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11 March 2021

How to grow clematis

Clematis are one of the most popular climbing plants, loved for their colourful flowers.
Pink clematis
CT
Ceri Thomas

You can grow clematis in the ground or in containers, and by planting varieties from the diverse groups that flower at different times of the year, you could be enjoying the delights of clematis for many months. 

Which? Gardening magazine grew a range of popular varieties to see which would give us the best display throughout the year.

How to grow clematis: month by month

January February March April May June


PRUNE GROUP 2&3 PRUNE GROUP 2&3/ PLANT PLANT PRUNE GROUP 1

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July August September October November December




PLANT PLANT



Best clematis varieties

Which? members can log in now to see the full results and which are our Best Buy varieties. If you're not a member, join Which? to get instant access.

Best Buy spring-flowering clematis
What it looks like Variety name Height x spread
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120 x 140cm
This variety is part of the Forsteri group of clematis and is a scrambling, evergreen plant. Its long stems and attractive, shiny, dark-green leaves form a dense and quite large mound. When the flowers open, it transforms from a rather untidy specimen into a firm favourite. The mass of creamy white, cup-shaped flowers cover the plants, forming an impressive display that really stands out. Grow with C. x cartmanii ‘Avalanche’ or ‘Joe’ to get seedheads
What it looks like Variety name Height x spread
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300 x 70cm
This variety has slightly twisted, violet-blue outer sepals over a bell-shaped cream centre that creates flashes of light through the blue. The colour stands out against the soft, dark foliage. Peak flowering only lasted a few weeks in our trial, but was followed soon after by a second flush of smaller flowers and then the occasional bloom through to the end of September. It also holds an RHS Award of Garden Merit (AGM).
Best Buy early to mid-summer flowering clematis
What it looks like Variety name Height x spread
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210 x 175cm
A variety with real presence, thanks to its strong growth and the intense colour of the flowers. In our trial, we found that by mid-June it had a dense canopy of foliage, which created a backdrop for the plate-like blooms. Each petal is dark-red with a wide central bar of deep pink; the golden stamens provide a good contrast. July sees seedheads and more flowerbuds, the latter heralding a second flower display at the end of August. Pruning group two.
What it looks like Variety name Height x spread
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200 x 115cm
In two separate trials, we’ve found this compact variety equally good for growing in a pot or the ground. Its mauve flowers, with a pronounced silvery bar, a thick ruff of lilac stamens that resemble rolled petals and bright, yellow-green centres, have an exotic look. Equally impressive is that it is capable of nine weeks of flowering in two main flushes in early and mid-summer with a few sporadic blooms in between. Pruning group two.
What it looks like Variety name Height x spread
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130 x 80cm
A great candidate for growing in a container, this compact variety wows thanks to the masses of large flowers that last for four to six weeks in early to mid-summer, before it finally runs out of steam by August. Grown in the ground, the bushy plants are tall enough to cover a short obelisk and the mass of elegant, upward-facing white blooms with their fine purple edge make a great display. Pruning group three.
What it looks like Variety name Height x spread
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185 x 45cm
There’s no shortage of purple-flowered clematis varieties, but this stood out in our trial. The large single flowers were plentiful, and the golden stamens made a lovely contrast to the velvety sepals. After six weeks of flowering, a good seedhead display carried on the show. Despite its name, this variety isn’t wild and untidy, but will neatly clothe an obelisk and would be compact enough to grow in a container. Pruning group two.
What it looks like Variety name Height x spread
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130 x 65cm
The dark-purple flowers of this variety, splashed with irregular markings of mauve and white, were quite a talking point in our trial. There were lots of flowerbuds and a few blooms in June, then the main show got underway and lasted the best part of July. Although it didn’t reach its full height in our trial, it can grow up to three metres tall and is best grown in the ground. Pruning group three.
Best Buy mid- to late-summer flowering clematis
What it looks like Variety name Height x spread
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250 x 75cm
This clematis has delicate leaves and flowers with an absolute profusion of small, deep violet-purple blooms with contrasting cream stamens that make a hugely colourful display. A worthy Best Buy, it’s been awarded an RHS AGM, too. It can reach five metres in height and when we grew it, it coped well with a cold spring and very hot summer. The first flush of flowers were quickly followed by fresh blooms and flowering lasting a total of 13 weeks. Pruning group three.
What it looks like Variety name Height x spread
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250 x 50cm
The large flowers of this variety have bright yellow stamens that stand out against the dark-pink, slightly ruffled sepals and give a beautiful display, even as they age and fade to a lovely plum colour. In our trial, this variety was undaunted by hot weather, producing a succession of buds and blooms, which lasted for 17 weeks from late May to the end of September. The plants are tall but slender and would suit a tall obelisk or arch. Pruning group three.
What it looks like Variety name Height x spread
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200 x 50cm
The individual blooms of this RHS AGM variety are quite small, made from four to six widely spaced, dark-pink sepals. Despite their diminutive size, there are so many flowers they combine to make a real splash of colour, set off against the bright green leaves. The flowers keep coming, too, giving a great display that lasts up to 12 weeks. It’s a vigorous and healthy variety that shrugged off a slight fungal infection affecting other varieties in the trial. Pruning group three.
What it looks like Variety name Height x spread
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150 x 70cm
The flowers on this variety are a sophisticated pale blue, with rounded petals and pale stamens. They make a wonderful display that lasts for up to 17 weeks throughout the summer, producing wave after wave of blooms that in our trial started off quite large and then got smaller as the season progressed, possibly due to some very hot, dry weather. Although it didn’t grow to its full height in our trial, it can reach three metres tall. Pruning group three.
Best Buy patio clematis for growing in a pot
What it looks like Variety name Height x spread
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120 x 70cm
The gorgeous cobalt blue of these very large flowers really stood out in our trial. Even when they faded to a more washed-out pinky blue, they still looked lovely. The sheer flower power was also outstanding: our plants were almost completely cloaked in vibrant blooms at their peak. Bushy plants, which didn’t attempt a getaway, and silky, spidery seedheads also added to their charms. This is a fabulous plant from all angles.
What it looks like Variety name Height x spread
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150 x 70cm
The large and remarkable double flowers were arranged in two layers, with the flat violet-blue petals below and paler quilled petals above. Our plants were bushy with a neat shape that they kept all season, making them ideal for pots. Blooms opened from the top to the bottom of the red stems, too, so we got a really good covering of flowers, and they managed three full flushes in one season.
What it looks like Variety name Height x spread
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100 x 75cm
Pale and certainly interesting, this variety had large, blush-pink flowers that faded first to white with a pink stripe, and finally to pristine white. Rain didn’t mark them despite their pale colour, but they did turn brown once they were finished, so you might want to do some deadheading. Our plants also had a full, bushy shape and a good covering of graceful flowers, which makes them absolutely ideal for growing in a pot.
What it looks like Variety name Height x spread
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100 x 70cm
This variety produced masses of stems and was really bushy. We added some chicken wire around our second pot and easily trained the plants to trail down it, in addition to climbing. The delicate violet shade of the large flowers, though not very unusual, looked pretty. There were masses of flowers and we got a second bonus flush later in the summer.
What it looks like Variety name Height x spread
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110 x 75cm
Described as ‘divine’ by one assessor, the huge, deep velvet-red flowers developed evenly all round our plants in a way that would make this variety a stunning focal point from any angle. The backs of the flowers were a pretty pink, and the lush, healthy growth filled out around the supports nicely. It also had attractive spidery seedheads and a second flush of flowers in August.

