How to protect your trees from frost

In Northern California, we don’t get brutal winters. But we do get frost. A hard freeze in late December or early January can drop into the mid-20s overnight, and that’s enough to kill a young citrus tree, damage a newly planted ornamental, or burn back tender growth on trees that leafed out too early during a warm spell.

I’ve lost two trees to frost in twenty years. Both times, the forecast said 35 degrees and the actual low hit 27. Both times, I could have saved them with thirty minutes of prep the evening before. Here’s what I do now, and for a step-by-step frost protection guide, we go even deeper on each method.

How frost actually damages a tree

Before getting into protection, it helps to understand what frost does at the cellular level. When temperatures drop below 32 degrees, the water inside plant cells freezes and expands. Those ice crystals puncture cell walls from the inside out. When the sun comes up and everything thaws, the ruptured cells collapse. That’s why frost-damaged leaves look water-soaked first, then turn brown or black as the dead tissue dries out.

Young trees and fresh growth are most vulnerable because their cell walls are thinner. Mature wood has thicker, lignified cells that resist ice crystal damage. This is why a frost rarely kills an established oak but can wipe out a two-year-old lemon tree in a single night.

Know what’s at risk

Not every tree in your yard needs protection. Focus your time on the ones that actually face danger.

Established native trees handle Northern California frost just fine. Valley oaks (Quercus lobata), coast live oaks (Quercus agrifolia), and California buckeyes (Aesculus californica) have dealt with cold snaps for thousands of years. They don’t need your help.

The trees that need protection are:

Young trees in their first two winters. Any tree planted in the last 12 to 18 months has a shallow, undeveloped root system. A hard frost can damage roots in the top 2 to 3 inches of soil, killing the tree before it ever gets established. If you planted bare root trees last January or February, their first winter is the critical one.

Tropical and subtropical species. Citrus, avocado, and ficus are the big three in Northern California yards. But cold tolerance varies by species and even by variety:

• Meyer lemon (Citrus x meyeri): Handles 28 degrees for a few hours when mature. A young one planted last spring might die at 32.
• Eureka and Lisbon lemons: More cold-sensitive than Meyer. Damage starts at 29 to 30 degrees.
• Washington navel orange: Hardy to about 24 degrees once established. The toughest common citrus for NorCal yards.
• Hass avocado (Persea americana): Leaf damage at 30 degrees. Bark splitting and branch death below 26.
• Mexican lime (Citrus aurantiifolia): The most cold-sensitive citrus you’ll find at a nursery. Damage starts at 32. Don’t plant one unless you have a south-facing wall.

Trees that leafed out early. A warm January can trick deciduous trees into breaking dormancy weeks ahead of schedule. If new growth is out and a hard frost follows, those new leaves and buds will burn. Japanese maples (Acer palmatum) are especially vulnerable to this. So are fruit trees like peaches and plums that bloom early in zone 9.

Recently transplanted trees. Even a cold-hardy species is more frost-vulnerable during its first winter after transplanting. The root system is disrupted and the tree can’t move water efficiently. Give any transplanted tree extra protection for two winters.

Understand your yard’s microclimates

Frost doesn’t settle evenly across your property. Cold air is heavier than warm air, so it flows downhill and pools in low spots. A tree at the bottom of a slope might sit in air 5 to 8 degrees colder than a tree 30 feet uphill. I have a Meyer lemon at the top of a gentle slope in my backyard and it’s never had frost damage. My neighbor’s identical tree, 40 feet downhill, gets hit every year.

Spots that get frost protection for free:

• Against a south or west-facing wall. The wall absorbs heat during the day and radiates it at night. This alone can keep the air 3 to 5 degrees warmer.
• Under a patio cover or eave. Overhead cover blocks radiative frost (the kind where heat escapes straight up into a clear sky). Clouds do the same thing, which is why overcast nights rarely produce frost.
• Near concrete or brick patios. Thermal mass stores daytime heat.

Spots where frost hits hardest:

• Low ground at the base of a slope. Cold air ponds here.
• Open areas away from structures. No stored heat, no overhead cover.
• North-facing exposures. Less sun during the day means less stored heat to release at night.

When you’re choosing where to plant a new tree, think about frost pockets. Putting a citrus tree in the lowest, most exposed part of your yard is asking for trouble.

Water the ground before sunset

This is the single easiest thing you can do and most people don’t know about it. Water the soil around your trees thoroughly in the late afternoon, before the temperature drops.

Moist soil holds heat better than dry soil. Water has a high specific heat capacity, meaning it absorbs a lot of energy during the day and releases it slowly at night. As the temperature falls overnight, that stored heat radiates upward and creates a slightly warmer microclimate around the trunk and lower branches. It’s not dramatic, maybe 2 to 3 degrees of difference, but 2 degrees can be the gap between leaf scorch and a dead tree.

