How to grow dahlias

purple dahlia

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 and thousands of different cultivars.

They 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 care and over-wintering.

Growing Dahlias

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, the 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.

Stored dahlia tubers can be replanted once the risk of frost has receded in spring.

Dahlia tubers

Dahlia tubers

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.

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.

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. See this separate article on deadheading dahlias for some important deadheading tactics.


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.

Dahlia mixed border

Summary

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.

Buy dahlias

For a great selection of dahlias, check out what is on offer at DirectGardening.com – you’ll find quality plants at great prices.


check the great prices at DirectGardening.com

References

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

Comments

  1. Thank you for the info, very helpful! Dahlias have always been one of my favorite flowers, this is the first year I have planted them. Happy growing!

    • Thanks Chelle. I hope you enjoy growing dahlias this year. Good staking and frequent deadheading go a long way to maximising their display.

  2. Thank you for the clear instructions… Took the page for deadheading into garden with me so I could see exactly what to do. Can I use the deadheaded buds to grow more dahlias? ( I am very new to gardening!)

    • Sorry Leigh, I neglected to respond to your question.
      The answer is that you can’t use the buds you’ve cut off.
      When you dead head, the idea is to remove the spent flower before any seeds set. Because the plant’s evolutionary purpose is to create seeds in order to maintain its species in existence, the plant is then encouraged to create more flowers and, if left to itself, seeds. As gardeners we interrupt this sequence between the end of flowering and the setting of seeds so that we get the flowers that we desire, but the plant doesn’t get the seeds it desires.

      Hope this helps. All the best.
      Martin

  3. susan oliver says:

    Hello
    Thanks very much for your extremely useful tip on how to differentiate between a spent dahlia flower bud and a bud yet to open. The deadheading task s so much easier now. I appreciate your expertise and the generosity with which you share it.

  4. I am a new dahlia grower. I just wanted to know if anyone had any pointers, tips, or suggestion for me. Thanks in advance

  5. This is my first in growing dahlias. I bought them in a bag at a box store. I didn’t plant them til the new stem was growing well. When I pulled the bulbs out of the bag there were cattail like things hanging from the main bulb. Are these cattail things also bulbs? Do plants grow from each one? I planted 5 bulbs in 1 container. I didn’t know they would grow so big. They are beautiful! The leaves are turning yellow around the outside of the leaves. It’s not an all over yellow, yet. I know I have something eating on other plants. I am going to use a homemade insecticidal soap made from unscented castile soap. It’s not supposed to kill everything. I saw the info on deadheading which is where I started. Thanks for the info. Sorry this is so long. Any help would be appreciated. Peggy

    • Hi Peggy, sounds like you’re on the right track. The cat-tail things are probably the tubers. Dahlia’s don’t have one bulb they have a number of tubers. The yellowing might mean they need a feed. But as the plants grow, some leaves do die off. You just need to remove them. Best of luck

  6. hi again!
    you mentioned that dahlias are prone to powdery mildew and mosaic virus.
    any tips on what might help with that?

    thank you

    • Hi Sandra
      For powdery mildew – try to maintain air circulation around the plants. Water around the roots, not from above. Don’t overfertilise them. You can also try spraying with a solution of one part milk, two to three parts water. If you want to go non-organic, you can use a fungicide.

      Mosaic virus is fatal. You need to remove infected plants and destroy them in case they infect other plants.

  7. Hi Martin, Thanks for this very usefull site and I have learnt a lot about my Dhalias. I have been growing Dhalias in pots for a few years but this year the plants have lots of healthy growth but no or very few flowers! Very disappointing. Do you have any ideas of why?
    I always mix the potting compost with my garden compost as Dhalias are hungry plants. We feed and water regularly and they are in a sunny spot. The tubers are kept in the garage over winter. I wonder if they are a little old? Probaly 5/6 years or older.
    Any help gratefully received as I love Dhalias!
    Thanks, Fran

    I

    • Hi Fran
      It could be you’re feeding it with too much nitrogen. Nitrogen promotes leafy growth. The received wisdom is that potassium promotes flower growth, but there’s some debate about that: see here . I suspect the answer is a balanced feed, though there are some theories about stressing the plant to make it flower (e.g. here). It’s true that this can promote flowering, though I’m not sure how good it is for the plant in the long run.

  8. Norman Sigel says:

    Does one cut some of the new growth at the very base of the new Dahlia?

  9. Jennifer Bozarth says:

    Growing dahlias for the first time and loving them. I was having some issues and came to read up on the answers. Thank you so much for being so helpful and informative. I hope for many months of beautiful blooms to come.

  10. Linda Lathan says:

    Good morning, Martin–I have been disappointed for a number of years with no success planting Dahlias. I am thinking that I got old bulbs at the store, and/or they were not planted in the best spots. I started a new border that receives full sun this spring, and bought my bulbs at a couple of different local stores, and this year I am so excited to say they are very lovely and healthy. I did, however, find my biggest and strongest plant lying down on the ground after one of the few rains we have had this summer. (I thought I had it stakes well!) It has been very hot and dry, but I religiously watered and fed my babies. I live in NY, and my Dahlia foilage is beautiful and healthy and there are lots of buds, but they take forever to open. The anticipation is special with this type of flower, but they bloom so late, that I am now worried about the first frost! In any event, these gorgeous plants have added a whole new dimension to my summer, and provide color when my other perennial blooms have pretty much faded. I find Hibiscus is fun and keeps vibrant late summer also. I spot a beetle in my Dahlias here and there but have hung a bag to help that problem. Anyway, I am hopeful that I can store them successfully over the winter and enjoy them again next summer. I have gleaned so much information from your site, and I thank you for all of your excellent advice. I am wondering if buying new bulbs from a catalog might be a good idea. I absolutely love true red, but find that my bulbs seldom are the color shown on the packaging picture, and what I though was red turned out to be dark pink or purple! Wow–Sorry to drone on so.

    Thanks so much! Linda in NY

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