We had an idyllic weekend in Oxfordshire recently. We took our young daughters to gather elderflowers from the hedgerow to make cordial. I also sowed more salads, carrots and radishes in the raised beds in our kitchen garden to ensure a steady supply through the summer.

It doesn’t get any better than these blissful, balmy days. I spent ages gazing with a critical eye at the 65ft-long border, which I planted last year. It is really beginning to knit together and I’m particularly happy with the way the grasses are softening the gaps between the roses and their neighbouring flowering perennials.

After much pondering, however, I have come to the conclusion there is still a lot to be done. On first sight, all appears well – the colour combinations look lovely and there is a good mix of foliage shape and texture – but this initial fav-ourable impression soon gives way to a sense that something is missing. It strikes me that much of the planting is of a similar height, which, despite the range of plants, makes it look a bit dull.

In fairness, it’s a little early to pass judgment, as many of the taller grasses reach their full height later in the season and will continue to do a good job during the autumn. Nevertheless, I need to break up the monotony of the horizontal plane by adding plants that draw the eye upward and add a bit of oomph in midsummer.

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Using the vertical plane in the garden is important – especially in a small one. If you can’t spread out, then go up. There are obvious ways to lead the eye skywards. Covering boundary walls, fences and trellis with climbers and wall shrubs, and giving buildings – unlovely potting sheds, garages, railings and bin stores – a secondary purpose as supports for a multitude of upwardly mobile plants will immediately add layers of interest to the garden.

Of course, these structures may not happen to be exactly where you want them, which is why pergolas, arches and other purpose-built plant supports are invaluable. The best are solidly made and will last for years. Many are decorative in their own right, needing minimal adornment from the plant to which they play host. They also add interest to the area in winter, when the border has died down.

I have just taken delivery of a tall wooden obelisk from Harrod Horticultural. Painted in a muted pale green, it will be the centrepiece of one of a series of new “rooms” I am planning in the garden for next year. Other plant supports are more businesslike, designed to be entirely obscured by stems, foliage and flowers.

Supports can be placed wherever height is lacking and, if used with annuals, can be moved around each year as the mood takes you. They need not cost the earth, either: a simple tripod made with long hazel sticks, tied together at the top, makes a good anchor point for a variety of shrubby and twining climbers.

I was going to place a series of these along the length of my own border and cover them with clematis and sweet peas, but changed my mind midway through planting because they just didn’t look right against the towering hornbeam hedge that forms the backdrop.

It was the right decision for this particular setting, but I need to find another way to add the height that is so obviously lacking. My first instinct is simply to do it with plants. I have already planted what will become pretty large shrubs in the border – including a pair of plummy-leaved cotinus and a lilac – but they are still small and will need several years to make a real impact.

In the meantime, I’ve decided to use some fast-growing perennials to fulfil this role. Some grasses, most obviously, Stipa gigantea (the name rather gives its size away – it grows to 8ft), and plants such as Cephalaria gigantea and Crambe cordifolia are undoubtedly tall, but they remain airy enough to retain a degree of transparency – so you can use them towards the front of the border without blocking out what is growing behind.

Where bulky height is required, look for fast-growing perennials with shrub-like proportions: Macleaya cordata, for example, which grows to 8ft, with a large clump of attractive foliage.

Alternatively, superslim spikes and spires are particularly useful for breaking up uniformity – just a few verbas-cums or lupins can do the trick when it comes to adding a vertical accent. These spear-like plants have the benefit of being tall but skinny. As they take up little ground space, they can be slotted between lower-growing neighbouring plants. These will in turn hide any unattractive foliage when the taller interlopers have finished flowering and the plant is past its best.

My border already contains plenty of medium-height plants with this vertical habit: purple-headed alliums pierce the planting like giant dressmaking pins, while quietly elegant foxgloves glow at the shaded end of the border running beneath the ash tree that marks its end. Really making its presence felt, however, is a bold clump of 5ft slender-stemmed Cirsium rivulare, the rich burgundy thistle-like flowers of which echo those of the neighbouring Rosa‘Munstead Wood’, a highly scented, velvety English rose named after Ger-trude Jekyll’s garden in Surrey.

It’s these big boys that really make an impact – and that’s what has been lacking. So, I’m on the lookout for delphiniums and in the market for cardoons. The days of one-level monotony are numbered. This time next year, the border will be a rollercoaster ride of thrilling ups and downs.

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