Guide to Growing Dahlias
Updated: Mar 20, 2021
Dahlias are easily my favourite flower of all time but they do tend swing back and forth on the fashion pendulum. Back in the 1800's they were known as Georginas and were frightfully expensive. They were a real status symbol costing one hundred pounds a tuber. Wealthy families would plant large borders full of them and invite guests round for afternoon tea to show them off. By the 1950's they had come down in price and in the drab era of brown and beige lost their attraction. They were considered showy and vulgar and many of them were lost.
My mother had oodles of red pompom types in her garden when I was young (Which I actually detested due to my aversion to the colour red even from a young age) but nowadays there are so many varieties available to choose from that they do not need to be dramatic in a vulgar way. Café au Lait for instance is just a delight, creamy white luscious petals which work so well in tonal arrangements with peach and burgundy. Even just having one of them in a bouquet gives an aura of sophistication.
Dahlias are at their best in the early late summer & early autumn, in the UK this is August/ September time. They originate from equatorial regions such as Mexico, Columbia and Central America, where the days and nights are equal lengths. So the autumn equinox which falls in late September is the day on which they should be at their peak. In reality many regions in the UK will have already had a mild frost in some years. For our plot in Hampshire, in 2018 this was Sep 25th and in 2019 it was Sep 7th !!!!! eeeek!!!! but in 2020 we had a bumper year and it was so late at the end of October that I can't even remember the date. Frost protection in their peak month is the key to keeping them blooming for as long as possible.. Some varieties are also a little more tolerant than others. I have found Arabian night to be the most tolerant of colder temperatures (2-3 degrees) where all my others varieties will not go down below 3 degrees before they bigin to complain.
Frost protection early in the season is also key. We 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.
We generally take our tubers out of storage in early 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. Cafe au Lait being the slowest to get going and often not flowering until September!
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 forst has passed. It at this stage that you can also try your hand at propagation.
Dahlias can be propagated in spring by taking cuttings. These cuttings will often be much more vigarous than the plants formed from a tuber and you may be able to get 10-20 cuttings from just one large tuber - and they will flower in their first year. 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.
Snip off the lowest set of leaves and pop the cutting into a pot of light gritty potting compost. By placing it at 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 also use some hormone rooting powder on the tips to encourage root growth)
To conserve moisture seal it in a plastic bag, but ensure that you turn 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. 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.
Division Dahlias can be propagated at 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 is actually made up of several individual tubers with roots comeing 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.
If you have bought new Dahlias the beware as they will have already been divided by the farm which grew them ready for sale on to you, so it is unlikely that you will be buying a very large tuiber which has lots of eyes. Chances are that it will only have a few tubers and one or two eyes. You may be able to get a few cuttings from it in year 1 but you will need to wait at least another year to be able to divide it.
Collecting and 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.
Wait until the pod has dropped all of its ray petals, the 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 the seed from the rest of the pod. Allow the seeds to dry thoroughly before storing ready for sowing the following year.
In early spring to germinate the 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 in the garden until June.
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.
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 after the shoots appear or the cuttings have taken and formed several sets of leaves, pinch them back by simply sniping or pinching the stem off just above a lower 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.
Dahlias can be planted directly into the garden when the danger of frost has passed.