As a result, the gardens are never the same each year. Many areas are regularly cleared and re-designed, to provide a range of opportunities for the students to gain the necessary practical experience in horticulture. They could hardly have a more stimulating site on which to do their training. Threave is an extraordinarily diverse place, having a rose garden, an orchard, a woodland garden, an arboretum, rock and peat gardens, a nursery, a walled garden, herbaceous beds, a small heath garden, a formal garden, a dwarf conifer collection and a secret garden.
If you're an expert or gardening buff, there's more than enough variety and interest here for a full day's exploration. But even for the non-specialist, who may be less bothered about knowing or identifying the names of plants and flowers, there's still enough colour, beauty and diversity within the 60 acres to feast the eye for a morning or afternoon.
Part of the charm of the garden is that it's had to be created on sloping terrain with many natural rock outcrops. As a result, its design is largely informal, and its features can be seen and appreciated from many different angles and levels..
One of the most delightful areas is the secret garden, previously a heather garden. Full of winding paths and ornamental bridges, the core of this has been designed and executed by the students. In one corner there's an attractive sunken garden with a fine view of the local hills.
Similarly, at the edge of the small heath garden there's a tiny space not unlike that of many people's own gardens. Using slate, sandstone and pre-cast slabs the students have produced a charming paved area, with a juniper tree, a camomile lawn, and seats facing a wall resplendent with climbers.
A more conventional but no less impressive feature is the one-acre walled garden, still much as it was when it was built in 1870 to provide fruit and vegetables for Threave House, an attractive sandstone building which stands in the centre of the garden and now provides accommodation for the students. For them the walled garden provides plenty of work. Among the tempting fruit being grown are gooseberries, strawberries, pears and redcurrants, flanked by rows and rows of cabbages, French beans and other vegetables. On one side there's a miniature herb garden, with marjoram, chives and ginger mint; on another, several greenhouses.
The colourful rock garden with cotoneaster, jasmine and dianthus, is a gem, as is the peat garden, planted with primula, gentians and dwarf rhododendrons. Nearby, adding to the tranquillity of this corner, are a small pond with water lilies and a miniature waterfall, both created by students.
The diversity of Threave is reflected again in the formal garden, an area much worked on by students featuring a knot garden, a series of raised beds, a rose and clematis garden, a miniature topiary, and a parterre.
All this provides a wonderful range of possibilities for the students, who have to learn about soils, compost and manures, the safe use of tools and equipment, glasshouse cultivation, propagation, garden design, pest and disease control, lawns, fruit and vegetables, and arboriculture.
The course, lasting one year and open to those with some experience at college, consists of supervised practical work in the garden, as well as demonstrations and occasional lectures. Because of the problem of students' getting discretionary grants, numbers have now been reduced to six, and all students receive a bursary from the Trust.
The track record of former students is good: many are now assistant curators in botanical gardens, or in charge of gardens overseas - to the slight chagrin of the Trust, which would like to attract a higher proportion into its own gardens.
Threave lacks any form of interpretation, an omission which Duncan Donald, head of gardens at the NTS, hopes soon to rectify. "We're trialling different methods at the moment," he says. "We want to make it more of an educational garden in the widest sense, to explain why these things are being done. "
Threave Garden, Castle Douglas DG7 1RX, open all year, daily 9.30 to sunset. Visitor Centre April 1 to October 31, 9.30 to 5.30. Admission Pounds 3. 50, children Pounds 1. NT members free. Gardens of Scotland Open for Charity available from bookshops and from 31 Castle Terrace, Edinburgh EH1 2EL (Pounds 2.50). Gardens of England and Wales Open for Charity from bookshops and Hatchlands Park, East Clandon, Guildford, Surrey GU4 7RT(Pounds 3.50).