Understanding Arborvitae Species
Arborvitae (genus Thuja) encompasses several species, each with distinct characteristics. Understanding which species and cultivars you're growing is essential for proper care.
Thuja occidentalis (Eastern or American Arborvitae)
The most common and widely grown species in North America, especially the eastern and midwestern US. Native from Quebec to North Carolina, it's naturally adapted to cool, humid climates.
- Hardiness: USDA zones 2-7 (extremely cold-hardy)
- Typical form: Narrow, pyramidal, columnar, or sometimes dwarf
- Foliage: Soft, feathery, flattened sprays. Green to yellow-green in summer; bronzes to brown in winter (normal).
- Growth rate: 12-24 inches per year for standard cultivars; much slower for dwarf forms
- Best use: Hedges, screening, specimen plantings, bonsai
Thuja plicata (Western Red Cedar)
Native to the Pacific Northwest, a larger tree with a more open, picturesque form. Less widely grown than Eastern arborvitae but increasingly popular.
- Hardiness: USDA zones 5-8 (more tender than T. occidentalis)
- Typical form: Large, pyramidal, naturally open branching
- Foliage: Broader, flatter sprays; darker green, more lustrous
- Growth rate: 24-36 inches per year; grows larger overall
- Best use: Specimen trees, larger hedges, maritime climates
Key Differences
T. occidentalis is hardier, slower-growing, and produces more cultivar diversity (especially dwarf and colored forms). T. plicata is larger, faster-growing, and preferred in milder climates and Pacific regions. For most of North America, T. occidentalis is the standard choice.
Planting Arborvitae
Proper planting sets the foundation for decades of healthy growth. Arborvitae is relatively forgiving but performs best with thoughtful site selection and technique.
Site Selection
- Sunlight: Full sun (6+ hours) to partial shade. Arborvitae tolerates part shade but grows denser and more compact in full sun.
- Soil: Well-draining soil. Arborvitae dislikes heavy clay or waterlogged conditions but tolerates a wide pH range (acidic to slightly alkaline).
- Moisture: Consistently moist but not waterlogged. Avoid drought-prone, sandy sites without irrigation.
- Wind: Sheltered from harsh, drying winds, especially in winter. Wind increases desiccation (browning).
Planting Technique
- Dig hole: Dig a hole as deep as the root ball and 1-2 times as wide. The top of the root ball should be level with grade (not buried deeper).
- Prepare backfill: Mix native soil with compost (25-30%) to improve drainage and soil structure.
- Plant: Remove the plant from its container, loosen the root ball gently, and place in the hole. Backfill halfway, water to settle soil, then finish backfilling.
- Firm soil: Gently firm soil around the plant, but avoid compacting. Leave a slight basin to hold water.
- Water thoroughly: Water deeply to ensure good root-soil contact.
- Mulch: Apply 2-3 inches of wood chips or bark mulch around the base (not touching the stem).
Spacing
- For hedges: Space plants 2-4 feet apart, depending on cultivar width at maturity. Closer spacing creates a denser hedge faster.
- For specimens: Space according to mature width. A dwarf cultivar (3-4 feet wide) needs 4-5 feet; larger forms need 6-8+ feet.
Best Planting Time
- Spring (March-May): Ideal. Plants establish over the warm season.
- Early fall (August-September): Also good. Plants establish before winter.
- Avoid: Late fall or winter planting. Plants won't establish and often suffer winter damage.
Watering and Soil
Consistent moisture management is critical for healthy arborvitae, especially in the establishment phase and during drought or winter.
Watering Needs
- First year: Water deeply 1-2 times weekly during growing season if rain is less than 1 inch per week. The goal is consistently moist (not soggy) soil.
- Established plants: Arborvitae is drought-tolerant once established (2+ years), but performs better with consistent water. During extended drought, water deeply every 2-3 weeks.
- Winter: Watering in fall and winter prevents winter desiccation. If fall/winter is dry, water established plants every 3-4 weeks until ground freezes.
Soil Conditions
Arborvitae prefers:
- Well-draining loam or sandy loam (avoid heavy clay)
- pH 5.5-7.5 (slightly acidic to neutral)
- Moderate fertility (avoid over-fertilizing)
Poor drainage: If your soil is clay, amend heavily with compost, peat, and sand. Consider raised beds or installation in a berm if drainage cannot be improved.
Mulching
A 2-3 inch layer of wood chips or shredded bark around the base improves moisture retention and regulates soil temperature.
- Keep mulch 3-4 inches away from the stem to prevent rot.
- Refresh mulch annually in spring.
