Few plants deliver the intensity of true blue flowers that cape plumbago (Plumbago auriculata) produces. This South African native blooms continuously from spring through fall in warm climates, covering itself in clusters of sky-blue phlox-like flowers that attract butterflies and hummingbirds. It grows as an evergreen shrub or sprawling vine reaching 6-10 feet in frost-free zones, and works equally well as a container plant brought indoors during winter in cooler regions.
Growing Conditions
- Hardiness zones: 9-11 year-round outdoors. Grows as a perennial that dies back to the ground in zone 8 with mulch protection. Container plant in zones 4-7
- Sun: Full sun to partial shade. Blooms most profusely in 6+ hours of direct sun. Tolerates afternoon shade in hot climates (zones 10-11)
- Soil: Well-drained, moderate fertility. Tolerates poor soil, sandy soil, and slightly alkaline conditions. Does not tolerate waterlogged roots
- Water: Moderate once established. Drought-tolerant after the first year but blooms best with regular irrigation (1 inch per week during bloom season)
- Growth rate: Fast. Reaches 4-6 feet in the first growing season from a 1-gallon nursery plant. Mature size: 6-10 feet tall and wide if left unpruned
Planting and Establishment
Plant cape plumbago in spring after the last frost date in your area. Dig a hole twice the width of the root ball and the same depth. Backfill with native soil — do not amend heavily, as cape plumbago thrives in average to poor soil. Water deeply at planting and every 3-4 days for the first month to establish roots. Apply 2-3 inches of mulch around the base, keeping it 2 inches from the stem.
Space plants 4-6 feet apart for a hedge or screen. For faster coverage, space 3 feet apart and expect them to merge within one growing season. Cape plumbago sends up arching stems that root where they touch the ground — this spreading habit makes it an effective ground cover on slopes and banks. Train it as a vine on a trellis, fence, or arbor by tying young stems to the support structure. Without support, it forms a mounding shrub that spreads wider than tall.
Pruning and Maintenance
Cape plumbago blooms on new growth, so pruning encourages more flowers rather than reducing them. Hard prune in late winter or early spring (February-March in most zones) by cutting the entire plant back to 12-18 inches from the ground. It looks severe, but new growth fills in within weeks and the plant blooms heavily on the fresh stems.
Throughout the growing season, deadhead spent flower clusters to encourage continuous blooming. Light tip pruning every 4-6 weeks keeps the plant compact and bushy. Without pruning, cape plumbago becomes leggy and open — attractive as a vine but unkempt as a shrub. The stems have a mild sticky residue that can discolor light-colored walls and fences — position the plant at least 12 inches from surfaces you want to keep clean.
Container Growing
In zones 4-8, grow cape plumbago in a large container (14-20 inch pot minimum) with drainage holes. Use a standard potting mix amended with 20% perlite for drainage. Place in the sunniest available location outdoors from late spring through fall. Water when the top inch of soil dries — containers dry faster than garden soil, often requiring water every 2-3 days in summer heat.
Before the first fall frost, bring the container indoors to a bright, cool location (50-60°F is ideal). Cape plumbago is semi-dormant in winter and needs minimal water (every 2-3 weeks) and no fertilizer during this rest period. Some leaf drop is normal indoors. Move the plant back outdoors after the last spring frost and resume regular watering and monthly fertilization. Container plants typically reach 3-5 feet with annual pruning.
Fertilizing
Cape plumbago is not a heavy feeder, but regular fertilization during the bloom season produces noticeably more flowers. Apply a balanced slow-release granular fertilizer (10-10-10 or 14-14-14) in early spring as new growth begins. Follow with monthly applications of liquid bloom fertilizer (15-30-15 or similar high-phosphorus formula) from April through September. Stop fertilizing in October to allow the plant to harden off before winter.
Over-fertilization — especially with high-nitrogen formulas — produces lush green foliage at the expense of flowers. If your cape plumbago grows vigorously but blooms sparsely, switch to a high-phosphorus fertilizer and reduce the application rate. In poor soil, cape plumbago actually blooms better than in rich, amended soil — the slight stress encourages reproductive growth (flowers) over vegetative growth (leaves).
Common Problems
- No flowers: Usually insufficient sun (needs 6+ hours) or excess nitrogen fertilizer. Move to a sunnier spot and switch to bloom-boosting fertilizer
- Yellowing leaves: Overwatering or poor drainage. Check soil moisture before watering and improve drainage in the planting area
- Whiteflies: The most common pest on cape plumbago. Spray with insecticidal soap ($5-$8) or neem oil ($8-$12) every 7-10 days until controlled. Yellow sticky traps ($5 for a 10-pack) near the plant reduce adult populations
- Frost damage: Stems blacken and foliage drops after a hard freeze. Cut damaged growth to the ground in spring — the root system survives to 20°F and will send up new growth
