Flowers for Indian summers

Compare the weeks that a group of hardy annuals are in flower – cornflowers, sweet peas, marigolds and love-in-a-mist – with a group of half-hardy annuals, such as tobacco plants, cosmos, snapdragons and love-lies-bleeding. The half-hardies flower twice as long. That's why many of us grow them – the long-performing, so-called "bedding" plants.

The more I garden, the more I realise the same is true of perennials. The traditional and widely grown herbaceous perennials – peonies, delphiniums, Oriental poppies, lupins and phlox – flower for about a month or six weeks, whereas the less hardy, warmer-climate varieties can flower three or four times longer.

In terms of colour and weeks at centre stage, these slightly tender plants – alstroemerias, penstemons, asclepias, gauras, verbenas, cupheas, heliotrope, tender salvias and chrysanthemums, silenes, pelargoniums and arctotis – beat the first lot hands down. Add coleus and plectranthus for bold leaves.

All these plants come from hotter, brighter places where the growing season is longer. Alstroemerias, coleus, plectranthus and heliotrope hail from South America; chrysanthemums from Asia; asclepias, silenes, penstemons (more on these soon) tender salvias and cupheas from temperate and tropical regions of the Americas; and the verbenas, pelargoniums and arctotis from South Africa.

Brought over here, with our shorter growing season, they continue to behave as at home. I've had alstroemerias in flower since June and they're still going strong. Gaura lindheimeri, both 'The Bride' in white-flushed-pink and the deeper-coloured 'Siskiyou Pink', have been flowering in pots since May and usually keep going until Christmas and the same applies to all.

I've been adding more of these plants at Perch Hill, so, along with dahlias, the garden looks as floriferous in autumn and early winter as it does in spring and summer. It would be dull now without them. Most of these tender perennials make excellent cut flowers.

One of the good things about climate change is that many have been overwintering happily in our garden. Outside, only 10 years ago, they would not have survived.

My 10 favourite tender perennials

For flowers

Alstroemeria 'Apollo' (Award of Garden Merit)

A white alstro with a dark-yellow speckled throat. This is one of the very best, long-flowering (June to November) lovely and reliable as a garden plant and an excellent, very long-lasting, cut flower.

Gaura lindheimeri 'The Bride'

Ethereal, wide, pink-stemmed bushes covered in delicate white and pink butterfly flowers. This is ideal and low maintenance for pots, flowering well until winter. If you want a taller variety for borders, go for the species Gaura lindheimeri, which is almost twice the size. Both are best and easily grown from seed.

Cuphea viscosissima

An unusual tender perennial, often grown as an annual, with deep magenta-crimson flowers from August until late autumn. There is a big clump of this in the rose garden at Sissinghurst that looks exotic, yet fits well with more traditional English plants. This can be grown from cuttings or seed.

Heliotropium arborescens

There are various varieties of this stocky, tender perennial group, but it's hard to beat the highly scented, purple-flowered, dark-leaved type. It flowers long and looks good in a container or the ground. It can be grown from cuttings or seed.

Salvia greggii 'Stormy Pink'

One of the very best plants in my garden at the moment, a variety unique to Derry Watkins, of Special Plants. This salvia is a soft, yet not wishy-washy, smoky apricot-pink, with a lovely grey calyx behind the petals (hence 'Stormy'). It flowers from June until late October. It is easy to grow and seems reliably hardy with me, thriving in a sunny position with good drainage.

Salvia involucrata 'Bethellii'

An amazingly exotic-looking plant, with ball-like buds, which open to wonderful pink trumpet flowers. This is borderline tender, but will survive outside if you can find it a sheltered spot in well-drained soil. It can stand cold or wet, but not both. On heavy soil, add plenty of grit and in the late autumn, mulch with bark chippings (about 4in deep) and then lay a sheet of polythene to keep the crown dry.

Verbena 'Diamond Merci'

This seems to be the best dark burgundy verbena, deep, rich and velvety with good, bright, healthy-looking foliage and neat growth. It flowers until at least Christmas, but is not as strong as some. There are newly bred hybrids in this colour that are better doers, but don't have quite the depth of colour or texture of 'Merci', now very hard to find.

Verbena rigida

Verbena rigida is a bright purple form, shorter and stouter, with a larger flower than V. bonariensis, and perfect for lining a path. This is an unmissable plant, flowering from June until November. When it's over, don't tidy plants. These are tuberous perennials like dahlias and borderline hardy so their tops will help give good winter protection. Cut plants down in April when the coldest weather is over and they will soon reshoot.

For foliage

Plectranthus argentatus

I've been growing Plectranthus argentatus with its soft, velvet leaves of bright silver in pots at Perch Hill for the past three years and it has become one of my favourite late-summer and autumn plants. I like it best kept well-clipped and quite compact and give it a regular light hair cut.

Plectranthus ciliatus

This won't flower here, but its dark green leaves and purple stems make up for this from midsummer until Christmas. Glamorous, exotic-looking and easy to grow – my current favourite container plant.