Rooftop rarity

A web of intrigue surrounds the discovery of a rare orchid growing on a grass roof at a Swanage sewage treatment site.

Keen-eyed locals told ecologists at Wessex Water that an early spider orchid was spotted on the roof last summer, so they were on the look-out this year, according to a press report.

It seems the roof has only existed for 10 years and the orchids take that long to flower. But it’s a mystery how they got on the roof – whether seed was in the soil or blown in from nearby.

Ellen McDouall, senior conservation ecologist at Wessex Water, said: “We don’t know how they got on the roof – whether seed was in the soil or blown in from nearby. Thankfully the roof of the sewage treatment works is under no particular operational pressure, so we are hopeful we will be able to actively manage the land for the benefit of the plant.”

The UK’s early spider orchid populations are restricted to parts of southern England, and the plant is regarded as rare. However, where the flowers do grow, they can do so in significant numbers. Purbeck limestone cliffs are one of three UK strongholds for the species, the others being Kent and Suffolk.

It belongs to the intriguing group Ophrys, whose flowers evolved to look like specific insect species, but they emit an enticing cocktail of chemicals. These mimic pheromones released by the female of the species and lure males to the flower.

The orchid has distinctive yellow-green to brownish green petals and sepals. The lip looks like a large, furry spider and is purple-brown in colour with a velvety appearance with a patch of bluish or violet markings in the centre, known as the ‘mirror’ or ‘'speculum’. The marking may take the form of a capital or sometimes an X.

The early spider orchid is a short-lived perennial herb which mainly reproduces by seed, and whose populations fluctuate from year to year. Up to 70% of the plants flower in the first year that they appear, and most die afterwards, but some may live for up to 10 years. About half of the plants are dormant underground each year. Leaves grow from the tubers in late autumn and over winter, withering after flowering. It persists through the summer as underground tubers (6).

This orchid flowers in early April to early May, a good month before the aptly named late spider orchid (Ophrys fusciflora). Flowers are pollinated by bees of the genus Andrena, but only around 10% of the flowers produce ripe seed. Self-pollination does occur, but it is inefficient.

As the plant is rare, be warned. Anyone uprooting, cutting, selling or destroying the early spider orchid could face arrest, as it is protected under Schedule 8 of the Wildlife and Countryside Act.

It’s apt that the early spider orchid is also used as part of the Dorset Wildlife Trust's (DWT) logo. A DWT spokesman said: “This is a lovely find as the early spider orchid is nationally scarce.”

Lucy McCormick of Wessex Water agrees. 'It was a fantastic find and it has chosen a great location to root itself away from the hustle and bustle of life.'