Don’t soak the soil to the point of standing water. Just a deep, thorough irrigation. Run your drip system for an extra cycle or hand-water with a hose for five minutes around each tree. Focus on the root zone, from the trunk out to the drip line.

One thing to keep in mind: this works best when the next morning will be sunny. If a multi-day freeze is forecast, water before the first night and keep the soil moist throughout.

Mulch protects the roots

A 3 to 4 inch layer of organic mulch insulates the root zone like a blanket over soil. Wood chips, shredded bark, and composted leaves all work. Keep the mulch 2 to 3 inches away from the trunk to prevent moisture rot, but spread it out to the drip line if you can.

Mulch slows heat loss from the soil surface. The soil temperature under 4 inches of mulch stays 8 to 10 degrees warmer than bare, exposed soil on a freezing night. That difference keeps roots alive. For young trees with shallow root systems, it can be the difference between a tree that recovers in spring and one you’re pulling out.

If you don’t have mulch down yet, November is the time to spread it. But even throwing down a few inches the day before a freeze helps. Free wood chips from tree service companies work great. Call around or check with your city’s green waste program.

For young trees, mulch is non-negotiable. I keep 4 inches of wood chips around every tree I’ve planted in the last three years, all winter long. For more on helping young trees through their first seasons, check out our tree planting tips.

Cover vulnerable trees before the freeze

Covering a tree traps ground heat around the canopy and prevents frost from settling directly on leaves and branches. According to the International Society of Arboriculture (ISA), properly applied covers can raise the temperature around a tree by 4 to 8 degrees on a still night. That’s significant.

What to use: Burlap, old bed sheets, lightweight blankets, or commercial frost cloth (sold at nurseries and garden centers for $10 to $20 per roll). Burlap and frost cloth are ideal because they breathe, letting moisture escape while blocking freezing air. Frost cloth (also called floating row cover) comes in different thicknesses rated by the number of degrees of protection they provide. A standard weight gives about 4 degrees of protection. A heavy-weight frost cloth gives 6 to 8 degrees.

What not to use: Plastic sheeting. Plastic traps moisture against the leaves and actually makes frost damage worse. The condensation on the inside of the plastic freezes on contact with the leaf surface. If rain is expected on the same night as frost, you can lay a tarp over the fabric cover to keep it dry, but the fabric goes against the tree, not the plastic.

How to cover: Drape the fabric over the tree so it reaches the ground on all sides. You want it touching the soil so the warm air rising from the ground stays trapped underneath. For a small tree, it’s like throwing a bedsheet over a lamp. For a larger tree, you might need to drape fabric from the lower branches down to the ground.

Use stakes or a simple frame to keep heavy fabric from crushing branches. A couple of wooden stakes with the fabric draped over them like a tent works fine. Secure the edges with bricks, rocks, or landscape pins so wind doesn’t blow the cover off at 3 AM.

Remove the covers every morning. Once the sun is up and the temperature rises above freezing, take the covers off. Trees need sunlight and air circulation during the day. Leaving covers on during a sunny day creates a greenhouse effect that can actually stress the tree. Put them back on before sunset if another frost night is forecast.

Adding heat for serious freezes

When the forecast calls for temps in the low 20s or below, covers and watering might not be enough for your most tender trees. Adding a heat source under the cover makes a real difference.

Outdoor-rated string lights. The old-fashioned incandescent C9 Christmas lights generate enough heat to raise the temperature under a cover by several degrees. LED lights don’t work for this because they produce almost no heat. Wrap a strand or two around the trunk and lower branches, then drape the frost cloth over everything. I’ve used this trick on my Meyer lemon for years.

Clamp lights with incandescent bulbs. A 60- or 100-watt incandescent bulb in a clamp light pointed at the trunk generates meaningful warmth in a small space. Position it under the cover, making sure the bulb doesn’t touch any fabric. Keep the cord and connections dry.

Don’t use open flame or propane heaters. Too much fire risk, especially with fabric covers. And never run a generator or any combustion device in an enclosed space around your trees near the house.

Small trees and container plants

For very small trees and container plants, improvise. A 5-gallon bucket turned upside down over a young tree works in a pinch. Cut the bottom off a milk jug and set it over a small sapling. Upside-down flower pots work for very small plants.

Move container trees against the house on frost nights. The wall radiates stored heat and blocks wind. A south-facing wall is best. If you have a covered patio, that’s even better since overhead cover alone can prevent radiative frost.

For valuable container citrus, consider bringing them inside a garage for the coldest nights. An unheated garage stays above freezing when outside temps drop into the 20s. The tree doesn’t need sunlight for one or two nights.

