Read the Temperature in Your Broadleaf Evergreens

A quick lesson in plant biology

With a little experience, you can learn to tell how cold it is by observing the leaves of rhododendrons and acuba (the gold-dust plant). These plants start curling their leaves when temperatures drop below 28 degrees. The margins on both sides of each rhododendron leaf will curl noticeably downward. When temperatures fall from 15 to 20 degrees, both margins will curl into two tight scrolls attached by the mid-rib. The tips of acuba leaves will curl downward and become darker green to almost black.

As temperatures rise, the leaves uncurl. But they do not achieve their normal appearance until temperatures are above freezing.

Most plants have guard cells to regulate the amount of water lost from leaf tissues. Kidney-shaped guard cells surround each stomata, the microscopic openings mostly on the undersides of leaves that allow air to enter — important for photosynthesis — and moisture to exit so as to maintain turgidity. During drought or extreme cold, the guard cells close to prevent water loss.

As these species lack guard cells, they prevent water loss by curling their leaves. A curled leaf has a smaller surface than a fully expanded leaf.

Thus if you see your rhododendron curling its leaves during the summer months, the plant is asking for water. Dry acuba will exhibit wilting symptoms similar to other plants.

Fruits and vegetables you can grow in your garden

Learn how to feed your family from the Bay Gardener in Chesapeake Railway Museum’s Bayside Chat March 7 at 2pm at Northeast Community Center, Chesapeake Beach. Free: 410-257-2554.