Allason left baking for trade, setting up in Port Glasgow with his brothers in the years when Scotland's 1707 Act of Union had thrown open the markets of England's American colonies. He shipped goods across the Atlantic and grew wealthy enough to buy land in the Caribbean. The profits came from two cash crops above all - tobacco from Virginia and Maryland, and sugar from the West Indies - and both depended on the labour of enslaved Africans whose lives Allason and his peers treated as a line item. With that money he bought Flenders Farm, the family holding, and built Greenbank House in 1763. His fortunes turned during the American War of Independence, when the colonial markets that had made him rich became enemy territory.

Allason's reverses passed Greenbank to other hands. From 1796 to 1961, several generations of the Hamilton family owned the house and farmed its land. They kept it productive in the traditional Scottish country-house pattern - vegetables, fruit trees, kitchen produce, a few decorative beds near the door. Then in 1961, William P. Blyth bought the property and, with his wife, did something unusual. Instead of holding the gardens as working ground or simply letting them lapse, the Blyths transformed the 2.5-acre walled garden into ornamental planting on a scale and variety that would soon make Greenbank one of the most celebrated small gardens in West Central Scotland. In 1976, the Blyths gave the house, walled garden, and surrounding 16 acres to the National Trust for Scotland.

Greenbank's walled garden is built to mark the calendar. Spring brings bulbs and the white drift of apple and cherry blossom. Aubretia spills over walls. Dicentra hangs its bleeding hearts. Saxifrages bloom in the rock work. Summer is the long high season: hydrangeas in their slow shift of colour, primulas, dahlias coming up through July, the great rambling display of roses, philadelphus heavy with scent, azaleas and rhododendrons. Then come the high summer specialists - phlox, cosmos, echinops, sedum, lavatera, monarda - and the late perennials like helenium and sweet william that carry colour into autumn. The Trust also grows the less familiar: rodgersia pinnata superba with its great pleated leaves, echeveria gibbiflora metallica, and agrostemma. In the tea-room next to the garden, the National Trust keeps a substantial encyclopaedia of plants. Visitors can puzzle out anything they cannot name.

The walled garden is the formal heart, but Greenbank is also 16 acres of woodland, made beautiful chiefly by rhododendrons that flame in late spring and early summer. A small herd of Highland cattle - long-horned, shaggy-coated, looking like something out of a Walter Scott novel - lives in the wood. The house itself is a Category A listed Georgian building, sixteen rooms with barns and stables, and the garden as a whole is included on the Inventory of Gardens and Designed Landscapes in Scotland. It is, in other words, officially heritage. The honest account of how the original money was made should sit alongside the carefully labelled plants - because both are part of what Greenbank is.