Allium Care: Growing Ornamental Onions
Alliums, the ornamental members of the onion family, bring unique architectural form and vibrant color to the garden with their spherical, globe-shaped, or exploding-firework flower heads held aloft on sturdy stems. Blooming primarily in late spring and early summer, they bridge the gap between spring bulbs and summer perennials. They are generally easy to grow, deer and rodent resistant (due to their oniony scent), drought-tolerant once established, and make excellent cut or dried flowers.
Common Types of Ornamental Alliums
Alliums offer a wide range of sizes, shapes, and colors:
Large Globe Alliums (Bloom Late Spring/Early Summer)
- Produce large, dense spheres (purple, lilac, white) on tall stems (2-5 ft). Foliage often fades before/during bloom.
- Examples include: 'Globemaster' (giant violet), 'Purple Sensation' (deep violet), A. cristophii (Star of Persia - airy metallic lilac), A. schubertii (firework-like), 'Mount Everest' (white).
Drumstick & Smaller Alliums (Bloom Early to Mid-Summer)
- Smaller flower heads (oval, cylindrical, clusters). Bloom slightly later, foliage may persist longer.
- Examples include: A. sphaerocephalon (Drumstick Allium - oval, green to red-purple), A. caeruleum (Blue Globe Onion - true blue), A. moly (Golden Garlic - yellow stars, spreads), A. unifolium (pinkish-lilac stars).
Late-Blooming Alliums (Bloom Mid-Summer to Fall)
- Extend the Allium season, often with attractive persistent foliage.
- Examples include: A. 'Millenium' (rosy-purple globes mid-summer, clump-forming plant not bulb), A. tuberosum (Garlic Chives - edible leaves, white flowers late summer, can self-seed aggressively), A. thunbergii 'Ozawa' (pink-purple flowers early fall).
Planting Allium Bulbs
Most ornamental alliums are grown from bulbs planted in the fall.
When to Plant
- Fall (September - November): Plant bulbs before the ground freezes hard. Allows roots to develop over winter.
Ideal Growing Conditions
- Light: Full sun (6+ hours daily) is essential for strong stems and best flowering.
- Soil: CRITICAL: Requires well-draining soil. Sandy loam is ideal. They will rot in heavy, waterlogged soil. Amend clay soil to improve drainage or use raised beds. Average fertility is fine.
- Spacing & Depth:
- Large bulbs (e.g., 'Globemaster'): Plant 6-8 inches deep and 6-12 inches apart.
- Small bulbs (e.g., A. sphaerocephalon): Plant 3-4 inches deep and 3-6 inches apart.
- Plant pointy-end-up.
Planting Technique
Sun and drainage are key. Consider companion plants to hide fading foliage.
Loosen the soil and improve drainage if necessary (amend heavy clay with compost/grit). Mix in bone meal or bulb fertilizer below the planting depth (optional).
Ensure holes are deep enough (2-3 times bulb height for large bulbs, 3-4 inches for small).
Place bulbs pointy-end-up at the bottom of the hole, spaced appropriately. Cover with soil, gently firming.
Water the area well once after planting to settle the soil and encourage rooting. Fall rains usually suffice afterward.
Alliums like A. 'Millenium' are often sold as plants, not bulbs. Plant these in spring or fall at the same depth they were in the pot.
Watering Alliums
- Water well once after fall planting.
- Provide water in spring during active growth and flowering if rainfall is insufficient (about 1 inch per week).
- Once flowering is finished and foliage begins to yellow, stop watering. Allium bulbs prefer to be dry during their summer dormancy. Established plants are very drought-tolerant when dormant.
Fertilizing Alliums
- Alliums are not heavy feeders.
- Incorporating bone meal or a balanced bulb fertilizer into the soil below the bulbs at planting time is beneficial but optional.
- A light top-dressing of compost or balanced fertilizer in early spring as growth emerges can be applied if soil is poor. Avoid high-nitrogen fertilizers.
After-Bloom Care
Foliage is Key!
- CRUCIAL: After flowering, allow the foliage to continue growing until it yellows and dies back naturally. Do NOT cut it off while green, braid it, or tie it up. The leaves are essential for photosynthesis, which replenishes the bulb's energy stores for next year's bloom.
- Once the foliage is completely yellow or brown, it can be easily removed.
Deadheading
- Removing spent flower heads is optional. It doesn't affect bulb health or rebloom. Some find dried seed heads attractive (A. cristophii, A. schubertii). Remove to prevent self-seeding if desired (A. sphaerocephalon).
Division
- Divide infrequently, only if clumps become overcrowded and flowering diminishes (every 5+ years). Dig clumps in late summer/fall after foliage dies back. Gently separate offsets and replant immediately.
Troubleshooting Common Problems
Alliums are generally pest and disease resistant due to their oniony nature.
Bulb/Root Rot
Symptoms and Management
Failure to emerge, or weak growth that yellows and collapses. Bulb is soft and mushy.
- Cause: Poor drainage, planting in waterlogged soil.
- Prevention: Ensure excellent drainage! Amend heavy soils or use raised beds. Avoid planting in low spots where water collects.
- Treatment: Usually fatal. Discard rotted bulbs. Improve drainage before replanting.
Poor Flowering / Failure to Emerge
Possible Causes and Solutions
- Poor Drainage/Rot: See above.
- Insufficient Sun: Need full sun for energy storage and flowering.
- Foliage Removed Too Early: Cutting green leaves prevents bulb recharging.
- Bulb Quality/Age: Old or poor-quality bulbs may fail. Buy from reputable sources.
- Overcrowding: Clumps may eventually need division if flowering declines.
- Squirrels/Rodents (Rare): While generally resistant, occasionally desperate critters might dig bulbs.
Conclusion
Ornamental Alliums are spectacular additions to the garden, offering unique forms, vibrant colors, and resistance to pests like deer and rabbits. Their ease of care makes them suitable for gardeners of all levels. Remember the keys to success: plant bulbs in fall in a sunny location with excellent drainage, and resist the urge to remove the foliage until it has completely died back naturally. Enjoy their dramatic floral display!
