Rearguard action: There's no slacking at the back in Emma Townshend's borders

In ye olden days of yore, gardens possessed herbaceous borders, great big wide strips of earth either side of the lawn with plants arranged in rows according to height, like a school photo. At the very back, peering over the rest, there would be the floral equivalent of the 5ft 11in schoolgirls of the netball team: delphiniums, hollyhocks and flowering shrubs such as lavatera, all traditional cottage favourites, providing a colourful backdrop for other planting.

These days, though, delphiniums are deliciously vintage. And even possibly old hat. This is partly down to cash: even in distinguished National Trust gems, budgetary concerns are a big issue, and labour-intensive plants such as delphiniums (which need digging up and dividing every few years, annual staking and constant vigilance against pests) are necessarily few and far between.

But gardeners still want their borders to have the wow factor. So it's no surprise that canny horticulturalists have cottoned on to alternatives that require less fuss. Pondering a planting of drifting and tasteful grey and blue sages in Hyde Park at the weekend, I noticed that by August, the flower bed will tell a whole different story, with a row of glorious Ricinus communis coming into full effect.

With its reddish-brown stems, deeply veined bronze leaves and bonkers-looking red pompom flowers, Ricinus is just the kind of spectacular plant to fill the back border and put a smile on visitors' faces. (Its seeds also contain an extremely effective poison, so in gardens with kids or dogs present, plants are best deadheaded immediately, before seeds appear.) They bloom from July and will go right through to first frosts, providing excellent value for money, if you can actually lay your hands on a plant – it's so newly trendy, no big nursery is selling it yet. To grow from scratch, the best choice of varieties and advice comes from Jungle Seeds, which offers 10 seeds for £2 (jungleseeds.co.uk).

The tropical floras are increasingly being raided for other high-impact plants to provide back-of-the-border oomph. The tall, pink-tassled Persicaria orientalis was much admired at the Inner Temple flower show three years ago, and some enthusiasts are still waiting to get hold of seed. You can, however, see it at Great Dixter, where Fergus Garrett is clearly higher up the waiting list: in recent years he's used 120 plants in the main border of what he calls "Kiss me Over the Garden Gate", its old-fashioned common name.

I'd better finish with one you can actually buy in the shops – and there's no back-of-the-border plant finer than the South American Salvia guaranitica, ever-more fashionable and familiar to many garden visitors, even if it's just as "that amazing blaze of blue". Growing well above head height if pampered, the flowers are as blue as a medieval sky in a book of hours; add in the tall stems and it's a back-row show-stopper (£9.99 from crocus.co.uk). 1

High and mighty

Three more attention-seekers that won't behave on the back row...

1. Angelica gigas

Like a gigantic beetroot-coloured cow parsley. Hmm, I'm not making it sound nearly as good as it is. £7.99, crocus.co.uk

2. Lobelia tupa

Great big curled scarlet flowers on spire stems make this plant, also known as "Devil's Tobacco", up to 6ft tall. £7.50, burncoose.co.uk

3. Perovskia 'Blue Spire'

Not quite as tall as the others, but still a stately and delicious blue haze through which to view a summer afternoon. £9.99, crocus.co.uk