Ornamental Alliums, often called flowering onions, provide a striking architectural element to the garden with their tall, slender stems and signature globe-shaped flower heads. These popular bulbs, relatives of garlic and chives, typically bloom in late spring to early summer, bridging the gap between spring and summer. Planting them in the fall is a prerequisite for successful flowering because the bulbs must undergo vernalization, a sustained period of cold dormancy. This necessary cold period, ideally lasting 10 to 15 weeks at 32 to 42 degrees Fahrenheit, triggers the internal changes required for the bulb to produce its spring bloom.
Selecting Bulbs and Site Preparation
Planting should occur after the soil temperatures cool below 60°F, generally from September through November, but before the ground freezes solid. This timing allows the bulb to establish a robust root system before winter dormancy. When selecting bulbs, choose specimens that are firm and plump, discarding any showing signs of mold, softness, or cuts, as these are unlikely to thrive.
Alliums perform best in full sun, meaning at least six hours of direct sunlight daily, which promotes stronger stems and vibrant blooms. Excellent drainage is the most important soil requirement, as these bulbs are susceptible to rot in soggy conditions. If your garden has heavy clay soil, amend the planting area by digging in organic matter, such as compost or well-rotted manure, to improve porosity and aeration.
The Mechanics of Planting
Use a garden trowel or specialized bulb planter to create individual holes. The general rule for planting depth is to bury the bulb two to three times its height; this anchors the tall flower stalks and protects the bulb from temperature fluctuations. For example, a two-inch bulb should be planted four to six inches deep, though larger varieties like ‘Giganteum’ may require six to eight inches.
The bulb must be placed in the hole with the pointy end facing upward toward the sky. This pointed tip is the future growing point for the foliage and flower stalk. Spacing typically ranges between four and eight inches apart, allowing for good air circulation and a visually dense cluster of blooms. Once the bulb is positioned correctly, gently backfill the hole with the excavated soil, lightly tamping the surface to remove air pockets.
Winterizing and Initial Care
Immediately after planting, thoroughly water the area to settle the soil and initiate the development of the root system. While alliums are drought-tolerant once established, this initial watering ensures good soil-to-bulb contact. Consistent moisture is important during the autumn, but avoid overwatering, which can lead to rot before the bulb enters winter rest.
Once the ground has frozen completely, apply a two to three-inch layer of organic mulch, such as straw or shredded leaves, over the planted area. This mulch provides insulation to maintain a consistent soil temperature, preventing the bulbs from being heaved out by repeated freeze-thaw cycles. Alliums are generally unattractive to deer and rodents due to their onion-like scent. Expect the first shoots to emerge in the spring once the soil warms, signaling the end of the required cold dormancy period.