Your gardening questions answered: What’s the best way to prune my unruly purple potato vine?

How and when should you prune Solanum crispum ‘Glasnevin’? I have a plant that has grown very vigorously – it flowered beautifully last year and now I’m nervous to cut it back, but the branches are so heavy it’s falling off the trellis. FB, Dublin

A native of South America, Solanum crispum “Glasnevin” is a hardy, fast-growing, semi-evergreen, sun-loving wall shrub (reliably evergreen in milder, sheltered Irish gardens) with long, lax stems that are covered with dense clusters of small, scented, golden-eyed, purple- blue flowers on and off from late spring until early autumn, sometimes even into early winter.

An ornamental member of the potato family (its common name is the Chilean potato vine), this particular variety was first spotted growing in the National Botanic Gardens in Dublin almost 150 years ago as an unnamed seedling that showed promise. Hence its varietal name of “Glasnevin”.

Lovely as it is, its naturally untidy, vigorous growth habit means this sprawling plant occasionally needs a hard prune to keep it in check, especially if it’s growing in a mild, very sheltered urban garden. But rest assured that despite its exotic appearance, Solanum Glasnevin will take even ruthless pruning in its stride. In fact I’ve seen plants cut back hard to within a few feet of ground level that have subsequently regrown.

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This is the best time of the year to prune it, using a sharp, clean secateurs to cut back any unwanted growth and to shorten side-shoots to three-four buds. Don’t worry too much about getting it badly wrong… a good-natured, fast-growing shrub, it won’t be too fazed by whatever you do to it.

To add additional seasonal colour, you could interplant these with a few of the many deciduous kinds of clematis

To help prevent it from becoming too unruly in the future, make sure to regularly tie the young shoots with garden string against trellis or horizontal training wires, resisting the urge to weave the stems underneath the trellising/wires as this will eventually cause damage.

But do bear in mind that by its nature, Solanum Glasnevin is a naturally vigorous, untidy, large plant that can’t be forever corralled neatly against a trellis and can eventually reach a height and spread of 6m-8m x 1m-2m. So, if keeping it to a manageable size proves to be just too much of a challenge, then perhaps consider some other slower-growing, more compact alternatives? Evergreen examples suitable for a sunny, sheltered garden include many kinds of clematis such as the spring-flowering Clematis “Avalanche”; star jasmine (Trachelospermum jasminoides), whose dense, intensely perfumed clusters of starry white flowers are produced throughout summer-early autumn; and many kinds of Ceanothus, a very decorative, blue-flowering woody shrub that can be easily trained against a sunny wall or trellis.

To add additional seasonal colour, you could interplant these with a few of the many deciduous kinds of clematis such as varieties of Clematis viticella (late summer-flowering) or any of the large-flowered summer-blooming hybrids, all of which can be found for sale in most good Irish garden centres.