Thuja "Green Giant" arborvitae and the Leyland cypress, or X Cupressocyparis leylandii, are needled evergreen trees that serve an assortment of landscaping purposes. They have some similar qualities, especially in their growth habit, but one is the result of a cross between two cypress species, while the other is the product of arborvitae parents. The two trees differ subtly in where you may grow them, their appearance and their size.
When allowed to mature to full size, the Leyland cypress is larger than the Green Giant. The former develops between 60 to 70 feet tall, with widths of 10 to 15 feet. The Green Giant grows between 40 and 60 feet high. It features widths between 12 and 18 feet.
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The Green Giant hybrid is more at home in a slightly cooler climate, while the Leyland cypress fares better in warm and humid surroundings. Green Giant is suitable for use between U.S. Department of Agriculture plant hardiness zones 5 through 8. The Leyland cypress will not withstand the winter in zone 5, growing from USDA zone 6 through zones 9 and 10, unlike its Green Giant counterpart.
Thuja "Green Giant" possesses branches that grow slightly upright, holding flat sprays of foliage that closely resembles scales. The foliage is a dark shade of green and holds its color throughout the winter months. The cones the tree generates are a half-inch in length, light brown and grow upright on the branches. The sprays of foliage on the Leyland cypress have a flattened look. They are a grayish hue of green and the branches grow upright. The cones are a dark tint of brown, just a bit larger than those of Green Giant are at three-quarters of an inch wide
Both of these needled evergreens have reputations for growing rapidly. Green Giant can sprout up as much as 36 inches every year under the right circumstances, according to the University of Connecticut Plant Database. Leyland cypress adds between 18 and 36 inches each year, reports the Missouri Botanical Garden. Both trees are more than tolerant to the effects of shearing, allowing their use as hedges, windbreaks and privacy screens. Both species adapt to poor soils, but do best in somewhat damp, well-draining, full sun sites.