Dahlias have become the rockstars of the flower world with a cult following for some specialist breeders and it’s not hard to see why. They are relatively easy to grow but do require a little attention in the beginning.
Frost protection early in the season is key. I lost around two thirds of our tubers a few years ago due to me forgetting to bring them in on a frosty night at the beginning of the season. Luckily we managed to salvage enough to start again and it also gave us the chance to buy a few new varieties to trial.
I generally take our tubers out of storage in early springtime (UK - May). Some varieties take much longer to get going and you can usually start to see signs of bud growth as soon as the weather warms up a little - variety dependant.
Once out of storage we pot them up into shallow trays of compost with the crown just above the soil level to get them going. We keep them warm and slightly moist to encourage growth. These trays are kept in a greenhouse and protected from frost. You can use a little bottom heat to get some of the more stubborn ones started if needed and once they show signs of growth they are potted on individually into large pots and generally kept indoors until the risk of frost has passed. It's at this stage that you can also try your hand at propagation.
Taking Cuttings
Dahlias can be propagated in spring by taking cuttings. These cuttings will often be much more vigorous than the plants formed from a tuber and you may be able to get 10 cuttings from just one large tuber Simply pot them into shallow trays as above. When your Dahlia tuber has sent up a handful of stems and they’re about three inches tall, cut some of these shoots off where they meet the tuber’s crown. If you can get a little bit of tuber to come off with the cutting all the better as these will have a higher success rate.
You will then need to put the cut stem into a medium for it to grow. You can use seed compost, mixed with sand, Root Riot cubes or neat perlite. The medium needs to have an open structure to create the right air and moisture levels and support rapid root development. I have often found very low success rates with compost mixes just because they are either too wet and rot, or too dry and they wilt so I prefer neat pearlite or Root Riot cubes. Each grower will have their own way with success and its good to experiment to see what works for you.
Snip off the lowest set of leaves, dip into hormone rooting powder and pop the cutting into the edge of the pot so that the roots grow touching the pot’s sides you will also increase the success rate as it will encourage budding at the roots. You can put several cutting around teh edge of the pot but if you are using cubes then just one per cell.
To conserve moisture seal in a plastic bag, but ensure that you open the bag every few days to release any build up of moisture. It will only take a few weeks for the cuttings to root at which point you should remove the bag. You can then pot them on to grow into bigger plants before plating them out in the garden. Planting then too soon will mean they are susceptible to pests such as slugs. Plant out when all risk of frost has passed.
Cuttings can be tricky and if you keep them too wet or too dry then your success rate will be low. It is actually far easier to propagate by division, especially for a beginner.
Dividing Dahlias
Dahlias can be propagated before planting time by dividing the tubers. It is important to know the different parts of the tuber and know what they are for to be able to divide them. Each tuber clump is actually made up of several individual tubers with roots coming from them and a crown at the top where they all meet. Dahlia stems will grow from buds or eyes on the crown.
Using a clean sharp knife separate the tuber into pieces. Each piece must include at least one swollen, dangling section and a piece of the main stem with at least one eye. Be sure to include at least one eye in each division otherwise the tuber will not have any way of sprouting.
If you are having trouble identifying the eyes then you can wait until they begin to sprout new growth before you divide the tuber. Plant the divisions in individual pots with the eyes at the top and the dangling swollen part at the bottom, keep them warm and slightly moist. They need to be kept frost free They will soon begin to sprout
You will also need to look out for any form of disease at this early stage. Crown gall is a common problem in Dahlias. It is caused by bacterium Rhodococcus fascians, which enters the plant through wounds in roots or stems and stimulates the plant tissues to grow in a disorganised way, producing dense clusters of distorted, leafy shoots. If you find any of your dahlias displaying signs of gall then they will need to be destroyed to prevent further infection of other plants.
To learn more about storing Dahlias over winter check out our page Storing Dahlias.
Collecting & sowing Seeds
As Dahlias have an octoploid tendency they have 8 sets of chromosomes !! This allows for large variants from seed. So do not expect plants grown from seed to be identical to the parent plant as they will have cross pollinated.
To collect open pollinated seeds wait until the pod has dropped all of its ray petals, the seed pod will be brownish green and the seeds inside should be a grey/dark brown colour. Cut off the pod and let it dry before separating out the seed. Allow them to dry thoroughly for a few days before storing ready for sowing the following year.
Seed can also be hand pollinated but is very much an expert sport, with amateur and professional breeders all across the globe mixing genetics to create the most beautiful new variants. To learn the basics of Dahlias breeding check out our guide to Breeding Dahlias.
To germinate seeds sow thinly on the surface of a well drained seed compost and cover with a light sprinkling of compost. Water lightly and keep damp at around 21 degrees. Pot the seedlings on into individual pots once they have their true leaves and harden off, plat out when all danger of frost has passed. Do not be tempted to sow the seeds too early as they detest cold weather and should not be planted out too soon, as young plants they are much loved by slugs so may need some protection.
These small seedlings will have grown small tubers by the end of the summer so they will need to be lifted and stored over winter. The following spring they can then be treated like tubers.
Caring For Dahlias
Whether sowing seeds, dividing or propagating from cuttings it is important to encourage the plant to make more branches and promote a bushy shape to give more blooms. So once plants have formed several sets of leaves, pinch them back by simply sniping or pinching the stem off just above the second pair of leaves. This will encourage the pant to form new stems from the node just below.
If more than five shoots sprout from the tuber remove them so that only five remain. It may seem counterintuitive but in the long run this will actually result in more flowers than if you had allowed additional stems to continue growing. It will also improve air circulation through the mature plant.
Feeding Dahlias
Adding organic matter such as home made compost, farmyard manure, and chicken manure to the soil before planting is the biggest key to success. Once plants are a good height and forming buds avoid anything too high in nitrogen as it will encourage leaf growth at the expense of flowers.
A liquid plant food which is high in Phosphorus and Potassium and low in Nitrogen is best used as a foliar feed, sprayed on to the leaves allows the plant to take in nutrients quickly, but can also be used in watering. Then towards the end of the growing season a feed of a high potassium fertiliser will help the tubers plump up and ripen, improving their prospect of keeping well during their dormancy.
Harvesting Dahlias
Dahlias do not have a very long vasr life, especially the dinnerplate varieties. Which at best will manage 3 days. Small pom pom varieties will last upto 5 days if cut at the right stage.
Cutting in the early morning or evening is best. Select blooms which have only just opened as these will give the longest vase life. If you are unsure then checking the back of the petails for signs of ageing is a good trick.
Ensure that you cut deep into the stems just above a pair of leaves. This will encourage the plant to send out long stems and they will form from the leaf joint just below where you have cut.
Plunge immediately into room temp water and place then into a cool dark place for a few hours to condition.
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