Plant guide: Dahlias

The dahlia’s popularity has been steadily on the rise in recent years thanks in no small part to gloriously photogenic varieties such as ‘Cafe au Lait’.

Flowering from Summer right up until the first frosts, available in a huge range of colours, and rapidly increasing in number each year, the dahlia has a huge amount to offer any gardener. Once established, they’re a fairly straightforward plant to care for, but there are a few tips that help them perform to their best…

Latin name: Dahlia

There are about 40 different species, with the garden dahlia being the most common: Dahlia pinnata.

Plant type: Herbaceous perennial

Dahlias will grow back year after year, but their foliage dies right back down below the ground during the winter months, where they stay dormant as a tuber.

Growing conditions

Dahlias need a sunny site (at least 8 hours of sun a day), with plenty of air space. In colder, wetter positions they can be affected by frosts over winter, and the tubers would need to be lifted and brought inside for the winter months. If they’re growing in a warmer, lighter soil, they should be okay staying in the ground in all but the coldest winters.

Avoid planting dahlias in windy or very wet sites.

Dahlias range in height from around 30cm to more than a 1m, so select a position in the bed according to the height of the variety. Taller varieties are suited to positions towards the middle or back, whereas lower growing varieties could be positioned towards the front of the border.

As the plants are herbaceous (and may need lifting out entirely) remember that they will leave patches of bare earth during the winter months, so plant them alongside evergreens and shrubs that will provide interest during this time.

Dahlias also look stunning grown in containers, with the added benefit that these can simply be moved to a frost-free place for the winter.

How to grow

Dahlias can be planted either as a dormant tuber – which are readily available to buy in Spring – or as a potted plant already in growth in the Summer.

If you’re planting tubers, these usually arrive in early Spring, often before the last frosts. In order to make sure the growing plant isn’t killed by frosts, it is safest to plant the tuber into pots, where it can be grown for a month or two in a sheltered location, before going into its final position in the ground in May time. Use a large pot – at least 2 litres – and place one tuber per pot, in a general multi-purpose compost. Grow on in this position until May and then it’s ready to plant in the ground.

Alternatively, you can store the tuber when they first arrive, ready to plant them directly into the ground in mid-April. If you’re storing them, keep them in the bags they were dispatched in, and keep them in a dry, frost free place, away from radiators and away from bright light.

In mid-April, once conditions are warm and dry, and the last frosts have passed, they are ready to go into the ground. Dig a hole about 30cm deep, add in some well-rotted organic matter, and place the tuber in the bottom. Space tubers about 60cm apart from each other. Water in well and continue to water regularly in any dry periods.

If you’re planting out tubers that you have grown for a month or so in a pot, dig a hole the depth of the pot and a little wider. Remove the plant and soil and place in the bottom of the hole, making sure that the level of the compost is the same as the level of the surrounding soil. Back fill around the edges and water in well.

Add a general purpose fertiliser to the soil around the plant after planting, following the instructions on the bottle. Always take care when using a fertiliser not to apply too much, as solution that has not been diluted properly can scorch the roots (and ultimately kill your plant – definitely a time when you can have too much of a good thing!)

Care:

Pinching out dahlias ensures that the plant grows bushy and healthy, and helps to encourage more flowers. There are lots of different recommended ways you can do this, from reducing the number of stems, to pinching out when the plant is still in very early growth. We tend to keep it very simple though, and wait until the plant is producing its first flower. Once you see the bud appear, cut down the stem with the flower bud on the end, removing it just above a leaf. This stops the plant’s energy going into flower production and helps it to put on more robust growth, and healthier, bigger flowers later.

It’s also a good idea to restrict the number of stems growing from each plant. Every gardener has a different number of ideal stems, but anything between 5 and 10 sturdy stems should be good. (As a general rule, the bigger the flower, the fewer the number of stems…) First remove any that look spindly or unhealthy. Then cut back any spares, making sure that the remaining stems are well spread out. This may feel counter-productive, but it encourages the plant to focus its energy on some really strong, bushy growth, instead of lots of weaker stems.

Most dahlias need to be staked, to prevent their stems breaking under the weight of the flower heads. This can be something as simple as a square of bamboo canes around the plant, with some string tied between them, or a purpose bought plant support, such as a round metal cage. Whatever you’re using, make sure to put the support in when the plant is still small, so that it can grow up into it. Trying to add the support to an already large plant can cause more damage than assistance!

Dahlias are very productive plants, creating many flowers over a short time, so they benefit from extra feeding. A potassium rich plant food (potassium helps with flowers and fruit) such as a tomato feed, applied every fortnight, once the flower buds start to form, will help provide the extra energy the plant needs.

Be sure to water well in any dry and hot periods. Give the plants a really good soaking when you do water them – always onto the soil and not onto the leaves…

Once dahlias are in flower, the more flowers you remove from the plant, the more they will make! Cut flowers to bring inside the house, and make sure to deadhead any flowers that have gone over. See our guide to deadheading for more info.

Finally, the biggest care decision is what to do over winter. In colder parts of the country, or on wet soil, it will be best to lift the tubers and store them somewhere dry and frost free over winter. To do this, cut the stems off and gently tease the tubers out of the ground with a fork – being careful not to pierce into any of them. Remove any decaying tubers, check for any signs of pests or disease, and shake off soil. Trim the stem back to approximately 10cm and store the tubers in boxes or crates, ideally packed in with something dry, such as sand.

However, if you live in a warmer part of the country, and the soil isn’t too wet, that might all sound like quite a lot of hassle (as well as requiring storage space like a shed or a garage). If this is the case, you can leave your tubers in the ground and risk it! They should be okay on all but the very coldest years, when they might suffer from really hard frosts.

Uses:

Dahlias are a very popular cut flower – they look stunning either on their own, or mixed with other flowers in a bouquet.

Only cut dahlia heads when the flower has already opened – the bud won’t continue to open once it has been cut off. It’s best to cut early in the morning, when it is still cool, and the flower stems aren’t drooping. Cut the stems as long as you need for the vase, and cut back on the remaining plant to just about a set of buds or leaves.

Once cut, condition the stems by leaving them in a small container of very hot, almost boiling water for an hour – this helps the flowers to last in the vase for longer. See our guide on conditioning cut flowers for more info.

Grow with:

Dahlias look fantastic growing with other late Summer perennials and annuals, such as amaranthus, asters or ammi visnaga. A more modern looking plant combination is to grow dahlias alongside grasses, such as Pennisetum, where the huge dahlia flower heads provide a real contrast to the thin grass stalks…

Our favourite varieties:

We have a real soft spot for the ‘pom pom’ varieties: Wizard of Oz, Jowey Nina and Downham Royal, which have a range of pink to purple shades of flowers and look especially good planted together.

We would also always recommend growing a few single varieties for the bees: ‘Bishop of Dover’ is a particularly beautiful one, with white flowers fading out to pale pink at the edges held above striking dark burgundy leaves.

Of course, no dahlia guide would be complete without mention of Instagram’s favourite – ‘Café au lait’... It’s popular for good reason, it’s such an incredibly beautiful flower, with vintage creamy-peach colours that look especially stunning in photographs