No room for shrinking violets in the woodland wild flower competition

Thursday's Guardian Country Diary is about the dog violet Viola riviniana, one of the earliest woodland plants to come into broom.

Spring offers a narrow window of opportunity for woodland wild flowers that are pollinated by insects. Bloom too soon and there won't be many insects about; bloom too late and the overhead tree leaf canopy will have plunged the woodland floor into deep shade, restricting the plant's capacity to grow vigorously and fill its seeds.

Dog violets are one of the first species to bloom. Their nectar is in a spur behind the flower and is mostly accessible to long-tongued bees. An early display of attractive flowers advertises the flowers to these insects, that are few and far-between in woodlands in late March and April.

By the time that May arrives the rest of the woodland flora - wood anemones, wild garlic and bluebells - has transformed the woodland into a sea of flowers. Then dog violets face two challenges - intense competition for pollinators and shade from other species that have grown up around them.

Dog violet's answer to the challenge is to switch to a different type of flowering called cleistogamy, where flower buds develop and never open, self-pollinating in the bud stage and producing seeds without help from insects. They produce these cleistogamous flower buds, unnoticed, right through the summer.

A cleistogamous flower bud of dog violet. It will self-pollinate and produce a seed capsule without ever opening.

There are dog violets that escape the constraints of woodland life and flower in more open habitats like hedge banks, and its these that always produce the best and longest floral display. We found this lovely example growing in a crevice in a boulder beside the river Tees downstream from Egglestone. It's one of the finest violet floral performances that I've ever seen.

I suspect that the seed must have washed down river during floods and lodged in this crevice, where it's rooted in the cool moisture of the rock fissure but exposed to full sunlight that has enabled it to bloom so profusely, even when the mosses covering the rock withered in a week of warm, dry weather.