How to Grow Giant Onion

Giant onion perennial plant with puffball of green buds and purple star shaped blooms on thin stalks

The Spruce / Evgeniya Vlasova

The tallest of all ornamental onion species, the giant onion produces 6- to 8-inch purple blooms made up of many star-shaped flowers. These captivating flowers are supported by 4- to 5-foot tall stalks that appear from a basal rosette of leaves, measuring about 18 inches long. This foliage dies back before the flowers bloom, leaving only the iconic stalk and puffball blooms to stand out among the garden.

Whether you're a fan of onions or not is irrelevant, since giant onions aren't edible. However, they're still frequently planted in gardens for their whimsical, Dr. Seuss-like vibe. The blooms last for a long while, attract bees and butterflies, and are great for cut flowers. It is best to plant giant onions in the fall. Giant onions are toxic to people, dogs, and cats.

Botanical Name Allium giganteum
Common Name Giant onion, giant ornamental onion
Plant Type Perennial
Mature Size 4-5 ft. tall, 2 ft. wide
Sun Exposure Full, partial
Soil Type Loamy, sandy, well-drained
Soil pH Acidic, neutral, alkaline
Bloom Time Spring, summer
Flower Color Purple
Hardiness Zones 4-9, USA
Native Area Asia
Toxicity Toxic to dogs and cats, toxic to people

Giant Onion Care

Giant onions are easy to care for and do not require much attention when planted in desirable conditions. Because of the foliage’s natural onion-like scent, these plants are rabbit- and deer-resistant. They are not often affected by many diseases or pests, though onion flies may visit these plants. Keep your garden beds clean and tidy to avoid this problem. The giant onion’s tall size makes them susceptible to high wind damage. Stakes may be required to support these tall flowers. 

To grow giant onions, plant the bulbs in the fall around 6 weeks before the ground is frozen. Plant 5-6 inches deep and 9-12 inches apart, keeping the pointed end upward. Proper spacing ensures adequate airflow and healthy bulbs. To transplant or divide, wait until the fall when the foliage naturally dies back. It is good to do this every 3-4 years to avoid overcrowding. Before planting, however, be sure to research your area’s regulations. Though not invasive, some areas restrict the use of alliums in the garden because of white rot. Idaho is one such state. 

Giant onion plant with tall stalks and puffball-shaped blooms in flower garden

The Spruce / Evgeniya Vlasova

Giant onion plant with tall green stalks and purple puffball with tiny blooms

The Spruce / Evgeniya Vlasova

Giant onion perennial plant with purple star-shaped blooms and green buds closeup

The Spruce / Evgeniya Vlasova

Light

Giant onions like lots of sunshine, though morning sun and afternoon shade may be best for hotter climates. Typically, the more sunshine these plants receive, the taller the plants will grow and the larger the blooms will be.

Soil

Well-draining, rich soil is best, though they can grow in almost any soil condition as long as it is well-draining. These plants benefit from compost, since it balances soil moisture levels and improves drainage. Compost also enriches the soil with many key nutrients. 

Water

The giant ornamental onion has average watering needs and is drought tolerant. Because of this, natural rainfall will most likely provide enough water for these plants. These plants handle under-watering much better than overwatering. Too much water can cause the bulbs to rot. 

Temperature and Humidity

Giant onions can be grown in zones 4 to 9, but prefer a hot, sunny location with average or low humidity. Cool, wet, humid climates can easily lead to bulb rot. For cooler zones, such as zone 4, it is important to place your giant onions in a sunny place with good airflow.   

Fertilizer

Giant ornamental onions thrive in nutrient soil. Throughout the growing season, you can use a well-balanced fertilizer to improve soil conditions. This will encourage more blooms and healthy growth, setting the plant up for successful future growing seasons as well.

Propagating Giant Onions

Giant onions are most often propagated by dividing bulbs. This makes propagation very easy. Here’s how to do it: 

  1. Wait until the fall when the plant’s foliage has died back naturally. 
  2. When this occurs, use a garden shovel to gently dig up the bulbs. You will most likely find multiple bulbs clumped together. 
  3. Using your hands, gently separate the bulbs from each other. 
  4. Plant each bulb at least 9-12 inches apart. Amend the soil with compost before replanting. 

Take note that you can also grow Allium giganteum from seed. However, since these plants take several years to mature and flower, propagation by division is usually preferred.

Potting and Repotting Giant Onion

The giant ornamental onion can be grown in large containers to show off its striking appearance in any spot with bright sunshine. When choosing a pot, be sure that it has good drainage holes. Fill the pot with well-draining, rich soil. If planting bulbs, be sure the bulbs are not touching each other.

If the giant onion begins to outgrow its pot, simply dig out the plant and divide the bulbs in the fall. Replant the desired plants into the pot and transfer the other bulbs to another area. 

Overwintering Giant Onions

For mature plants, leave the foliage and allow it to die back naturally. This will help nourish the bulbs. Cover the ground with mulch to help insulate the bulb.

For newly planted bulbs, add a layer of mulch or compost over the top of the bulb. Throughout the winter, be sure the bulb does not dry out. If it dries, water it every month or so. 

Article Sources
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  1. North Carolina State University Extension Office. "Allium giganteum.Ncsu.edu. N.p., n.d. Web.

  2. Cope, R. B. “Allium Species Poisoning in Dogs and Cats.” Aspcapro.org. N.p., n.d. Web.