Emerald Green' is one of the more popular arborvitae cultivars. They grow fast up to 20 feet tall. They can withstand cold temperatures and rarely have problems with pests and diseases. These trees are best planted in the fall when they will experience minimal heat stress. Their foliage consists of flat sprays of glossy bright green needles, plus urn-shaped cones about an inch long that turn reddish-brown in the fall.