastic milk cartons with the bottoms cut out work quite nicely. Cover larger plants with blankets, sheets, drop cloths etc. Use stakes or pvc hoops to hold cover away from the foliage in a tent-like formation that falls almost all the way to the ground. Leave the bottom of the covering open to the ground. Keeping the cover off the foliage will prevent the frost laying on it from transferring cold to the plant's leaves and freezing them. This may not be possible with larger trees and shrubs, so cover the bulk of the foliage as much as possible. Many purchased covers offer a drawstring at the bottom. Do not cinch the bottom of the cover shut because as the soil cools it will release its heat up into the opening of the covering and help to warm the plant. Weight the edges or clip the cover to the support structure so it doesn't blow off during the night. Plastic isn't a good material for the cover as it doesn't breathe. All of these coverings should be removed in the morning when the temperature has warmed.
If frost is the normal state of things in your area you can purchase plant frost protection covers specifically designed to be left in place throughout the season. They can be cut to size and have a large weave. They are designed to allow sunlight and heat to enter during the day, and protection from heat loss and frost damage during the night.
If frost is predicted, bring small container plants and any hanging pots inside in the evening and then put them back out in the morning. Place them inside to a cool, not a heated location like a garage, garden shed or basement, after the sun sets in the evening. These potted and hanging plants are more susceptible to cold and root damage because they are surrounded by air and not down in the insulating soil. Put the plants back out after the frost in the morning.
Air movement is important to frost protection. In Napa Valley the sound of vineyard fans are common in winter months. This air movement can actually raise the air temperature as much as 2-7 degrees. Small home gardeners don't have huge fans, but we can move the air in our gardens using small electric fans placed on a raised platform to blow over frost sensitive plants. Remember, safety first, use only equipment that is rated for outdoor use.
If a frost warning is missed, what's a gardener to do? Wait. Wait. Wait. Frost damaged plants often display softened translucent leaves as well as blackened and/or brown foliage and stems. This is a result of the cold air actually freezing the cells in the leaves of the plants. As the liquid expands as it freezes these cell walls will actually burst releasing the liquid when it thaws. The last frost date for our area is around April 15th. Wait until the weather warms before cutting away any frost damaged foliage. Frost damaged leaves and branches actually help protect the new spring plant growth from further frost damage. After the last frost, wait at least a month before removing dead foliage to give the plant a chance to come back.
I have quite a few "walking wounded" in my garden as I live in a cold sink microclimate and get more frost than is usually predicted for our area. Being very fussy about my garden's neatness and perfection, these damaged plants are really a sore spot. I plan on taking my own advice and waiting for the spring growth to see which ones have survived my miscalculations. Each gardening adventure is a learning experience.
Napa Master Gardeners are available to answer garden questions by email: mastergardeners@countyofnapa.org. or phone at 707-253-4143. Volunteers will get back to you after they research answers to your questions.
Visit our website: napamg.ucanr.edu to find answers to all of your horticultural questions.
Please see the our website napamg.ucanr for events including the Las Flores Community Center where instructional events are planned for the last Saturday of the month through 2022. Topics will center on research-based horticultural and climate change issues by featuring water saving plants and soil care. Join us each month at the Las Flores Learning Garden 4300 Linda Vista Ave. for these public education events.
Photo credits: Mel Kendall
Frost prevention information links:
UC ANR https://mgsantaclara.ucanr.edu/frost-avoidance-and-dealing-with-damage/
https://ucanr.edu/sites/mgscc2016/files/360387.pdf
UCMG Napa County https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=25850
UCMG Sacramento County https://sacmg.ucanr.edu/Frost_Protection/