Winter Desiccation Prevention
Winter desiccation (browning in late winter) is common in arborvitae, especially in harsh, windy climates. Water thoroughly in fall before ground freezes. In severe climates, wrap young plants with burlap to shield from drying wind.
Pruning and Shaping
Arborvitae responds well to pruning and can be shaped into formal hedges, screens, or natural forms. Proper technique ensures dense growth and avoids killing inner branches.
When to Prune
- Timing: Late spring to early summer (May-June in the North) is ideal. Pruning in this window allows new growth to harden before winter.
- Avoid: Fall pruning (August-September onward). New growth won't harden, leaving tender shoots exposed to winter damage.
- Winter damage: Prune out broken or damaged branches in spring after new growth begins.
How to Prune Without Damage
Critical principle: Never cut into bare (brown) wood. Arborvitae has green foliage on outer stems, but the interior is woody and leafless. Cutting into this interior growth typically doesn't regenerate—the branch dies.
- Prune only the outer, green foliage. Remove up to 1/3 of the plant's foliage in a season. This stimulates branching and maintains density.
- Use sharp tools: Hand shears for small branches, hedge shears or electric hedgers for larger surfaces. Clean cuts prevent disease.
- Step back frequently: Shaping is iterative. Remove a little, step back, assess, and repeat. This prevents over-pruning.
Shaping Formal Hedges
For a dense, formal hedge:
- In the first year, prune lightly to encourage branching. Remove the top 6-12 inches.
- In subsequent years, prune to maintain a pyramidal or rectangular form. Wider at the bottom, narrower at the top.
- Prune the top and sides evenly. Never leave the top wider than the base or lower branches won't receive light.
- Aim for a hedge 1-2 feet shorter than desired final height; growth will fill in over time.
Shaping Specimen Plants
For natural, specimen shapes:
- Remove crossing or damaged branches.
- Thin interior branches slightly to improve light penetration.
- Remove lower branches if you want a more open, tree-like form.
- Minimal pruning maintains the plant's natural character.
Dwarf Cultivar Pruning
Dwarf arborvitae cultivars often need minimal or no pruning. If they become dense and crowded, thin interior branches slightly in late spring. Avoid heavy heading back, which can leave visible pruning wounds.
Common Problems
While arborvitae is relatively hardy, several issues can arise. Early identification and treatment prevent serious decline.
Browning (Winter Desiccation, Spider Mites, Root Issues)
Symptoms: Foliage turns brown or bronze, starting in late winter and continuing into spring. May affect the entire plant or only sun-exposed sides.
Causes and solutions:
- Winter desiccation (most common): Dry, cold wind and sun cause moisture loss faster than roots can replace it, especially in sandy soil or with poor fall watering. Prevent by watering well in fall, ensuring mulch, and wrapping young plants in burlap in harsh climates. Prune out dead tissue in spring once new growth begins.
- Spider mites: Cause stippled, yellowing foliage and fine webbing in hot, dry conditions. Spray with water weekly or use horticultural oil. Improve humidity by watering regularly.
- Root rot (from poor drainage): Foliage browns, becomes dull, and doesn't recover. Check soil—if constantly waterlogged, improve drainage or transplant to a better-draining location. Trim out dead wood.
Deer Browsing
Symptoms: Ragged, browsed foliage, often on the lower 3-4 feet of the plant. More common in winter when other food is scarce.
Solutions:
- Install 6-8 foot fencing around vulnerable plants or areas.
- Use repellents (capsaicin-based sprays, rotten egg sprays, or commercial deer repellents). Reapply monthly or after rain.
- Plant in protected locations with human activity nearby.
- Grouping arborvitae with less-palatable plants may reduce damage.
Bagworms
Symptoms: Small, brown bag-like structures (1-2 inches) hanging from branches. Larvae inside feed on foliage from within the bags.
Solutions:
- Hand-pick bags in fall or winter before they pupate. Remove and destroy.
- Spray with Bacillus thuringiensis (Bt) in late spring when larvae are young and actively feeding.
- Monitor for new bags throughout the season.
Snow and Ice Damage
Symptoms: Branches break or bend under heavy snow/ice. Multi-stem columnar forms are especially prone to splaying.
Solutions:
- Wrap young, narrow plants with soft rope or burlap before winter to support splaying.
- Gently remove accumulated snow from branches to reduce weight (shake branches lightly rather than forcefully).
- Prune out broken branches once growth resumes in spring.
Propagating Arborvitae from Cuttings
Arborvitae propagates readily from cuttings, making it possible to clone rare cultivars. Propagation is one of the most exciting ways to grow arborvitae at home or develop stock for propagation business.
Best Timing: Late Fall/Early Winter
Take cuttings in November-December (late fall, early winter in the Northern Hemisphere). This semi-hardwood stage—where growth has firmed but hasn't fully hardened—optimizes rooting success.
