This may sound like I'm getting a little over-sensitive about gardening, but I often wonder whether sub-shrubs get the hump. There may not have been any intended maliciousness in this horticultural classification, but you've got to admit it sounds a touch belittling. Demeaning as it sounds, the term "sub-shrub" should be seen as a positive thing for the urban gardener. It actually means "woody perennial", something that occasionally makes them difficult to categorise in the botanical world. Lavender, thyme, caryopteris are all labelled as sub-shrubs. They provide a valuable link between shrubs and perennials, especially in loose borders of perennials where the more identifiable and larger-than-life shrubs might spoil the intended fluidity and movement within a scheme. Their relatively smaller size means that they have an obvious use for urban gardens where space is tight or where a light structure is needed.
My favourite at this time of year is the leadwort, Ceratostigma willmottianum, valuable ammunition for any urban gardener's artillery. A native from the rocky foothills of China, it provides electric-blue, wheel-like flowers, not unlike the tender plumbago to which it is related. The spiral of small, unremarkable, rough-textured leaves begin to flush red and rust in August, giving an extra kick when contrasted with the flower clusters that range from luminescent blue tones to plum-purple. For a plant that is largely understated for much of the year, it makes up for it from August to October.
Like any 10-syllable plant, Ceratostigma willmottianum is a great one for novice gardeners to have at their disposal. I was introduced to this plant by a client when I first started garden maintenance more than 20 years ago, and wasted no time rattling off the name to non-gardening friends whenever I got the chance - though, looking back, I'm sure they were more bored than impressed. It had been planted along a low, south-facing wall where the sun's warmth was trapped and released like a storage heater. Reaching a height between 60cm and 90cm, it would be cut back to the ground each spring, once new shoots could be seen emerging from its base. To this day, due to its sheltered position, it never fails to grow strong enough to flower. In exposed gardens, where it is not afforded such protection, flowers may come very late and, occasionally, not at all.
In the right hands, the plant has various medicinal uses, one of which has given rise to its common name, leadwort. Derived from the family name, plumbago, the plants of the plumbaginaceae family are said to be able to detoxify the body of lead. Those who use Bach Original Flower Remedies will know that the plant is a key ingredient to help inspire confidence and cure insecurity.
With its seeds germinating readily in paving cracks, it shows that despite being labelled a sub-shrub, leadwort is a survivor. It also gives a clue to its preferred growing conditions: medium to light soil with sharp drainage. In fact, it will withstand relatively poor soil, so it is a useful foil in a herb garden, and associates well with grasses on the wane, their wheaten colours accentuating the russet tones of the leadwort's leaves. Butterflies and moths swoon over the flowers and, planted next to the sedum 'Autumn Joy', it will dance to the last Indian summer sunset. The herbaceous variety C. plumbaginoides (another 10 syllables) is smaller (to 30cm) and can be used as ground cover at the front of a border. Bulbs growing through the early foliage help distract from their late-flowering habit. For those who enjoy a relaxed style of gardening, let it feel its own way as crevice planting, in drystone walling and as a decorative cushion for step risers.
Oh, I forgot to mention that ceratostigma varieties are resistant to deer grazing. Having planted some in various gardens in London I can certainly vouch for this. Not so much as a nibble in more than 20 years.