What frost damage looks like

Frost-burned leaves turn brown or black at the tips and edges. They look scorched, like someone held a lighter to them. Young growth wilts and goes limp within hours of thawing. Bark on thin branches can split lengthwise as the frozen wood expands.

The damage usually shows up the morning after a frost, but sometimes it takes two or three days to fully appear. Citrus leaves curl inward and look water-soaked before turning crispy. Avocado leaves go brown from the edges in. Japanese maple leaves turn black and papery.

On citrus, also check the fruit. Frozen oranges and lemons feel soft and mushy when you squeeze them. The juice vesicles inside have burst. The fruit is still safe to eat right away (frozen orange juice is still orange juice), but it won’t store and will rot quickly on the tree. Pick any frost-damaged fruit within a day or two.

Don’t panic and don’t start cutting. Here’s what to do instead.

After the frost: what to do (and what not to do)

Leave the damaged growth alone. This is the most common mistake. People see brown, burned leaves and immediately want to prune them off. Don’t. That dead tissue actually insulates the living tissue behind it. If another frost comes a week later, the damaged leaves act as a buffer that protects the branch underneath. The ISA recommends waiting until all frost risk has passed before pruning frost-damaged wood.

Wait until spring. Once new growth is clearly emerging and the frost risk is past (usually late March in most of Northern California, early April in foothill and mountain areas), you can prune away the dead material. Cut back to where you see green, healthy wood. For detailed pruning guidance, our guide on when to trim your tree covers timing for every common species.

Don’t fertilize. Fertilizer pushes new growth, and new growth is exactly what frost kills. Wait until the tree is actively growing in spring before feeding it. For citrus, that means your first February fertilizer application at the earliest, and only if the tree is already leafing out. For a broader look at caring for weather-damaged trees, our storm damage tree care guide covers what to do when the weather does its worst.

Water normally. Don’t overwater a frost-damaged tree, but don’t let it dry out either. Keep the soil consistently moist through the rest of winter. A frost-stressed tree is more susceptible to root rot, so good drainage matters more than ever.

Assess the trunk. On young citrus and avocado trees, check the trunk for vertical splits in the bark a few days after a hard freeze. This happens when the cambium layer freezes and expands. If you see splitting, don’t seal it. Keep the area dry and let it callus over naturally. If the split wraps more than halfway around the trunk, the tree probably won’t survive.

Which trees handle NorCal frost without help

If you’re tired of covering trees every January, plant species that laugh at our mild freezes. Here are the trees I’ve never had to protect in zone 9b:

• Valley Oak (Quercus lobata): Hardy to 0 degrees. Will outlive your house.
• Coast Live Oak (Quercus agrifolia): Evergreen, handles any NorCal frost.
• Japanese Maple (Acer palmatum): Hardy to zone 5 (minus 20). But protect fresh spring growth from late frost.
• Crape Myrtle (Lagerstroemia indica): Hardy to about 0 degrees. Drops its leaves and sleeps through winter.
• Eastern Redbud (Cercis canadensis): Hardy to zone 4. Bulletproof in NorCal.
• Chinese Pistache (Pistacia chinensis): Hardy to zone 6. Great fall color and zero frost concerns.

If you want citrus, plant Washington navel oranges or Satsuma mandarins (Citrus unshiu). Satsumas are the most cold-hardy citrus you can buy, handling temps down to 22 to 24 degrees. That’s well below what most of the Sacramento Valley sees, even in a bad year.

A simple frost prep checklist

The evening before a forecast freeze:

1. Check the forecast low and the duration of below-freezing temps. Brief dips to 31 are less dangerous than six hours at 27.
2. Water the soil around vulnerable trees in late afternoon, focusing on the root zone.
3. Spread mulch if you haven’t already. Three to four inches of wood chips out to the drip line.
4. Cover small and subtropical trees with burlap or frost cloth, all the way to the ground. Weight the edges.
5. Add incandescent lights under covers on your most valuable or vulnerable trees if temps will drop below 26.
6. Move container plants against the house or under a patio cover.
7. Bring any small potted citrus or tropicals into the garage if possible.

The next morning:

1. Remove covers after sunrise once temps are above freezing.
2. Check for damage but don’t prune anything.
3. Water again if the soil surface feels dry and another cold night is coming.

That’s thirty minutes of work that can save a $200 tree and five years of growth. Frost is predictable. The forecast gives you 24 to 48 hours of warning. Use that time and your trees will make it through winter just fine.

For more on protecting your yard from seasonal weather damage, including ice storms and heavy rain, those resources dig into the bigger picture of keeping your trees healthy year-round.