The plants to protect from frost fall into three groups. You need to cover warm-season crops like tomatoes and peppers. You also need to guard tender plants like basil and citrus trees. Soft-stemmed flowers fall into this group too.
I lost a whole basil patch one May night to a surprise 30°F (-1°C) dip. The forecast missed the cold by a wide margin. By sunrise the leaves looked black and limp. The smell of cooked basil hung over the bed all day.
You can sort tender plants into tiers by how much cold they can take. Tomatoes die at 32°F (0°C) since their cells hold no sugar to stop ice. Kale packs sugar into each leaf and holds firm down to 10°F (-12°C) with no cover at all.
The list below shows damage points for common garden plants. Data comes from UC Master Gardeners and Iowa State Extension.
Tender Tropicals and Annuals
- Basil and impatiens: Damage starts at 32°F (0°C). Death follows within hours of any frost on the leaves.
- Tomatoes and peppers: Leaves die at 32°F (0°C) since ice crystals burst the cells inside each leaf.
- Citrus limes and lemons: Limes burn at 29°F (-2°C). Meyer lemons hold down to 22°F (-6°C) for short spells.
Semi-Hardy Plants
- Lettuce and chard: Light frost down to 28°F (-2°C) causes minor leaf burn. The crown stays alive for new growth.
- Broccoli and cauliflower: Take 26°F (-3°C) with no cover. Flavor turns sweeter after a light freeze.
- Strawberry plants: Flowers die at 30°F (-1°C). Crowns hold up well below freezing on their own.
Cold-Hardy Champions
- Kale and collards: Take 10°F (-12°C) with ease. Leaves grow sweeter as starches turn to sugar.
- Spinach and mache: Hold under 15°F (-9°C) with light mulch in zones 5 through 8.
- Brussels sprouts: Keep flavor and texture down to 20°F (-7°C) for harvest through winter.
Most gardeners waste covers on plants that need no help. Save your time for the truly frost-sensitive plants in the tender list above. The hardy ones do fine on their own through a light freeze.
Build your own tier list for the warm-season vegetables you grow each year. Write down the lowest temperature each one can take without harm. Check the forecast low against your list before a cold night.
You should cover only plants rated above the predicted low for that night. Skip the hardy crops since they take care of themselves. This simple rule saves hours of work and keeps your fragile plants safe.
Keep your covers near the back door from October through May in most zones. You will move fast when a surprise freeze rolls in. The patch of basil I lost that one May night still stings, and your garden does not need to learn the same hard way.
Read the full article: Frost Protection for Plants: Complete Guide
