Showy Mountain Ash

Sorbus decora

Showy Mountain Ash is a cold-hardy, ornamental tree native to Canada, prized for its clusters of bright red berries, white spring flowers, and stunning fall colour. It thrives in Zone 2-5, making it well-suited for our local climate (Zone 3). Unlike European Mountain Ash, it is more disease-resistant and better adapted to prairie conditions.

Planting & Location

• Hardiness Zone: 2-5
• Mature Size: 20-30 feet tall, 15-20 feet wide
• Growth Rate: Moderate (~12-18 inches per year)
• Sunlight Needs: Full sun to partial shade (at least 6 hours of direct sun preferred)
• Soil Preference:
  • Prefers moist, well-draining loam but tolerates clay and sandy soils.
  • Performs best in slightly acidic soil (pH 5.5-6.5) but adapts to neutral soils.
  • Avoid overly wet sites—susceptible to root rot in poor drainage.
• Spacing: Plant 15-20 feet apart to allow full canopy development.

Watering

• Young Trees: Water deeply 1-2 times per week during the first 2 years.
• Established Trees: Water every 2-3 weeks in dry periods.
• Drought Tolerance: Moderate but benefits from occasional deep watering in hot summers.
• Winter Watering: If fall is dry, provide one last deep watering before the ground freezes.

Fertilizing

• First Year: No fertilizer needed. Focus on root establishment.
• Mature Trees: Apply a balanced slow-release fertilizer (e.g., 10-10-10) in early spring.
• Organic Alternative: Mulch with compost in spring for gradual nutrient release.

Pruning & Maintenance

• Best Time to Prune: Late winter to early spring (before new growth).
• How to Prune:
  • Remove dead, diseased, or crossing branches to improve airflow.
  • Maintain a strong central leader and remove weak growth.
  • Avoid heavy summer pruning, as it may stress the tree.
• Suckering: Occasionally produces root suckers—remove them to maintain tree form.

Pest & Disease Management

Common Pests:

• Mountain Ash Sawfly – Small larvae skeletonize leaves.
  • Solution: Handpick larvae, use insecticidal soap if severe.
• Aphids – Sap-sucking insects that cause leaf curling.
  • Solution: Encourage natural predators like ladybugs.
• Borers – Can attack stressed trees.
  • Solution: Keep trees healthy and avoid mechanical injury to bark.

Common Diseases:

• Fire Blight – Bacterial infection causing blackened, wilted branches.
  • Solution: Prune affected branches 8-12 inches below infection and sanitize tools. Avoid excess nitrogen fertilization.
• Powdery Mildew – White fungal coating on leaves.
  • Solution: Improve air circulation, apply fungicide if needed.
• Rust (Gymnosporangium spp.) – Causes orange spots on leaves and berries.
  • Solution: Remove nearby junipers (alternate host), apply fungicide if severe.

Winter Protection

• Young Trees: Wrap the trunk with tree guards or burlap in late fall to prevent sunscald and frost cracks.
• Berries & Wildlife: Birds love the berries in winter, adding seasonal interest. If you prefer to keep the berries longer, consider netting the tree in late summer.
• Mulching: Apply 2-4 inches of mulch around the base (but not touching the trunk) to protect roots from extreme cold.

Landscape Uses

Ornamental shade tree
Wildlife-friendly (berries attract birds)
Low-maintenance and disease-resistant
Beautiful fall colour (orange-red leaves, bright red berries)
Works well in small yards, urban settings, and mixed tree lines

Photo courtesy of Foothills Nursery