Thuja Propagation

by M.H. Dyer

A stately, fast-growing evergreen, thuja, also known as arborvitae (Thuja spp.) is often planted to create an attractive privacy screen around a family gathering spot or children's play area. Although hardiness varies among species, most types of thuja are suitable for growing in U.S. Department of Agriculture plant hardiness zones 5 through 8. However, some are hardy to USDA zone 2. Propagate thuja by stem cuttings or seeds.

1. Stem Cuttings

Thuja is propagated from the current year's growth when stems are firm but still flexible. Cuttings taken from older growth are less likely to root. Thuja requires a well-drained potting mixture such as equal parts peat and either coarse sand or vermiculite. Dipping the bottom of the stems in rooting hormone just before planting aids rooting. Plant six or seven cuttings in a 6-inch pot, just deep enough to keep the stems upright. Thuja requires warmth, and cuttings root best in consistently warm temperatures -- usually in about three months. If you have a heat mat, set the temperatures to 68 F. Otherwise, cover the pot with a plastic bag to create a warm, humid environment. Check the cuttings and mist lightly if the potting mixture feels dry. However, the bag keeps the potting soil moist for several weeks.

2. Seeds

Extract thuja seeds from female cones, which are upright with hinged scales, in autumn. Place the seeds in a plastic bag filled with moist peat moss and chill the seeds in the refrigerator until spring. Thuja seeds germinate well in a small container filled with a seed-starting mixture consisting of three parts peat moss, one part fine bark and one part vermiculite or perlite. Germination requires temperatures between 54 and 59 degrees Fahrenheit. Keep the seed-starting mix slightly moist but never soggy. Seeds that germinate successfully reach heights of 1 to 2 inches in six to eight weeks.

3. Hardening

Before planting outdoors, thuja seedlings grown in carefully controlled conditions must be acclimated to outdoor temperatures. The hardening off process requires about a week. Begin by moving the seedlings into a protected outdoor location for an hour. Add an hour the next day and then continue adding about an hour every day until the seedlings spend at least seven hours outdoors. Plant the hardened thuja seedlings in their permanent home.

4. Growing Requirements

Although thuja grows in partial shade, the plant performs best in full sunlight. Thuja tolerates nearly any type of soil, including acidic soil or soil that is consistently wet or dry and rocky. However, the best growth occurs in moist, well-drained, neutral to slightly alkaline soil with a pH 6.8 and 7.2. Amend the soil lightly with compost or manure before planting and then keep the area around the tree mulched with bark chips, pine needles or another natural mulch to keep the soil moist and cool. Don't allow mulch to pile up against the trunk.

5. Considerations

Wear gloves when working with garden soil and amendments to protect yourself from soil-borne pathogens.