Create a September Sensation

September 2018 border planner.jpg

Let dazzling dahlias and Verbena bonariensis brighten up your borders this month, says Helen Billiald

This planting scheme is inspired by the exotic garden at Great Dixter. It’s 25 years since Christopher Lloyd ripped out the old rose garden in order to create this exotic wonderland. Together with his gardening protégé, Fergus Garrett, they converted the once-formal space into a fantasy garden filled with colourful, jungly plants like this stunning combination of dahlias, verbena and ipomoea. The resulting late summer and autumn profusion inspired a seismic shift in planting styles across the country whose influence continues to resonate today.


Choose the right plants

Dahlia ‘Fascination’
Unflinchingly violet-pink dahlia with semi-double dark centred flowers against a backdrop of inky leaves. H and S 90cm (3ft)

Canna ‘Erebus’
Grey-blue lance shaped leaves emphasise this canna’s salmon pink flowers. H1.5m (5ft) S50cm (20in)

Verbena bonariensis
Little clusters of purple-lilac flowers on stiff branching stems, June to October. Airy look makes it a wonderful ‘threader’ through other plants. H1.8m (6ft) S45cm (18in)

Ipomoea lobata
Spanish flag is a fast-growing tender twining climber that’s treated as an annual in this country. Its tubular flowers mature from red to creamy yellow. Flowers from July to the frosts. H2.4m (8ft)

Pennisetum advena ‘Rubrum’
Dark red strap-like foliage and arching bottle-brush flowers that mature from rusty red to brown. Dubious hardiness, lift to overwinter. H90cm (3ft) S60cm (2ft)

Tetrapanax papyrifer
Large bold foliage plant with palmate leaves and white flowers in late autumn. Needs space, sends out suckers and not hardy for everyone. Typically, H2-3m (6-8ft) S2m (6ft 6in)

Get Planting
If you’re after a tip-top display you really want to spoil this planting. Give it your best, sunniest site sheltered from strong winds and be prepared to seriously improve your soil to ensure fertile growing conditions. Annual mulching with well-rotted organic matter as well as digging in further material at every planting will also help. Be prepared to water deeply during prolonged dry spells.

1. Order the dahlia and canna
Order the dahlia tubers and the canna rhizomes now and you’ll be able to forget about them until they arrive in the post next February or March. Pot them up and grow them on in a frost free greenhouse before planting out in late May. If you want to increase your stock then take cuttings from stout newly emerged shoots. Watch out for slugs and snails which can decimate young foliage.
‘Fascination’ is a relatively short cultivar but it’s still worth putting in a cane when you plant so that you have the option to support shoots with twine if needed. Deadhead every time you pass to keep the display coming. Once plants have turned black from the first frosts you can either cut them back and dig up the tubers to store in barely damp old potting compost in a frost free shed, or leave them in situ covered with a protective dry mulch.

Unless you live in a very sheltered part of the country and on free-draining soil, it’s not worth the risk of leaving plants in the ground over winter, even with a protective mulch. Instead, wait for the first frosts then cut back plants and lift as a single clump to store surrounded by old potting compost kept damp in a frost-free shed. Next spring large plants can be divided and potted up, making sure each new section has several growing buds.

 2. Sow the verbena
Verbena bonariensis is a doddle to grow from seed, you just need to give them an early start. Sow under cover in March using a warm windowsill or heated propagator at around 20C (68F).
A more expensive option is to pick up young plants in late spring. Once they’re established you’ll find they self-sow and seedlings start to pop up around the garden often in unusual yet welcome places like paving cracks or sections of gravel.
Plants are short-lived perennials and overwinter best on well-drained soil. A dry winter mulch helps get them through the coldest spells. Resist cutting back the top growth until new shoots are growing the following spring.

3. Sow the ipomoea
Sow seeds in mid-April to May in the greenhouse. Seeds are large and easy to handle with good germination rates so sow one seed to a pot and use a propagator to maintain a cosy temperature (around 21C/70F). Soaking or nicking seed isn’t essential. Pot on before planting out in early June when nights should have warmed up.
These plants will cope with shade as well as sunshine, just avoid too much nitrogen in the planting site or you’ll run the risk of over enthusiastic foliage with little in the way of flowers. They’re brilliant at rambling right through a planting, often travelling in unexpected directions.

4. Add the pennisetum
Give this tactile grass a front-of-border position so you can stroke its arching bottle-brush flowers as you pass. ‘Rubrum’ is sterile so there’s no chance of sourcing this plant from seed. Instead order plants from a specialist nursery ready for delivery in May or look out for them at your local garden centre. Youngsters are best planted out after all risk of frost.
Dig up plants once frosts are threatened and move them undercover into a frost-free greenhouse. Lift them as a single large clump, cut back the top growth by half and place in a large container, filling in any gaps with old potting compost: you’re not trying to encourage new growth now. Water sparingly over winter then in spring divide into smaller clumps and pot up in fresh compost ready to plant out in late May.

5. Establish the tetrapanax
Specialist nurseries will be your best chance of tracking down young tetrapanax. Plant out in early June allowing plenty of space from the start and further improve the ground with well-rotted organic matter – it prefers a well-drained site. 
In future years, severe winters will dictate how much top growth survives. Even if it’s cut back right to the ground by frost it will regrow from its roots the following year. You might also choose to coppice plants right back in early spring to encourage lots of low shrubby growth rather than a taller trunk. Remove unwanted suckers as they appear in spring: you can always pot them up as gifts for jungle-loving friends.