Emerald green arborvitaes will reach maximum heights of up to 15 feet. Their base will grow to about four feet wide, sloping into a pyramid shape toward the top. Trees planted in dense hedges will not often reach these maximum sizes. With a moderate growth rate of one to two feet per year, arborvitaes will reach their mature height within 10 to 15 years. If the trees are grown in ideal conditions, you can expect a lifespan of approximately 25 years.