Newsletter Articles
How to Protect Plants From Frost in Southern California
Frost is infrequent in Southern California, but a single cold snap can damage tender plants like citrus, succulents, and bougainvillea overnight. You can protect them by draping fabric, never plastic, over the plant to the ground to trap the soil’s heat, watering the soil beforehand, moving containers to shelter, and waiting until spring to prune any damage.
Quick answer: Cover tender plants with fabric, water the soil first, and move pots to shelter before a cold night.
- Frost is most likely on clear, calm nights, and cold air pools in the low spots of a yard.
- Cover tender plants with fabric such as a frost cloth or old sheet, draped all the way to the ground, and never let plastic touch the leaves.
- Leave the cover open at the bottom or remove it by day, since trapped heat under a closed cover can soar past 130 degrees F in the sun.
- Water the soil before a cold night, because moist soil holds and releases more heat than dry soil.
- Move potted plants together into a sheltered spot near the house or under eaves.
- Do not prune frost-damaged growth until spring, when you can see what is truly dead.
| Method | How to do it | Why it helps |
|---|---|---|
| Cover with fabric | Drape frost cloth or a sheet to the ground overnight; keep plastic off the leaves and remove or vent the cover by day. | Traps heat radiating from the soil around the plant. |
| Water the soil | Water the ground before a cold night, except around succulents. | Moist soil holds and releases more heat than dry soil. |
| Move containers | Cluster pots together in a sheltered spot near the house or under eaves. | Walls and grouping buffer potted roots from the cold. |
| Mulch the roots | Add a temporary mulch layer over low plantings, then remove it once frost danger passes. | Insulates roots, though bare soil better warms the air around a tree. |
| Wait to prune | Leave frost-damaged growth in place until spring. | Damaged growth shields the plant, and spring reveals what is truly dead. |
How Frost Damages Plants
Frost harms plants from the inside. UC Marin Master Gardeners explain that frost forms ice crystals inside plant cells, which cuts off water to the tissue and leaves foliage looking water-soaked before it shrivels and turns dark brown or black.[1] There is a real difference between frost and a hard freeze. UC Integrated Pest Management defines a freeze as air at 32 degrees F or colder, and frost as air warmer than 32 degrees F while the plant’s own tissue still drops to 32 degrees F or below.[2] How much harm is done depends on how cold it gets and for how long: Oregon State University Extension notes that established landscape plants are generally hardy to about 24 degrees F, while more tender plants can freeze anywhere between 24 and 32 degrees F.[3] Clemson Cooperative Extension adds that on some otherwise hardy species, root damage can begin around 28 degrees F.[4]
Knowing When Frost Will Strike
The riskiest nights are the still, clear ones. The University of Arizona Extension explains that calm, clear nights bring the greatest frost danger because there is no wind to mix the warm air rising off the ground with the cold air sinking onto it, and that cold air flows downhill like water and collects in low spots.[5] Colorado State University Extension calls these radiation frosts, which form on calm, clear nights with no cloud cover to hold in the day’s heat.[6] That is why the layout of your yard matters so much: UC Marin Master Gardeners advise against placing tender species in open areas or low spots where cold air settles, since elevation, nearby structures, and canopy all shape a garden’s microclimate.[1] Learning your yard’s warm and cold pockets, as our guide to garden microclimates explains, tells you which plants are most at risk.
How to Protect Your Plants From Frost
When frost threatens, the plan is to cover, water, and relocate. The most reliable step is to drape tender plants with fabric: UC Integrated Pest Management advises covering plants overnight with cloth rather than plastic, left open at the bottom so heat radiating from the soil can warm the plant, and taken off during the day.[2] The University of Arizona Extension says to drape the cover all the way to the ground with no gaps, and notes a frost cloth can protect to about 30 degrees F, with heavier fabrics down to roughly 20 degrees F, then remove sheets and blankets each morning once it warms under the cover.[5] If you do use plastic, UC Marin Master Gardeners say to hold it off the foliage with stakes so leaves never touch it.[1] Whatever you use, Colorado State University Extension warns the cover must be vented or removed by day, because the air under a closed cover can rocket past 130 degrees F in the sun.[6] Watering helps too: UC Marin notes that soaking the soil before a cold night (except around succulents) lets moist soil hold heat better than dry soil,[1] and the University of Arizona adds that firm, bare, moist soil absorbs and releases more heat than loose or heavily mulched soil.[5] Finally, cluster potted plants together in a sheltered spot near the house, where it is easy to move containers out of the cold.[1]
Which Plants Need Protection, and What to Do After a Frost
The plants to worry about are the tender ones. UC Marin Master Gardeners list citrus, tomatoes, geraniums, bougainvillea, fuchsias, and succulents as tender plants that frost can injure or kill.[1] Young citrus is the classic case: UC Integrated Pest Management recommends covering young, frost-sensitive citrus overnight with cloth and pre-watering the soil a few days ahead,[2] and the University of Arizona notes that wrapping the trunks of young citrus is a practical safeguard.[5] After a frost, the hardest advice to follow is to do nothing in a hurry. Clemson Cooperative Extension warns not to rush to prune or remove damaged plants, because some that look dead are not, and watering wilted frost-damaged plants will not revive them.[4] UC Marin agrees, advising you wait until the weather warms in spring to see whether new leaves sprout before cutting anything back, especially on palms.[1] University of Minnesota Extension adds a prevention tip: avoid late-summer pruning, fertilizing, and overwatering, since the soft new growth they push is the most frost-sensitive part of the plant.[7] For a garden that shrugs off cold with less fuss, build around hardy evergreen plants and shrubs with winter interest.
Not sure which of your plants are frost-tender or how to shield them? Stop by any Green Thumb Nursery and our team can help you spot the vulnerable plants and pick frost cloth and mulch to carry them through a cold snap.
Frost Protection FAQ
How do I protect my plants from frost?
Cover, water, and relocate. UC Integrated Pest Management advises draping tender plants overnight with cloth, not plastic, left open at the bottom so soil heat warms them, plus watering the soil beforehand and moving pots to shelter.[2]
What can I cover plants with for frost?
Fabric, not plastic on the leaves. The University of Arizona Extension recommends draping frost cloth or a sheet to the ground, noting frost cloth can protect to about 30 degrees F and heavier fabrics to roughly 20 degrees F.[5]
Should I water plants before a freeze?
Yes, except around succulents. UC Marin Master Gardeners note that watering the soil before a cold night helps, because moist soil holds heat better than dry soil.[1]
What plants need frost protection in Southern California?
The tender ones. UC Marin Master Gardeners list citrus, tomatoes, geraniums, bougainvillea, fuchsias, and succulents as tender plants that frost can injure or kill.[1]
Should I prune frost-damaged plants right away?
No. Clemson Cooperative Extension warns not to rush, because some plants that look dead are not; wait until spring and new growth to see what is truly dead before pruning.[4]
Sources
- [1] UC Marin Master Gardeners, Frost: ucanr.edu
- [2] UC Integrated Pest Management, Freezing and Frost Damage to Citrus: ipm.ucanr.edu
- [3] Oregon State University Extension, Gardening in Central Oregon (EM 9662): extension.oregonstate.edu
- [4] Clemson Cooperative Extension, Cold Damage: hgic.clemson.edu
- [5] University of Arizona Extension, Frost Protection: extension.arizona.edu
- [6] Colorado State University Extension, Frost Protection and Extending the Growing Season: extension.colostate.edu
- [7] University of Minnesota Extension, Protecting Trees and Shrubs in Winter: extension.umn.edu
Sign up for the Green Thumb Nursery newsletter for seasonal Southern California gardening tips.