Why late fall? Late-season cuttings root during winter under cool, humid conditions. By spring, rooted cuttings are ready to pot up and establish.
Taking Cuttings
Size and selection:
- Cut 4-6 inch terminal (tip) shoots using sharp pruners.
- Choose healthy, vigorous growth from the outer canopy.
- Include a small section of older wood at the base—a "heel." This is the best rooting section.
Preparation:
- Remove lower foliage, leaving 2-3 leaves at the tip. Foliage below soil level will rot.
- Lightly wound or score the base of the cutting (optional but helpful). Make 2-3 shallow cuts along the stem base.
- Dust the base with rooting hormone powder (IBA—indole butyric acid—is standard). Tap off excess powder.
Rooting Medium and Conditions
Medium: Well-draining, sterile, and aerated. Mix equal parts peat moss (or coconut coir), perlite, and sand. Or use commercial rooting medium.
Containers: Small pots or propagation trays with drainage holes. Fill with pre-moistened rooting medium.
Humidity: High humidity is critical. Place cuttings in a plastic dome, propagation chamber, or humidity tent. Condensation inside the dome indicates proper humidity.
Temperature and light: 60-70°F is ideal. Bright, indirect light (avoid direct sun, which heats and dries the medium). A shadehouse or north-facing bench under lights works well.
Moisture: Keep medium evenly moist (not waterlogged). Mist cuttings once or twice daily if humidity drops.
Rooting Timeline
Roots develop slowly over 8-12 weeks (sometimes longer in very cool conditions). Check for rooting by:
- Gently tugging the cutting. Resistance indicates rooting.
- Peeking at drainage holes or sides of medium for visible roots (if using clear containers).
- Observing new growth. Rooted cuttings often flush new foliage as rooting completes.
Post-Rooting Care
Once roots are 1/2 inch or longer:
- Gradually reduce humidity: Remove the humidity dome for 1 hour daily, gradually increasing daily exposure over 1-2 weeks.
- Pot up: Transplant rooted cuttings into 2-4 inch pots filled with potting soil (not rooting medium).
- Shade and hardening off: Keep potted cuttings in bright, indirect light and high humidity for 2-3 weeks. Gradually acclimate to normal greenhouse or outdoor conditions.
- Overwintering: If rooted cuttings are potted up in late winter/early spring, grow them in a cold frame or nursery bed through the summer, then plant out in fall.
- First growing season: Keep soil consistently moist and protected from intense heat. Fertilize lightly every 4-6 weeks. Plants should reach 6-12 inches by fall.
Cultivar Selection Guide
Thuja occidentalis has hundreds of cultivars, each with distinct form, color, size, and hardiness. Choosing the right cultivar for your intended use ensures success and satisfaction.
For Screening and Formal Hedges
- DeGroot's Spire (DeGroot's Spire): Ultra-narrow columnar form, 20-25 feet tall, 3-4 feet wide. Perfect for tight spaces and narrow screens. Zone 3-7.
- Zebrina (variegated): Columnar form, 20 feet tall, 4 feet wide. Golden-yellow variegated foliage that brightens hedges. Zone 3-8.
- Emerald Green (Smaragd): Dense, narrow columnar, 15-20 feet tall, 3-4 feet wide. Very popular. Zone 3-8.
For Specimen and Accent Plantings
- Rheingold: Dwarf, mounding form, 4-6 feet tall, warm golden-bronze foliage year-round. A collector's favorite for its color. Zone 2-8.
- Filiformis: Dwarf, fine thread-like foliage, 4-6 feet tall. Unique texture ideal for collectors. Zone 3-8.
For Rock Gardens and Containers
- Rheingold: Dwarf, 4-6 feet tall, mounding form with golden-bronze foliage. Excellent for containers. Zone 2-8.
- Filiformis: Dwarf, 4-6 feet tall, fine, filiform (thread-like) foliage. Unusual texture. Zone 3-8.
For Unique Texture and Color
- Filiformis: Ultra-fine, thread-like foliage. Very fine texture, unusual and prized by collectors.
- Rheingold: Golden-bronze foliage year-round. Warm color that intensifies in winter.
- Zebrina: Golden variegation creates striped effect. Bright, distinctive color.
Cold-Hardiest Cultivars (Zone 2)
If you live in extremely cold climates, these cultivars offer superior winter hardiness:
- Techny, Degroot's Spire, Rheingold, Filiformis, and many standard cultivars are rated Zone 2 or 3.
- Check cultivar labels for cold-hardiness ratings. Most named cultivars are hardy to at least Zone 3.
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