Renewal Pruning an Arborvitae

About Arborvitae

• There are three commonly grown species of arborvitae and hundreds of named cultivars, though the plants have only two main growth habits, pyramidal or globe shaped. Oriental arborvitae (Thuja orientalis) grows up to 20 feet tall, with a pyramidal shape, and is suitable for U.S. Department of Agriculture plant hardiness zones 6 through 11. Giant arborvitae (Thuja plicata), or Western red cedar, commonly grows 50 to 75 feet tall in USDA zones 6 through 8 A. American arborvitae (Thuja occidentalis), or white cedar, is hardy in USDA zones 2 through 7 and reaches 25 to 40 feet tall.

Renewal Pruning

• Renewal pruning is a multi-year process that cuts shrubs down to size by removing one-third of the tallest, oldest branches each year for three years running. In the case of arborvitaes, remove up to one-quarter of the longest, tallest branches back to the trunk or a side branch to control size, making it a four-year process. Arborvitaes are best pruned in late winter to early spring. This takes advantage of the growth cycle of the plant, which begins its new season of growth in spring.

Renewal Exceptions

• Not all arborvitae can recover from drastic pruning. In most cases it is not recommended to remove more than 20 percent of the height of any arborvitae at one time. Arborvitae shrubs growing in full sun are better able to regenerate new growth from buds under the bark in the joint where branches meet than are arborvitaes growing in shade. Older arborvitae growing in the shade, especially, and oriental arborvitae should not be pruned past the shell of green growth on the outside of the plant -- they may not grow back.

Considerations

• Prune only with sharp, sterile tools to avoid tearing plant material and spreading disease and fungus from plant to plant on pruning tools. Household antiseptic cleaner, rubbing alcohol or diluted bleach can eliminate pathogens without degrading the metal of the tool. Wearing gloves when pruning arborvitae is recommended, because contact with its foliage can cause an itchy rash.