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Why big rhododendron leaves roll up in the cold

November to March

On a hard-freezing morning, the leaves of some big-leaved rhododendrons roll inward from the midrib and hang down. As the air warms above freezing they slowly unroll again.

Why this time of year

These rhododendrons keep their leaves through the cold months, so a hard freeze and the thaw after it can be watched on the same shrub.

How to notice it

  1. On a very cold morning, find a big-leaved rhododendron near a house or fence and look at the leaves. Note whether they hang down and are rolled inward from the sides. That curling is called thermonasty.
  2. Check the same shrub again once the air is above freezing, and see whether the leaves have slowly opened. Drooping on its own can also mean the shrub is short of water.
  3. Compare mornings: the colder it is, the tighter the curl. If you have an outdoor thermometer, check it, but remember the leaf's own temperature is what the plant responds to.

Try this with kids: Is the rhododendron leaf rolled up tight or open and flat? What do you guess the weather is, and does your thermometer agree?

Word to know: “Thermonasty” means curling and drooping of leaves set off by cold.

Sources

  • rhododendron.orgAmerican Rhododendron Society: normal for some rhododendrons to droop and curl at around 32F; the lower the temperature the tighter the curl.
  • cals.iastate.eduIowa State University news on Rajeev Arora's research: leaves roll inward from the midrib one or two degrees colder than freeze initiation; in the morning as air rose above freezing the leaves slowly unfurled. Though it is not common, it does happen in several species of broad-leaved Rhododendron; cold-induced thermonasty is curling and drooping of leaves.
  • archive.rhododendron.orgNilsen and Tolbert (Virginia Tech), J. American Rhododendron Society v47n2: curling at a narrow leaf-temperature range above the leaf freezing point; the cue is leaf temperature. Leaf temperature required to elicit curling 0 to -2 C; little to no curling in some species; leaf angle movement primarily stimulated by water stress.
  • scholar.lib.vt.eduSame Nilsen paper (VT host): critical temperature varies among species; winter curling stimulated only by temperature.

Checked against these sources on October 9, 2026.

In the guide for November, December, January, February, March

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