A cold hard frost can be devastating to some plants in the garden. While some plants can come back and leaf out after the frost season, others may not be as fortunate. Damage by frost to plants occurs when they transpire moisture from their leaves but can't replace the lost moisture because the soil is dry or the water in the soil is frozen. In general, there are a few basic principles one should know and follow that will go a long way to protecting frost sensitive plants.
The warning signs for frost are still air, (tree branches motionless and chimney smoke going straight up); no cloud cover (stars easily visible, very bright); low humidity (windshields and grass dry); and low temperatures (45 degrees or less at 10PM). If you notice these danger signs at bed time, get any at-risk container plants under a porch roof or eaves, or in a garage.
Above all else, make sure the soil around the plant is moist when frost is expected. Moist soil holds and releases more heat than dry soil, which will create a more humid environment around the plant when the frost pulls moisture from the leaves. "Ice off" plants by spraying them with water. This will ensure water will freeze on the outside of the leaves rather than the moisture inside, which causes cells to rupture."
However, DO NOT hose down plants in the morning after a frost. Plants should be allowed to unthaw naturally and gradually to avoid rupturing plant cells in the leaf tissue. If covering a plant with blankets, sheets or plastic, make sure the fabric does not come in direct contact with plant foliage. Rather, create a tent-like structure that covers the plant without resting on the foliage.
Do not prune frost sensitive plants until danger of frost has past (March in San Jose ). In a sense, let the frost do the pruning for you. If your plant is injured from frost, leave the damaged burnt leaves on the plant so as to protect the foliage underneath it. Premature trimming may stimulate tender new growth that will be damaged by later frosts or cause frost to continue burning healthy tissue . You may also end up cutting out more than is necessary, mistaking still-alive growth for dead. Prune the damaged foliage only after the frost season is over and new growth has started to appear.
Come in and pick up your FREE copy of Almaden Valley Nursery's Frost Protection Guide. It has a complete list of plants that are susceptible to frost along with a list of products and information that can be used in the fight against frost. If you still have any questions as to which plants you should protect, just ask one of our garden experts. We'll be glad to help make Jack Frost get lost.
Matt Lepow, Owner, CCNPro, B.S. Ornamental Horticulture