How cold is it? In Boston today, it was cold enough for the rhododendron leaves to begin to curl.
Rhododendron can be used as a natural thermometer if you learn the signals their leaves are giving you.
In this photo taken midwinter the leaves were beginning to tighten into their most protective roll.
Garth Wedemire over at www.rhododendron.org says, "Leaves of rhododendrons respond to cold by first drooping...then curling up.
- At temperatures above freezing, the leaves extend at approximately right angles from the stems.
- Just below freezing, the leaves droop downwards...but remain flattened.
- At 25° or thereabouts...the leaves droop and curl, and by 20°, they are curled so tightly that they look like green pencils. Once the temperature drops below 20°, they can't curl up much tighter, and only a non-living thermometer will do."
He says this is a defensive reaction designed to prevent moisture loss from the leaves. These plants will show some drooping and curling in extreme heat as well.
Details:
Here's the page with Garth Wedemire's article.
http://www.rhododendron.org/news/plantsv10n3.htm
Here's a selection of books for city gardeners with small spaces.
