VENUE: Mells Walled Garden.  Does the planting in your garden run out of steam half-way through the season? Designing a garden full of plants that look attractive all year requires some forward planning. On this day long workshop we share the key tips and techniques that professionals use to keep gardens looking great all year round. We focus in particular on herbaceous perennials, the flowers that we rely on for months of colour in the border. At the end of…