Top Of The Crops - Dahlia

Welcome to the top of the crops! Today you will be learning how to grow dahlias.

Growing Dahlias in a Polytunnel

Dahlia come in many different varieties and can be attractive in many gardens. They are also popular for use as cut flowers. While Dahlia are fairly easy to grow, they do need some protection during the colder months in the UK. This is where a polytunnel can really come in handy – in giving dahlia protection from frost. 

Key Information

Dahlias are tender flowering perennials that originally come from Central America and Mexico. These plants, while not winter hardy, are extremely popular for cottage gardens and other garden designs. 

They are prized for their large, bold flowers which bloom over a relatively long period – often through summer and right up to the first frosts. There are numerous cultivars available. They are often grown for use as cut flowers for household displays. 

However, these plants are also an edible crop. The flowers and tubers of different varieties can vary a lot, and not all taste that great, but all are technically edible. If you can find a variety with tasty tubers, these are a worthwhile crop in their own right. And the petals of dahlias can be pretty additions to a salad. 

The Preferred Conditions for Dahlias 

In order to thrive, Dahlias need to be placed in the right location. To grow them successfully, you need to think about the environmental conditions that they require. The basic growing needs for dahlias are:

  • Full sun.

  • A reasonably sheltered position.

  • Fertile, moist yet free-draining soil.

How to Grow Dahlias

The first step in growing dahlias is deciding which particular dahlias you would like to grow. Once you have chosen a variety or varieties, you will need to decide whether you will grow your dahlias from potted plants, dormant tubers, root cuttings, or from seed. 

Buying Dahlias

There are four main ways that dahlias can be purchased. You can buy:

  • Potted plants, in summer. 

  • Dormant tubers, widely available from garden centres and plant nurseries in spring.

  • Rooted cuttings, from specialist dahlia nurseries in spring. 

  • Or dahlia seeds to sow indoors before transplanting to their final growing positions. 

Where to Plant

When choosing a place to plant dahlias, the most important thing is to make sure you meet their basic growing needs and select a place in full sun, that is warm and sheltered, with a moist yet free-draining, fertile soil or growing medium 

Most dahlias will do best in the ground or a raised bed, but smaller bedding dahlias can also be grown in large containers. 

When to Plant

When dahlias are planted out depends on the form in which they were purchased. 

Potted plants purchased in containers should be planted as soon as you get them home or they arrive. 

Stored tubers should be planted in containers in spring and kept indoors to start growth early, then transplanted to their final growing positions in late May or June, depending on the location and the weather in a given year. 

If you do not have the space to start them off indoors or under cover, then you can plant them directly into their growing positions in mid to late April. 

Seeds should be sown in early to mid spring in a propagator. In late May or June then can then be hardened off and placed into their flowering positions. 

How to Plant Indoors

Plant dormant tubers or rooted cuttings into pots of peat-free multipurpose compost in spring. Make sure that the tubers are the right way up, with the linked section uppermost with the sausages or 'fingers' pointing downwards. 

The linked section at the top should be just below the surface of the growing medium. It is from this point that shoots will emerge. 

Keep them in a bright and frost-free location and water regularly to maintain moist but not waterlogged conditions. 

How to Plant Outside

Remember to harden off your dahlias before you plant them outside. Prepare the growing area where you plan to plant your dahlias, making sure that it is weed free and high in organic matter. Add compost or well-rotted manure to enrich the site. 

Potted plants and young plants grown from tubers should be planted to the same depth that they were at in their previous container. The top of the tubers, from where shoots emerge, should be just at the soil surface. Plants should be spaced around 60cm apart. 

Rooted cuttings should be planted a few millimetres lower than they were in the container and spaced around 50-60cm apart. 

Water the plants in well and then add a thick layer of organic mulch over the top of the soil to help conserve soil moisture. Make sure, however, that the mulch does not touch the base of the stems or they may rot. 

Planting Dormant Tubers Outdoors

If you do not, as mentioned above, have space to start dormant dahlia tubers indoors, you can plant them directly into their flowering positions in mid to late April. Roots can begin to form and by the time the shoots emerge, the garden should be reliably frost-free. 

These plants will likely just be a little behind those started indoors/under cover. Again, make sure you water in well and apply a mulch of homemade compost or well-rotted manure. 

Care Tips for Dahlias 

The key things to remember when caring for dahlias is that they are hungry and thirsty plants, that require plenty of water and nutrients throughout the growing season. They are also tender so you will need to consider how to care for them over the winter months. 


