Ever wonder why rhododendron shrubs wilt in the cold? It’s not dying, it’s surviving!

Freezing temperatures cause the water inside the leaves to burst. That’s why most plants drop their leaves before the coldest winter months arrive. Some plants, like pines and spruces, stay green all winter. They have small waxy, needle-shaped leaves that can resist deep winter freezes.

Rhododendrons are one of the few plants with large leaves that stay green in our climate all winter long. Instead of losing leaves, rhododendrons lose the water in their leaves during cold temperatures.

The process is called thermonasty! “Thermo” is related to temperature and “nastic” describes how plants can move in response to their environment. When the weather drops below freezing, rhododendron leaves start to wilt. Below 25, the leaves start to curl. Rhododendron leaves curl up so tightly below 20 degrees, the plant looks practically bare!

This adaptation protects the leaves from the strong winter sun. The strong winter sun causes the water to evaporate and when temperatures are below freezing, rhododendrons can’t absorb water to replace what is lost. So instead, they take the water out of their leaves. Problem solved!

You’ll notice other broadleaf evergreen plants also have thermonastic leaves. And some flowers, like tulips, are also thermonastic! They will close on cold nights to protect their delicate reproductive system and open wide on warm days to invite pollinators.