Alliums, commonly known as ornamental or flowering onions, are striking additions to any garden, recognized for their unique spherical blooms and architectural height. This plant genus also includes culinary staples like garlic, onions, and chives. Proper spacing is the most significant factor for both visual impact and successful harvest. Correctly separating the bulbs prevents health problems and ensures each plant receives the necessary resources for vigorous growth. The required distance varies dramatically depending on the specific variety, ranging from an inch apart for a dense crop to a foot or more for a giant bloom.
Why Spacing is Critical for Allium Health
Properly spacing Allium bulbs establishes a healthy microclimate that influences the plant’s long-term survival and vigor. A primary reason for separation is the mitigation of fungal diseases, particularly bulb rot. When bulbs are packed too closely, the soil remains consistently moist and restricted air circulation creates a damp environment where fungal spores thrive.
The lack of air movement around dense foliage also increases the risk of foliar problems like downy mildew. This disease spreads quickly through congested plantings, weakening the bulb’s ability to photosynthesize and store energy. Overcrowding forces plants to compete intensely for limited soil nutrients and water. Adequate spacing ensures each developing bulb has unrestricted access to the resources required to produce a robust flower stalk or a large edible bulb.
Specific Spacing for Ornamental Varieties
Spacing for ornamental Alliums is determined by the mature size of the bulb, height, and spread. Large-scale varieties, which produce dramatic, globe-like blooms, require substantial separation. For imposing plants like ‘Globemaster’ or ‘Purple Rain,’ which can reach heights well over three feet, a minimum spacing of 8 to 12 inches is recommended. This distance accommodates the expansive root system and provides clearance for flower heads to develop without obstruction, ensuring stability and preventing disease transfer.
Medium-sized ornamental Alliums, such as Allium aflatunense ‘Purple Sensation,’ are planted closer together to create a cohesive drift of color. These bulbs require a spacing of 4 to 6 inches, which allows for good visual density while providing sufficient air flow and nutrient access. This tighter arrangement allows individual flowers to merge into a single field of color.
Dwarf and small Allium species, including low-growing varieties like Allium moly (Golden Garlic), require the tightest spacing. These bulbs only need 1 to 3 inches of separation to form dense, naturalized clumps. Planting them closely allows for the rapid formation of a continuous mat of color, often used as ground cover or in rock gardens. If specific variety data is unavailable, the general rule is planting bulbs two to three times their own width apart.
Density Requirements for Edible Alliums
Spacing for edible Alliums is governed by the desired size and yield of the harvested crop. Garlic, cultivated for its compound bulb, is typically planted 4 to 8 inches apart. This distance allows each clove room to swell into a full-sized head. The closer end of the range favors slightly smaller, but more numerous bulbs. When planted in rows, a separation of at least 12 inches between rows ensures access for weeding and hilling.
Bulbing onions and shallots, grown for a single, large underground bulb, require moderate separation to prevent overcrowding. Transplants are often set out with a spacing of 3 to 4 inches, while larger bulbing onions need 5 to 6 inches apart to reach maximum size potential. Planting too closely results in many small, underdeveloped bulbs, while wider spacing encourages fewer, but larger, bulbs.
Non-bulbing edible Alliums like scallions (bunching onions) and chives are planted for maximum density to encourage the growth of tender leaves and stems. Scallions are often planted 1 to 2 inches apart, or thinned to an inch after direct seeding, promoting a dense stand of green stalks. Chives, grown from divisions or clumps, do not need individual separation, but the entire clump should be divided every few years and replanted to ensure consistent air circulation and vigor.
Practical Planting Strategies and Arrangement
Implementing spacing begins with calculating planting depth, as this influences horizontal separation. A general principle is to plant the bulb at a depth of two to three times its own diameter, providing necessary anchorage and access to consistent soil moisture. Deeper planting, especially for larger bulbs, often allows for slightly tighter horizontal spacing because the bulb is better stabilized and less prone to being pushed out by frost.
When planting, measure from the center of one bulb to the center of the next, rather than measuring the distance between the edges. This center-to-center measurement ensures consistent application of the calculated spacing. For a soft, informal appearance, naturalizing is used, where bulbs are scattered and planted where they fall to create uneven drifts.
To maximize visual impact, bulbs are typically grouped in odd-numbered clumps of five to seven, rather than planted in formal rows. Even when planting in clumps, the spacing rules for individual varieties must be maintained to prevent fungal issues and resource competition. Using a garden trowel or auger to create individual holes ensures the bulb is placed at the correct depth and separation, avoiding inconsistent spacing encouraged by digging a large trench.