When watering dahlia remember that they need moist but not waterlogged conditions. Tubers can easily rot if things get too wet, so good drainage is immensely important. Excess water must always be able to drain away freely. 

In dry and hot weather dahlias are likely to need additional watering, even when growing outdoors. Under cover, you will of course have to ensure that water is consistently delivered to meet the needs of your plants. 

Remember that plants in pots will need watering more frequently than those growing in the ground since containers will tend to dry out more quickly. Always try to water below the plants and to keep the foliage and flowers dry where possible. 


Since dahlia are vigorous plants that grow relatively quickly, they do best with additional feeding. So in addition to using organic mulch materials, you should also feed the plants once a fortnight with an organic, potassium-rich liquid plant feed between July and early September, or once a week if growing in a container. 


Most dahlias except the very smallest dwarf bedding cultivars need some support, as they grow quite tall and can become top-heavy. Damage is likely for unsupported plants, even in sheltered conditions, especially after heavy rain. 

Dahlias with smaller flowers can be supported by bamboo canes, or within enclosing fences of canes and twine around clumps, while those with larger and heavier flowers typically need sturdy stakes to which they can be tied as they grow. 


It is a good idea to deadhead dahlias regularly during the blooming period. This not only keeps your garden looking neat but also ensures that plants focus on producing more flowers rather than on creating seeds. 

To deadhead dahlias, simply snip off the individual flowers, cutting back to a leaf joint lower down the stem. 

It can sometimes be difficult with dahlias to distinguish between faded flowers and flower buds once the petals have dropped. The key difference is their shape. Flower buds are rounded while the spent flowers have a more pointed appearance. 


Dahlias will have their foliage die back as soon as the first frosts arrive. Once the foliage dies back you have a couple of choices about what to do next. 

In milder areas, or perhaps in polytunnels, the tubers can potentially be left in the ground over winter, and mulched for protection. 

In most areas, however, when growing dahlias outside, the tubers should be lifted and stored over the winter months. It is not cold but rather wet conditions in winter that can prove the most problematic. In most parts of the UK, therefore, lifting tubers is the best policy to avoid them rotting in wet winter conditions. 

Leaving Tubers in the Ground

  • The riskier choice but the easiest too. 

  • Suitable for milder areas, polytunnel growers, and/ or areas with light, free-draining soil. 

  • If you choose this method, simply cut down the dahlia stems, and cover the crowns with around 15cm of compost or a coarse mulch like bark chippings to protect them from frost. 

Storing Tubers Indoors

  • The safer choice, but involving a bit more work. 

  • The better option for colder, wetter areas and/or those with heavy clay soil. 

  • If you choose this method, dig up the dahlia tubers before the first hard frost. Dust or shake off as much soil as possible and cut back the stems to stumps around 5-15cm long. 

Trim off any damaged tubers but otherwise do not split the clump. Place the tubers in shallow boxes or trays filled with dry potting compost or sand and store them in a dark, cool, frost free location. 

Common Problems for Dahlias

Dahlias can fall prey to a number of pests and diseases. Common pests include slugs, aphids, capsid bugs, earwigs, caterpillars, and red spider mites. 

Powdery mildew can be common in dry conditions, so keep your plants well-watered and make sure humidity does not drop too low. 

Mosaic virus and tomato spotted wilt virus can occasionally cause blotchiness and stunted growth for dahlias, and leafy gall can lead to distortions of leaf shoots, often close to the ground. 

Top Tips for Growing Dahlias in a Polytunnel

Dahlias can potentially be grown as an edible crop in a polytunnel, as well as being grown for ornamental reasons. Learning about different varieties will help you to choose ones that taste good as well as looking good. 

Ensure good ventilation and avoid overcrowding your dahlias. They do best when not overcrowded. You can also thin out stems to prevent overcrowding of established clumps. 

Feed with a potassium-rich feed to boost flowering, and take care not to provide too much nitrogen as this can lead to poor flowering but lots of leafy growth.  

Pinch out the growing tips on dahlia when the plants are around 40cm tall to encourage bushiness. 

If you wish to use dahlias as companion plants to draw in pollinators for other nearby crops, choose dahlias with single flowers as these are the most pollinator-friendly varieties. 




Ford, W., (2023) The Coffee Ground Hack That'll Make Your Dahlias Thrive. [online] Available at:  

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