Why are the leaves turning yellow?

It is normal for the foliage to die back as the plant prepares to bloom.

Do I need to dig them up in winter?

In zones 4 to 8, they are hardy and can remain in the ground year round.

Plant guide

Giant Onion.

Allium giganteum

Pronounced AL-ee-um jy-GAN-tee-um

Also known as Giant Allium, Ornamental Onion, giant onion

This striking ornamental onion produces massive, globe shaped flower heads that stand tall in the garden. It is a fantastic choice for adding vertical interest and architectural flair to your borders. You will love how these blooms attract pollinators throughout the late spring.

At a glance

Care level: Easy
Plant type: Bulbous perennial
Light: Full sun
Indoor light: Direct sun
Water: Every 7 to 14 days
Mature size: 4 to 5 feet
Growth rate: Medium
Hardiness: USDA 4 to 8
Bloom time: Late spring to early summer

Care

Plant your bulbs in well draining soil during the autumn months to ensure a strong root system. Expect the foliage to yellow around the time the flowers open, which is a natural sign the plant is storing energy for next year. Keep the area relatively dry once the plant goes dormant in summer.

Watering: Water consistently during the active growing season but keep dry while dormant.
Feeding: Apply a balanced bulb fertilizer once in early spring as new growth emerges.
Pruning: Remove the spent flower stalks after the seeds have matured or the stalk turns brown.
Repotting: Divide the bulbs every few years if they become too crowded or flowering declines.
Soil: Well draining sandy or loamy soil
Soil pH: 6.0 to 7.5
Temperature: -20 to 35°C
Humidity: 40 to 60%
Drought tolerance: Moderate
Lifespan: Perennial, typically 3 to 5 years with good care.
Deer and rabbits: High

Signs it needs repotting

Bulbs are crowded and flowering has decreased.
Roots are growing out of the drainage holes.

Safety

People: Ingestion of the bulbs can cause gastrointestinal distress in humans.
Pets: This plant is toxic to cats and dogs if ingested.
Children: Keep bulbs out of reach of children as they can be mistaken for edible onions.
Contact allergens: Handling the bulbs may cause mild skin irritation in sensitive individuals.
Airborne allergens: The pollen is generally not a significant airborne allergen.
Edibility: While some alliums are edible, this ornamental species is not typically consumed and may cause digestive upset.

How to identify it

Large spherical purple flower clusters
Tall sturdy leafless flower stalks
Broad strap like basal leaves
Leaves often wither before flowering

Height: 0.9 to 1.5m
Spread: 0.2 to 0.3m
Leaves: green, deciduous
Flowers: purple, violet, 10 to 15cm

Problems to watch for

Bulb rot · Fungal decay caused by overly wet soil conditions.
Thrips · Tiny insects that can cause streaking on leaves and flowers.

By symptom

Plant problems by symptom

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Growing it

Propagation: division, seed
When to plant: autumn
Overwintering: Leave the bulbs in the ground and apply a light layer of mulch if you live in a colder climate to protect against extreme frost cycles.
In containers: They grow well in deep pots with excellent drainage, though they are best planted in groups for visual impact.

Division

1. Wait until the foliage has completely died back in late summer.
2. Carefully dig up the bulb clump without damaging the roots.
3. Gently separate the offsets from the main bulb.
4. Replant the offsets immediately at the same depth they were previously growing.

Plant these behind lower growing perennials so the base of the plant is obscured when the leaves start to yellow.

Grows well with

Peony · Their lush foliage helps hide the fading leaves of the allium.
Catmint · The soft blue flowers provide a lovely color contrast to the purple globes.

Similar species

Star of Persia · It has a similar spherical shape but with more open, star shaped florets.
Drumstick Allium · It offers a smaller, egg shaped flower head that blooms later in the season.

Science

Family: Amaryllidaceae
Genus: Allium
Order: Asparagales
Class: Liliopsida
Native range: Central and southwestern Asia.
Conservation: Not of conservation concern.

This plant thrives in sunny, well drained meadows where it can dry out during its summer dormancy.

Value to wildlife: nectar source
Pollinators: bees, butterflies
Uses: ornamental border, cut flower, dried flower arrangement

Through the year

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Flowering

Fruiting

Foliage

Ecological indicators

Salinity: 2/9
Soil reaction: 6/9
Light: 8/9
Temperature: 5/9
Moisture: 4/9
Nutrients: 5/9

Taxonomy resolved against GBIF.

Questions people ask

Why are the leaves turning yellow?: It is normal for the foliage to die back as the plant prepares to bloom.
Do I need to dig them up in winter?: In zones 4 to 8, they are hardy and can remain in the ground year round.

Where the name comes from

The genus name Allium is the Latin word for garlic, while the species name giganteum refers to its impressive height.

Good to know

The flower heads can reach up to 6 inches in diameter.
They are excellent for drying and using in winter floral displays.
The foliage often begins to yellow and fade just as the flowers start to open.

Also in

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This guide was generated with AI on October 19, 2026 and checked against GBIF taxonomy. It is general information, not horticultural or veterinary advice. Read about how we build these guides.