Magnolias are attractive, ornamental trees in garden landscapes, but the look may change if an early or late frost nips them. Magnolias grow as large shrubs or small trees, and there are many different species that produce flowers in yellow, purple, red, pink or white. Freeze damage may seriously harm your magnolias; it could prevent flowering and may even be fatal.
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Magnolias are commonly regarded as warm-region plants, but some species are cold hardy up to United States Department of Agriculture Plant Hardiness Zone 3. Magnolia trees that go dormant during winter protect themselves against cold by halting growth, but a late-season or early-season temperature change may cause freezing damage. A light freeze occurs when temperatures fall between 29 to 32 degrees F. Light freezes kill tender plants. Moderate freezes occur at 25 to 28 degrees F, and may cause heavy damage to plants. Severe freezes, which occur at 24 degrees F and lower, will cause damage to most plants. Kobus magnolia, Loebner magnolia and star magnolia (Magnolia stellata), are cold-hardy growers, but they flower early and may be damaged by late-season freezing weather. Some early-flowering types of saucer magnolia (Magnolia x soulangeana), may also be damaged by late freezes.
Freeze-damaged areas of magnolia trees may become soft and limp, eventually dropping from the tree after they blacken and shrivel. Flower buds exposed to freezing temperatures will dry up and fail to bloom, and flowering could be prevented all over the tree for the remainder of the season. Flower buds are sensitive to cold temperatures, and are easily damaged by even light freezes.
Unless freezing temperatures affect the roots of the magnolia, plants will recover after sustaining damage. The roots of the star magnolia, for example, are damaged at 23 degrees F. Don't prune off the damaged wood until spring, when the plant leafs out. Once the weather warms, apply nitrogen-heavy fertilizer to freeze-affected trees that have been established at least five years.
Plant trees in a sheltered spot to avoid damage from freezing temperatures. The western and southern exposures are usually the warmest. Planting trees near buildings also provides protection from chilling winds and weather. Keep magnolias watered, even during winter, to keep them strong and healthy. Dehydrated, dry trees are more likely to become damaged by freezing weather. Protect the soil around the tree with mulch to provide warming insulation around the root zone. If severe freezing weather is expected, you can cover the tree with sheets, blankets or a commercial frost cloth to keep the tree warm. Cover the plant from the top all the way to the ground to trap heat around the tree. The covering should be removed in the morning when temperatures rise above 50 degrees F.