Before we think about how to grow dahlias, we might ask ourselves why should we grow dahlias?’ In other words, do dahlias repay the attention that we are required to give to them?
According to the late Christopher Lloyd, whose Great Dixter garden featured multiple dahlia cultivars, dahlias are best in in late summer, when they are valuable in: “stiffening a garden that might easily dissolve into an amorphous froth of Michaelemas daises, and when the mellow quality of autumn sunlight best suits their warm colours.”
This is undoubtedly true, but I think there is even more to dahlias than that. They are, to my mind, one of the best all round garden plants.
Having been a stalwart of many a 1950s and 1960s garden, they gradually fell out of favour. But now, like Eames chairs and G-plan sideboards, these retro garden stars are back.
Dahlias originate from the mountainous areas of Central America and Mexico. There are about 30 species of dahlias, including dahlia pinnata and dahlia coccinea, and thousands of different cultivars or varieties.
Dahlias are tuberous perennial plants, grown in gardens for their showy flowers which last from midsummer to late autumn.
Most dahlia cultivars have mid to dark green leaves, although some modern varieties have dark red-black foliage.
Here I’ll give you the full year round rundown on how to grow dahlias, from planting in spring, through summer dahlia care and over-wintering.
As noted, dahlias originate from the warm, dry areas of Central and South America and, as with all plants, the place of origin gives us a strong clue as to the conditions in which dahlias thrive.
Soil for dahlias
Dahlias will grow in most soils, as long as the soil (like the hillsides of their home) is well drained and not prone to water-logging. Dahlia tubers are liable to rot in waterlogged soil.
That said, dahlias do need plenty of water, so it is important that the soil is reasonably water retentive. Pure sand or very free-draining soils that easily dry out are not ideal.
Well drained soil that is also water retentive – wouldn’t we all like to have such soil in our gardens? The fact is, if you don’t have the perfect soil, it won’t stop you growing dahlias and, if you are willing to put in a bit of effort to improve the soil that you have, you can grow outstanding plants.
The key to improving any soil is the addition of organic matter. This means adding well rotted manure, garden compost, mushroom compost or commercially available soil improvers.
It is a good idea to start improving the soil for your dahlias at the end of the growing season in Autumn. In this way you begin the process of replenishing the soil with the nutrients that have been used up in the summer’s growth. This means identifying where you are going to grow next year’s plants.
If you lift your dahlias for the winter (more on this later) and intend to grow again in the same spot the following year, then dig in as much organic matter as you can into the area you have lifted the tubers from.
If you leave your dahlias in situ over trhe winter, mulch thickly over and around the site where your tubers are. Use good organic matter, such as garden compost, for the mulch and and it will break down over winter to improve the soil.
For more on mulching, read this post on the best mulch for dahlias.
After this preparatory work you can then leave the site until Spring. Wait until the soil begins to warm and is not too heavy and wet and then turn over the top layer of soil. This doesn’t have to be a deep dig, more just a loosening of the first few inches of soil to undo the compacting that may have occurred with the snows and rains of winter.
At this time it is also a good idea to add some organic fertiliser ( I use bonemeal or blood, fish and bone) at about 4oz per sq yard (100g per sq metre). Sprinkle over the surface and then fork it in gently. You might also want to add some organic slug pellets (see Pests and Diseases, below).
How much sun do dahlias need to grow?
Fundamentally, dahlias need a position in full sun (remember those Central American hillsides where they originate from). They need warmth and light in order to put on the huge amount of growth that they do over the course of 3 or 4 months of summer. So dahlias planted in the shade of trees or shrubs, or hard against a boundary wall or fence, will struggle. They may grow, but they will not thrive.
That said, dahlias can stand some partial shade, especially afternoon shade where temperatures are very warm. They can also grow in cooler climates. I grow dahlias in Scotland, and whilst they don’t do as well as the dahlias I grew when I lived further south, in London, they will do okay. Although, where temperatures are lower, the more sun you can give your dahlias, the better.
It is a good idea to plan where you are going to plant out your dahlia tubers. You need to give some thought to the characteristics of the particular varieties you intend to grow. You can place some canes for staking in the proposed position of the plants, leaving enough room for anticipated growth.
Most cultivars, other than the bedding varieties, will require staking. It is advisable to get a sturdy frame in place early in the growing season and to tie in stems as the plants grow out.
Pay attention to the height and width that your dahlia variety will eventually reach – this information should be available from the plant catalogues or websites of dahlia sellers – and plan your planting accordingly.
If planting in a mixed border with other plants (or planting different dahlia varieties), you’ll also need to take account of the size of surrounding plants and make sure that the smaller plants are not obscured by the taller ones.
A mistake that is easily made (in other words, it’s one I sometimes make) is to under-estimate the size of different plants, with the result that by mid-summer some of your favourites are hidden by taller plants in front.
