Note: the following was written in January, 2009
It seems that we have already experienced more than our fair share of cold weather this winter. If you live in the Valley and leave home each morning around 6 a.m., you will have noticed a layer of frost on your windshield several times during the past few weeks. Gardeners are concerned with frosty weather because of the damage it can do to frost-sensitive plants.
There are three primary strategies employed to address the phenomenon of cold weather in the garden:
Location, terrain, microclimate: Cold air, like water, moves downhill. The safest place to plant frost-sensitive plants is on a slope. Witness the commercial avocado orchards planted on slopes in the Santa Paula and Temecula-Fallbrook areas. Even though frosts can occur in these locales, the likelihood of damage to the trees growing there on sloping terrain is small. By the same token, planting in a low garden spot endangers cold-sensitive plants since freezing air, like water, can settle there.
Open exposure to the sky, as opposed to an overhanging patio roof or tree canopy, also poses a danger to frost-sensitive plants. Heat absorbed by the ground during the day will protect plants at night as long is there is a barrier overhead to keep the heat, which radiates upwards after sunset, from escaping to the sky. I have seen tropical plants such as Dracaenas that were planted under trees survive a freezing night because of the protection afforded by overhead foliage.
Placement of plants near buildings may offer a significant measure of protection from the cold. Exterior walls, like the ground, absorb heat during the day and radiate it outward at night. Heated houses or rooms also leak warm air through their walls so that plants situated near these walls will be a few degrees warmer at night.
Physical protection or covering: Wrapping woody plants, especially small tropical trees, in burlap is a commonly used strategy for protection from the cold. Other materials used to cover plants include bed linen, newspaper, mosquito netting and floating row cover (spun bound, polyethylene fabric also used for protection of vegetable seedlings from insects and from bad weather of all kinds). You can also build a four-sided plastic “cage,” covered at the top, that you place over a sensitive plant when a cold night is forecast. For individual seedlings or small plants, many of which are sensitive to cold only in their early stages of development, black plastic nursery stock containers in one-, five- or 15-gallon size provide frost protection when turned upside down and placed over cold-sensitive species. All protection devices should be removed in the morning. Of course, keeping plants in containers at all times, whether on a patio or in the ground, allows you to move them into a greenhouse, garage or other enclosed structure when a freezing night is forecast.
Water application: When water freezes, or even as it approaches the freezing point, it releases heat. Thus, water applied through sprinklers or drip irrigation on a frosty night increases the heat in a garden. Although keeping the sprinklers on during night seems an excessive price to pay for plant protection, the cost of running a drip irrigation system is minimal. Similarly, soaking or flooding a garden in advance of a frosty night could slightly increase garden temperatures. Some gardeners cut the tops off empty soda or milk containers and fill them three-quarters full with water. Prior to a predicted cold snap, these containers are placed around young tomato plants, for example, their water giving off heat as it freezes during the night.
Correction: Last week’s column erroneously identified the forest at the Robinson Gardens in Beverly Hills as consisting of kentia palms when, in fact, it comprises the largest collection of king palms (Archontophoenix cunnighamiana) outside of their native Australia. King palms, whose trunks are green in their youth, are noted for luxurious foliage that detaches on its own so that pruning is never required. You can arrange to visit the Robinson Gardens, which are open to the public, by calling (310) 276-5367.