How to grow penstemon
Overview
Penstemon digitalis is a herbaceous perennial grown for its tall spikes of tubular white or pink flowers and neat foliage. It thrives in full sun and well-drained soil, and is valued for filling gaps in borders with long-lasting colour.
Varieties
- Penstemon digitalis 'Husker Red' — Bronze-red foliage and white flowers; popular in Australian gardens for its striking colour contrast.
- Penstemon digitalis 'Dark Towers' — Deep purple foliage with pink flowers; a standout for sunny borders.
Soil Preparation
Penstemon prefers neutral to slightly alkaline soil (pH 6.5–7.5) with excellent drainage. If soil is heavy clay, amend with coarse sand and well-rotted compost. A handful of lime can be added to acidic soils to improve performance.
Growing Guide
Plant Penstemon in full sun with free-draining soil; avoid heavy clay or waterlogged sites. Space plants 30–45 cm apart. Dig a hole twice the width of the root ball, backfill with compost, and water in well. Mulch lightly to retain moisture but keep mulch away from the crown. In cooler zones, plant in spring after frost; in subtropical zones, plant in autumn for winter–spring bloom.
Sowing
Sow seeds on the surface of seed-raising mix and barely cover; they need light to germinate. Keep moist at 18–22°C; seedlings appear in 14–21 days. Direct sow in spring or start in punnets; transplant when 5–10 cm tall. Space 30–45 cm apart.
Care
Feed with a balanced slow-release fertiliser (e.g., 10-10-10) in early spring. Cut back spent flower stems to the base after the first flush to encourage a second bloom. Divide clumps every 2–3 years in autumn to maintain vigour and prevent woody centres.
Companion Planting
Penstemon pairs beautifully with Salvias, Echinacea, and ornamental grasses like Pennisetum. Avoid planting near moisture-loving plants such as ferns or impatiens, which may cause root rot in Penstemon.
Common Pests & Risks
Aphids cluster on new growth and flower buds; spray with a strong jet of water or use insecticidal soap. Spider mites cause stippled leaves in dry weather; improve air circulation and mist foliage. Slugs and snails may attack young shoots; use iron-based pellets or beer traps.
Troubleshooting
Yellow leaves: overwatering or poor drainage; reduce watering and improve soil. Brown leaf edges: wind scorch or low humidity; shelter from drying winds. Leggy growth: too much shade or rich soil; move to full sun and reduce fertiliser. Powdery mildew: poor air circulation; space plants wider and avoid overhead watering. No flowers: insufficient sun or over-fertilising with nitrogen; ensure full sun and use a balanced feed.