How we test clematis

We grow different varieties alongside each other so we can compare them as they grow. We assess the weekly through the growing season so we can see how they perform. We grow them for two years to see how well they do through winter as well as summer.

Caring for your plants

Planting

Preferably buy plants in 2-3L pots and plant at any time of year when the ground is workable, although the dry summer months aren’t ideal. Provide shade for the roots so they stay cool. Plant them so the rootball is 5-7.5cm under the soil so that buds underground can regrow if the top growth gets damaged or diseased and has to be cut back. Water regularly for several months after planting until they’re well established.

Feeding and watering

In fertile soil they may not need feeding. In poor soil use a general fertiliser, such as Growmore, in spring. Mulch annually with 5-7.5cm of well-rotted compost. Established clematis shouldn’t need watering unless the soil is very dry.

Pruning

Group one Trim back after flowering and tie in stems as needed. Cut back by around half to renovate if needed.

Group two Dead or weak stems can be removed or shortened before flowering in February or March. After flowering, some stems can be cut back by half to side-shoots or strong buds to encourage new growth and a second flush.

Group three In February or March, cut all stems to a pair of healthy buds around 20-30cm from the ground.

Growing clematis in a pot

  • Use a large pot to give roots room to grow; roughly 45cm in diameter and depth.
  • Use a Best Buy compost for containers and mix in a Best Buy controlled-release feed.
  • Avoid clematis wilt by planting deeply so the top of the roots are below soil level.
  • Buy a good-quality obelisk that will support your plant for a number of years; we found that cheap wicker obelisks soon broke. Ensure the diameter will fit your pot.
  • Topdress with fresh compost and controlled-release feed annually.
  • Water regularly; daily during hot weather. 
  • Most varieties will grow better if you site them away from potential cold winds. Strong sun can fade flower colours. Move the pot so the plant gets some midday shade if necessary.

Common growing problems

Greenfly

Greenfly can quickly build up and cause poor growth by sucking the plant’s sap. Treat by squishing any colonies by hand.

Read more about greenfly.

Clematis wilt

This is a fungal disease that causes the whole plant to wilt. Planting deeply will mean that you can cut back the whole plant and it should regrow from the base. Sometimes the cause isn't clematis wilt but actually physical damage such as slugs nibbling the stems at the base.

Green petal

Green petal is caused by cold spring weather while flowerbuds are forming. Some petals become bright green and distorted, but there are no detrimental effects to the plant. 

Read more about green petal.

Slime flux

This is a bacterial problem that can cause the plant to collapse and horrible ooze to come out of the base. Cut back the whole plant and it should resprout from the base.


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