cold frame 
noun
A frame with a glass top in which small plants are grown and protected without artificial heat.Sow seeds of herbaceous perennials and alpine plants in a cold frame or unheated greenhouse....
- Bulbs forced inside or grown in cold frames in portable containers can easily be placed into large, outdoor containers at flowering time.
- According to our thermometers, this combination was not as protective as cold frames covered with glass, but the plants didn't seem to notice.