The Freezing Temperature for Potted Magnolia

Chilling Considerations

• When growing magnolias in outdoor containers, understanding how air temperatures and soil temperature differ will help you succeed. When magnolias are planted in the landscape, surrounding soil acts as an insulating buffer against extremes. Soil protects magnolia roots from freezing and from dramatic temperature fluctuations that provoke cycles of freeze and thaw. In some locations, underground winter temperatures can be 35 to 45 degrees Fahrenheit higher than above-ground temperatures. Plant hardiness ratings assume plants will winter in the ground. For potted magnolias, the insulating impact of garden soil is lost.

From Hardy to Hardly

• Magnolia roots are less cold-hardy than branches and trunks. Plants that would survive subzero temperatures in the ground will be killed at much higher temperatures in pots. Saucer magnolia (Magnolia x soulangiana), hardy in U.S. Department of Agriculture plant hardiness zone 4 through 9, and star magnolia (Magnolia stellata), hardy in USDA zones 4 through 8, do well in containers. At the lower limit of their garden hardiness in zone 4, winter temperature extremes drop to 20 to 30 degrees "below" zero. When potted, both these magnolias can die when temperatures dip to 23 degrees.

Choosing Wisely

• To help your potted magnolias through winter, choose the right container upfront. Larger containers increase winter survival. The bigger the pot, the greater the temperature buffer provided by the soil around the tree. Water and nutrient reserves increase with larger pot size too. Ideally, a container magnolia should have 2 cubic feet of soil for every square foot of tree canopy. Containers with straight sides winter better than containers that slant out toward the top. Slanted containers tend to heave freezing soil up and out, while straight-sided pots keep it in place. The process of heaving subjects magnolia roots to greater temperature extremes.

Tolerance Tips

• Keep your potted magnolia healthy year-round so it enters winter at full strength. Create the warmest possible microclimate to help it through winter cold. Overwinter the plant in an area with extra shelter and warmth. Wind chill factor affects container temperatures, so choose sheltered locations close to buildings for wind protection as well as additional heat. North-facing locations protect from wind and premature thawing caused by warm winter suns. Wrap containers with insulation if more protection is desired. Straw provides good insulation too. Keep your potted magnolias off raised balconies or porches during winter. Air circulating below them makes these locations even colder.