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Seaside Rock: gardening in a south-westerly

22 April 2016

Lottie Allen, Head Gardener, St Michael's Mount discusses the challenges and intricacies of gardening on a tidal island.

The Mount

Within the natural harbour of Mount’s Bay, just 12 miles east of Land’s End, lies the iconic islet of St Michael’s Mount, a granite outcrop that has been home to the St Aubyn family since 1659. The islet is a tidal island cut off from the mainland for eight hours in every 12 subject to the variations of neap and spring tides; at the lowest low tide access to the Mount can be gained from the beach through the mouth of the harbour and at the highest low tide the causeway is impassable.

The island covers 8.5ha (21 acres) in total and is defined by three main areas (other than the castle atop): the weather-beaten and wild south-western third which is exposed to the prevailing Atlantic winds and waves; the northern third populated by a small community who live and work on the island as well as the necessary service buildings; and the south-eastern third, in the lee of the prevailing wind, where plants have thrived for centuries protected by shelter belts and the ready Cornish sunshine. On a calm still day the waters around the island seem only to make a half-hearted effort of lapping at the shore, and yet in the storms of February 2014, trees were lost and the causeway damaged, rendering it useless until emergency work linked its remaining parts together with concrete. Weather and tide are common to many Trust properties; on St Michael’s Mount, they add drama to a memorable visit and are an ever-present test to an exotic plant community, not to mention the resilient bunch of islanders.

Planting on the Mount dates back to the medieval period when the monks grew herbs for medicinal and culinary use. More recently, in around 1780, Sir John St Aubyn, 5th Baronet, built a Walled Garden at the request of his four daughters. Little is known of the planting at that time, nor whether the structure has since been altered; it may be that we have the original layout. The current three-tiered Walled Garden has a south-east aspect and is protected by a bank of elm trees on its lower south-western edge.  The elms have not been affected by Dutch Elm Disease although there height is constantly checked by the salt winds.

The Walled Gardens

The idea of viewing a garden from above isn’t new, but the sheer distance of 61m (200ft) between the Garden and Castle terrace, as well as the wider panorama of the sea, horizon and sky, makes for a dramatic setting. The challenge for Michael Harvey, the Mount’s Garden Designer, and the Garden Team is to use designs that are immediately visible from the Castle, yet dynamic enough to foster discovery from within the garden. The bold contrast comes from the use of foliage, texture and colour, and the floral relief provides the refined, subtle intimacy from within the garden.

The Top Walled Garden has been designed to reinterpret the wave patterns of the sea imitated by the contrasting foliage of silver-leaved Tulbaghia violacea ‘Silver Lace’ and the dark green of Parahebe ‘Porlock’. The stylised shapes of the pattern come from the windows of the Lady Chapel, seen from the South Terrace of the Castle where visitors can best view the design.

The conspicuous space of the Middle Walled Garden has recently been planted using a series of prairie-type plants. On the strength of the wave pattern in the Top Walled Garden, Michael decided to use a wave pattern here too. From above, the series of alternating stripes mimic another form of the rhythmic pattern of waves; specifically a reference to how successive waves lap the shore. And just like the Top Walled Garden, the contrast of foliage as opposed to flowers, is the main device employed to emphasise the pattern seen from above.

The Terraces

To the east and west, the Walled Garden is flanked by tiered terraces. The East Terraces were created from granite left over after the East Wing of the Castle was built in the Victorian period. The planting is laid out in drifts using foliage and flower colours like the pink Osteospermum jucundum, vivid blue Lithodora diffusa ‘Heavenly Blue’ and orange Gazania ‘Bicton Orange’ to accentuate the difference between the grass areas and plantings. Plants such as Elegia tectorum, Agave americana, Leucodendron argenteum and Coleonema ‘Sunset Gold’ punctuate the drifts and the walls are studded with species of Aeonium, Aloe and Echeveria. In recent years the use of succulents in the walls has been deployed to prevent rot and give more planting opportunities in the terraces. The success of ‘going vertical’ is obvious by the vigour of all the succulents including a flower spike from an Aloe polyphylla, not seen flowering in the garden before.

The West Terraces have recorded temperatures as high as 100F (37.78C) and, perhaps unsurprisingly, there is an area called the Hot Bed where succulents thrive. Here too, the plantings create the area’s character using the straw-coloured grass foliage of Chionochloa rubra and yellow daisy-like flowers of Euryops chrysnthemoides as well as self-seeded (but much wanted) pink Geranium maderense and the tall blue spikes of Echium pininana.

Gardening on the edge

Between the Castle and the Garden is a rock face interrupted by narrow ledges created by the natural weathering of the exposed granite bedrock. As the garden has developed alongside the essential inspections and work to make the rock face safe, the Garden Team has turned the challenge of keeping the ledges weed-free into a planting opportunity. Gradually, as the ledges are weeded, which includes the removal of local invasive Hottentot fig, they are being planted with Aeonium cuneatum and trailing Lampranthus. Work at such a height requires roped access as well as a head for heights and an ability to use a strimmer, hand fork or trowel at any moment whilst harnessed up. Members of the Garden Team have been trained to abseil and, with the help of a local contractor; the ledges are slowly being tamed to reflect the same type and style of plantings seen in the garden below.

A microclimate fit for exotics

Despite the salt air, storm-force winds, thin soils and occasional damp pockets fed by natural springs, the garden is exposed to a fantastic amount of light both directly from the sun and  also from rays reflected off the water below. Frosts are unusual on the island due to the mother rock of the Mount which absorbs heat during the day and releases it at night which, coupled with the surrounding salt air and winds, keep the succulent population from freezing and allow the plant community to not only live but thrive on this tiny island of treasures.