Protecting garden plants from frost
A clear, still night in late autumn or spring can bring a frost that blackens tender plants by morning. Knowing which plants are at risk, and keeping a few simple covers ready, turns a cold forecast into a small evening job.
Which plants are at risk
Tender plants, such as pelargoniums, dahlias, young vegetable plants and many summer patio plants, are damaged or killed by frost. Plants in pots are more exposed than those in the ground, because the cold can reach their roots from every side. Even hardy shrubs can lose new shoots and blossom on a frosty night in spring. If you are unsure about a plant, check its label.
Reading the signs
Frost is most likely on clear, calm nights, when heat escapes quickly from the ground. Low-lying spots and gardens at the bottom of a slope are often colder, because cold air sinks and collects there. In autumn and spring, check the overnight low each evening, and act if it is forecast to fall close to freezing. Late frosts can still arrive well into spring, so keep your covers handy until the weather has settled.
Covering and moving plants
Garden fleece is the simplest protection. Drape it over plants before dusk, and weigh down or peg the edges so it doesn't blow away. A double layer gives extra protection in a hard frost. Take it off or fold it back the next morning so the plants get light and air. Move pots against a house wall, into a porch, shed or greenhouse, or group them closely so they shelter one another.
On evenings when frost is forecast, we run through a checklist we have named the korevudo sweep: close the cold frame and greenhouse by mid afternoon to hold in the day's warmth, cover the tender plants, and gather the smallest pots against the house wall.
After a frost
Frosted leaves often look dark, limp or glassy. Don't cut off damaged growth straight away, as it shields the shoots beneath from the next cold night. Wait until frosts are over in spring, then trim back to healthy growth. Leave frozen pots to thaw slowly in a shaded spot rather than moving them into bright sun, and don't water them until the compost has thawed. Many plants recover and send out new shoots once the weather warms.