As we approach the end of February 2019, a few more punches from old man winter can be expected before spring. As I write this, snow and ice storms are or will be occurring in many sections of the U.S.
So, with perhaps one more bout with frost and freeze it might be appropriate to provide some information how gardeners can make their last stand.
You may already know that a freeze occurs when temperatures drop below 32°F, the freezing point of water. When water freezes inside a plant, it can cause plant cells to explode causing irreparable damage.
If you are an experienced horticulturist, then you probably are aware that plants react differently to freezing temperatures. For example, tropical and frost-tender plants cannot survive a freeze and grow only in warmer climates. Annual plants can’t survive a freeze, but can disperse seeds to replenish their species once the weather warms. Root-hardy perennials die during a freeze, but the roots survive in a dormant condition until spring. Fully hardy perennials, shrubs, and trees are dormant in winter and that minimizes their vulnerability to a freeze. They survive because sap is reduced and water is conserved. A late spring freeze may damage spring blooms and early foliage, but the plants themselves most often recover.
Frost happens on clear, still nights. As the air temperature nears freezing, the surface temperature of the plant can dip below freezing resulting in ice crystals forming like dew on warmer nights. Since temperatures vary just a few feet above the ground, frost can form even when temperatures are above freezing.
There are three types of frost –- hoarfrost, rime, and black frost.
Hoarfrost appears feathery white on chilly mornings. It occurs when water in the air deposits directly as ice crystals.
Rime occurs when water is deposited as dew or fog that then freezes and appears as a glaze.
Black frost occurs when plants are damaged and blackened by freezing temperatures, but frost doesn’t form.
Effects Of Freezing Temperatures On Plants
In the case of tender plants, it is not the frost or freeze that effects the plants, it is how cold it gets and for how long.
There are terms that assist gardeners in determining the severity of a freeze.
- Light frost or light freeze occurs when temperatures drop to 28°F for a couple of hours and harms only very tender plants when ice forms on the outside of the plant.
- Hard frost, killing frost, or moderate freeze occurs when temperatures drop to 25°F to 28°F for several hours, which results in damaged foliage and blossoms due to ice forming inside the plant.
- A severe freeze occurs when temperatures drop below 25°F for several hours causing damage to many plants, mostly due to drying.
It is suggested that you avoid stimulating tender plants with fertilizer until the freezing weather passes.
The plants most susceptible to a frost include:
- Houseplants and tropical
- Spring-blooming shrubs and trees like azaleas, rhododendrons, and cherry.
- Citrus trees
- Tender bulbs like dahlia and elephant ear.
- Warm season vegetables like tomatoes, corn, and peppers.
- Warm season annuals like impatiens, petunia, and geranium.
It is recommended that you cover shrubs with a blanket to protect them from a late spring frost.
Other ways to protect plants from frost or a freeze include:
- Frost-tender plants in containers should be brought indoors during cold weather.
- Dig up tender bulbs and store them in a cool dry place.
- Water plants thoroughly before a freeze to prevent drying and to add insulating water to the soil and plant cells.
- Cover tender sprouts overnight with an inverted bucket or flowerpot, or with a layer of mulch. Don’t forget to uncover them in the morning when the temperature rises above freezing.
- Cover shrubs and trees with fabric, old bed sheets, burlap, or commercial frost cloths. It is best to drape the cover over a frame so that the material doesn’t touch the foliage. Fabric covers assist in trapping heat from the soil. So be certain that the drapes go all the way to the ground. Uncover in the morning when temperatures rise above freezing.
After the freeze or frost, assess the damage to your plants. Hardy perennials, trees and shrubs may recover from a late spring freeze, even if visibly damaged. Frost-tender plants will not recover. So don’t plant them until you’re sure that freezing temperatures have ended.
To avoid damage, it is best to select and install plants that are hardy for your climate zone, or plant tender plants in containers that can be brought indoors.