Generally, when talking about plants, 10.0 to 18.3 degrees C is cool and anything less than 10 degrees C is considered cold. Many plants can thrive indoors with temperatures less than 10 degrees C as long as the plants are kept out of drafts and not over-watered. Many houseplants need cooler temperatures to flower.
Cast Iron Plant
The cast iron plant (Aspidistra elatior) is able to withstand temperatures down to -1.11 degrees C. Cast iron plants need low light levels and can be watered every eight to 10 days. Leaves are dark green with pointy tips, and the stems have an arch. Cast iron plants that are kept indoors will rarely have flowers, but if kept outdoors, the plant can have bell-shaped dark purple to cream-colour flowers. Plants can grow to heights of 2 feet.
Cyclamen (Cyclamen persicum) prefer temperatures between 4.44 and 10.0 degrees C during the night and temperatures below 18.3 degrees C during the day. These plants are colourful indoor plants with colours ranging from lilacs to dark red, pastel pinks to white. Flowers extend upwards off stems protruding from silver heart-shaped leaves. Cyclamen start growing from tubers and need to be watered from the bottom of the container or from the top around the sides of container to keep the water away from the crown of the plant. These plants need bright light when kept in cooler to cold temperatures.
Christmas cactus (Schlumbergia) will bud more when the temperatures are below 12.8 degrees C and it has less than 12 hours of daylight. Plants that are kept in rooms with temperatures between 15.5 and 18.3 degrees C will need to be placed in a closet for at least 12 hours a night to promote budding. When temperatures are kept above 21.1 degrees C, the plant will more than likely never bloom. Christmas cactuses have flat green leaves connected to each other with a thin thread like stem. Buds form at the end of the leaves. The flower blooms a few days after the bud appears and can stay in bloom for weeks during the Christmas holidays.