Written by Dr. Bradley Rowe
Various species of Sedum are the workhorses of extensive green roofs. However, it is not wise to rely on one genus in any landscape, regardless of whether it is located on a roof or at ground level. The genus, Allium, is one group of plants that can complement sedum or be grown with other plant species.
Alliums are herbaceous perennials that grow from bulbs, produce showy flowers on scapes (long, leafless flowering stems), and often have an onion-like odor and taste. Common species within this genus include onions, garlic, scallions, leeks, and chives. There are hundreds of wild alliums, many of which are cultivated for food and as ornamentals. In the wild or on a roof they usually naturalize under favorable growing conditions by self-seeding and through bulb offsets and rhizomes. Foliage generally persists late into the summer long after flowering.
From a design aspect, their upright growth habit adds a vertical element that complements the horizontal spreading form of sedum and increases the potential for biodiversity as the flowers attract butterflies, bees, and birds. Alliums have no serious insect problems as the sulfur compounds they produce apparently help ward off insect pests. Root rot may be a problem in waterlogged growing media, but poor drainage should never be a problem on a properly designed and installed green roof. Two alliums that are useful on green roofs are Allium cernuum and Allium schoenoprasum. Both have performed admirably in studies conducted at Michigan State University.
Allium cernuum(wild nodding onion) is native from Canada down to Mexico, and in its natural environment, it tolerates drought and shallow rocky soils which make it ideal for extensive green roofs. Nodding onion is suitable within USDA hardiness zones 4 to 8, performs best in full sun, but can benefit from some afternoon shade in warmer regions. It normally reaches a height of 30 to 45cm (12 to 18 in). Rising above its upright, flat, grass-like leaves are the scapes with clusters of pink to white, bell-shaped flowers that are organized into nodding clusters (umbels) that flower during the summer. The scape bends or nods down just below the flowers, a feature that distinguishes it from other alliums (thus the common name).