January and February are two months when a green roof can be expected to look at its worst. In previous blogs I have discussed evergreen planting, however bulbs can also make a really colourful difference.
Bulbs need at least 100mm substrate depth, and can be planted between plug plants either in drifts or scattered randomly across the roof. They will provide a splash of colour before any of the spring-flowering plants get going, and provide a source of nectar for foraging insects when there is little else around.
Tulipa tarda is currently the star of our office roof, and the Crocus purpurea is ready to unfurl and take over in the coming weeks. Muscari armeniacum is also a popular choice for biodiverse roof schemes. Generally drought tolerant hardy species are best, and dwarf varieties are particularly helpful for exposed areas.
Bulbs can be a very economical way of planting a roof if they are supplied in bulb form, however they must be ordered and planted before January and are thus subject to timely project management on site. Alliums which are later-flowering are also commonly used on roofs as part of wildflower and herb schemes, but are supplied as plugs and thus less sensitive to project delays.
So if your roof will be planted in autumn, the addition of a couple of tulips and crocuses in bulb form is my best recommendation for a high value, low cost improvement to your green roof planting scheme.