How to Grow and Care for Giant Allium

The tallest of all ornamental onion species, the giant allium 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 across. The foliage dies back before the flowers bloom, leaving only the iconic stalk and puffball blooms to stand out in the garden.

Whether you're a fan of onions or not is irrelevant, since giant ornamental onions aren't meant to be eaten. Moreover, they're popular in the garden for their whimsical, Dr. Seuss-like appearance. The blooms last for a long while, attract bees and butterflies, and add an eye-popping burst of color as cut flowers. It is best to plant giant ornamental onions in the fall.  While all alliums are edible, most ornamental varieties including the giant allium are generally considered too strong for human consumption. This plant is toxic to dogs, and cats.

Giant Allium Care

Giant ornamental 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 disease or pests, though onion flies may visit these plants. Keep your garden beds clean and tidy to avoid this problem. The plant's tall size and large bloom make it susceptible to high wind damage. Stakes may be required to support these tall flowers. 

To grow giant alliums, plant the bulbs in the fall around 6 weeks before the ground freezes. Plant 5-6 inches deep and 9-12 inches apart, keeping the pointed end of the bulb facing 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 states restrict the use of alliums in the garden because of white rot.

Light

Giant ornamental onions like lots of sunshine, though morning sun and afternoon shade may be best when grown in 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 ornamental 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 Allium

Giant ornamental 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 Allium

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 it 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 Allium

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.