Alliums in containers
Alliums love hot, dry conditions and this makes them great plants to grow in pots and window boxes. Make sure that the container is well-drained by putting lots of broken pots or pieces of polystyrene in the bottom. In large pots use strong-stemmed tall alliums such as the big-headed Allium ‘Globemaster’. Three or five bulbs planted in a two-foot wide pot with the silky Silver Feather Grass, Stipa barbata, is an impressive sight.
Lower-growing alliums are more suited to window boxes, troughs and small pots. Unlike most alliums, Allium molly will even be happy in shady corners. If you use a compost that contains plenty of humus or leaf-mould you could plant it with Phlox divaricata ‘Clouds of Perfume’ or another of the pale-mauve prostrate phlox. The pastel colours of the phlox will moderate the harshness of the allium’s yellow flowers.
A friend has window-boxes on a south -facing house that not only have to endure the ruthless heat of the sun but also the pollution of a busy street. One of the few plants that revel in this environment is the diminutive Allium senescens, which has tough narrow leaves that twist into a corkscrew as they grow. The flower stems are never more than six inches tall and the pink ping-pong ball flowers take their time to open. This means that from the tight dusky pink buds opening to them forming clear pink flowers you get six weeks of enchantment. It’s a very robust plant that never complains, however harshly it’s treated – mine grow in very little soil on the top of a low wall and thrive despite neglect