You can rely on the published information about the eventual sizes of your plants, but also use you own local knowledge. If your soil is gold and your climate is perfect, your plants are likely to grow bigger. If your soil is so-so and your climate is cool, plan for smaller plants.
For a complete guide on when to plant dahlia tubers, wherever you are, read this post:
Growing dahlias on over summer
Dahlias prefer fertile humus–rich soil with good drainage in full sun. They should be fed with a nitrogen rich fertiliser in early summer to promote bushy leaf growth.
Once the flower buds start to appear, in midsummer, they should be fed with a high potash/potassium fertiliser as this promotes flower growth.
One of the great attributes of dahlias is that they can flower for a long period between midsummer and the end of autumn. However, the key to extended flowering lies in deadheading, which is one of my favourite garden tasks. See this separate article on deadheading dahlias for some important deadheading tactics.
End of season dahlia care
Dahlias are borderline hardy. Flowers and foliage will be cut back by the first frosts. But the tuberous root system can survive winter temperatures down to around -5° C, if conditions are not too wet and a thick layer of mulch is applied.
Where winter temperatures are colder, or where otherwise desired, dahlia tubers should be lifted once the first frosts have hit.
The tubers should be cleaned up and, in the first instance, stored upside down so that water can drain away from the hollow stems. This lessens the risk of rotting of the tubers during storage.
Once the tubers have dried out, it is worth dusting them with an anti-fungal powder. They should then be stored in a dry growing medium or dry sand, preferably in a slatted wooden box to allow air to circulate, in a dry and dark place where temperatures don’t dip below freezing.
For a complete guide to how you can overwinter dahlia plants and tubers read this post.
Stored dahlia tubers can be replanted once the risk of frost has receded in spring.
Pests and diseases
Dahlias can be troubled by aphids, red spider mites, caterpillars, earwigs and slugs.
They can also be prone to powdery mildew and mosaic virus. Certainly in my experience, powdery mildew is the biggest potential problem and not one I’ve found very easy to deal with.
Conventional wisdom says that it occurs in hot and dry conditions where plants are tightly packed together. Therefore a good watering regime, mulching and wider spacing may help.
There is a bit of work in growing dahlias, but to my mind it is well worth it. Dahlias are large enough to add presence to a border or to stand alone as a featured plant. The multitude of different varieties offers countless different flower shapes and colours.
Best of all, dahlias provide us with a focus for the whole growing season. First, a sense of anticipation, as they gradually grow and bush out into their allotted space. Then, the grateful appreciation of their floral display and, finally, the careful attention we give to deadheading and pruning, as we seek to eke out their beauty for as long as the seasons will allow.
All the best nurseries and plant supplier will stock dahlias – either as tubers or as grown on plants.
Brickell C (ed), 1998, the Royal Horticultural Society A-Z Encyclopedia of Garden Plants Dorling Kindersley, London.
Buczacki, S., & Harris, K. M. (2005). Pests, Diseases & Disorders of Garden Plants (Collins Complete Photo Guides) Harper Collins UK.
Lloyd, C. (2001) The Well-tempered Garden Wiedenfeld and Nicholson, London
More on growing dahlias
You can also get much more guidance on growing dahlias in these posts:
- Taking care of dahlias: the trick with deadheading
- Dahlia pinnata: a dahlia original
- How to grow dahlias: the complete guide to dahlia care
- How to overwinter dahlia plants and tubers
- All you need to know about dahlia tubers and dahlia bulbs
- Dahlia varieties: your complete guide to all types of dahlias
- Dahlias in my garden: Six on Saturday
- Dahlia Wizard of Oz – Beautiful pink pompon dahlia
- Can you grow dahlias in raised beds?
- Do dahlias grow in Florida?
- Dahlia Wine Eyed Jill
- Can you grow dahlias in a tropical climate?
- Dahlia Ivanetti: magnificent magent dahlia
- Dahlia Night Silence – dusky pink dahlia beauty
- Visit a dahlia farm near you
- Best mulch for dahlias: a comprehensive guide
- Dahlia Islander: large, pink, loud, showy. What’s not to like?
- Heat tolerant dahlias: beat the heat with these 120 choice varieties
- When to plant dahlia tubers” solving the dahlia grower’s dilemma
Martin Cole has been an avid gardener for more than 20 years and loves to talk and write about gardening. In 2006 he was a finalist in the BBC Gardener of the Year competition. He is a member of the National dahlia Society.
He previously lived in London and Sydney, Australia, where he took a diploma course in Horticultural studies and is now based in North Berwick in Scotland. He founded GardeningStepbyStep.com in 2012. The website is aimed at everybody who has been bitten by the gardening bug and wants to know more.
Gardening Step by Step has been cited by Thompson and Morgan, the UK’s largest mail order plant retailer, as a website that publishes expert gardening content.
Check out my comprehensive step by step guide, with plain language explanations and ultra-useful images and illustrations. This is for you if you love dahlias and want to the best out of the dahlias you grow.