How to be successful with dahlias
Read this guide on how to be successful with the beautiful summer flowers.
Dahlias are beautiful and beloved flowers. The wonderful plant also goes by the name Georgina variabilis, but the correct Latin name is Dahlia. There is a small dispute about who first found the dahlia, or more correctly, who first named it. We from western Europe maintain the notion that the plant is named after a well-known botanist named Anders Dahl, who was a student of the equally famous Linne, who first named the Dahlia. But if we must be absolutely correct, it was a female CEO from the botanical garden in Madrid, who gave Anders Dahl a plant that she was the first person in Europe to have grown from a seed. She named it Dahlia after her good friend Dahl. The name Georgine originates from a German-Russian professor named Georgi.
The plant is originally from Mexico, Guatemala, and Colombia, and there exist over twenty wild species that have been cultivated into countless numbers of variants, types, and sorts. Some almost have a spherical form, and others are simpler with traditional petals, others look like a spiky cactus. The Dahlia roots have a number of qualities and have been used for both food and natural medicine. To this day, it is still possible to buy natural medicine from Mexico based on Dahlia.
How to be successful with Dahlias
You can place the roots in a bed in May. But if you jump the gun and place them in a pot in your greenhouse or another warm place, the Dahlias might bloom earlier. They can also grow in a pot or a large jar if you don’t want to plant them in the ground.
Soil and placement
Use good soil filled with nutrients, preferably with compost and manure. Place the roots in a sunny spot. They need at least six hours of sun to develop correctly, and they will also get more flowers in a sunny spot.
Planting depth and planting time
You can start the pre-sprouting process and transplanting when there is no risk of frost. The temperature of the soil must be above 12 °C; this is usually the case in the end of April. However, if frost still happens, cover the soil with isolating materials. Dahlias do not thrive when planted too early. The hole must be a little bigger than the tuber. Place the tuber in the hole making sure that the shoots are pointing upwards and cover with about 5cm of soil. When you see shoots appearing, cover them with additional soil until the hole is completely filled. Plant the tubers at a distance of approximately 40cm. If you put them too closely, they will easily cover each other with shade.
Fertilizer and water
Dahlias thrive with liquid fertilizer. Fertilize the first time after sprouting and then every 3-4 weeks from the middle of summer to early fall. Be careful not to over-fertilize, especially with high levels of nitrogen. Too much fertilizer often means little or no flowers and weak or rotten roots. There is no reason to water the soil before the plants rise above ground. Overwatering can also be the cause of rotten roots.
Other care tips for Dahlias
Pinch off the heads when they are about to wither, and new flowers will develop.
Preparing for winter
At the first sign of night frost, dig up the Dahlias. Alternatively, it might be an advantage to dig up the flowers before the frost appears. The later you dig them up, the more the flower will wither, and then it might become difficult to see what colour it used to be. You can also set up small signs to keep track of the plants and their colours. However, if there is frost, it will also be harder to dig them up. Dig them up with the flower and stem and cut them down to about 10cm after placing them in the box you want to store them in. Remember to add the signs, to keep track of the colours. If you cut them down first and then dig them up, it can easily become quite a mess.
The storage should be frost-free but cool. If you don’t have the opportunity to store them frost-free, place them in the coldest room you have. Rather a bit too warm than the risk of frost. Dahlias don’t mind laying in the dark, so packing them up in some insulating material is also an option. Make sure the roots get air and check on them from time to time. If they start to shrink, give them a little water. Any possible watering usually happens around January or February. Some store them in boxes with a little dried sphagnum. The dried sphagnum will, to some degree, protect against evaporation.
Planting in pots
Don’t pick the tallest sorts; they will easily tip over in a pot. Up to a meter is fine though. Choose a large pot and remember to water often but let the ground dry out between watering.
If you have Dahlias in your garden or know someone who does, it is possible to make your own Dahlias. Just sow the seeds from the flower. It is important to let the flower sit until the seeds are ripe. The seeds will not develop as the mother plant but will become slightly different in colour. If you want a completely identical plant, use one of the roots from the mother plant or buy seeds from a nursery. Sow the seeds in boxes inside in mid-March. Cover them with about 1cm of soil. The seeds germinate quickly and must stand in the light. Give them fertilizer from late April. If you have the opportunity to place them in a greenhouse, do so, but watch out for frost. You can also plant them in a bigger pot on the way. From June 1st, it is okay to start transplanting.
Dividing the roots
Roots can be divided in spring or fall. It is easier in the spring because the small black shoots are visible. Each root should consist of a tuber and a neck, and a crown. The roots should not be divided during the first couple of years, but when the root has become larger. The larger it gets, the more the blooming will suffer. You can also make cuttings. Cultivate the tuber in February and take a cutting when there is a 10cm shoot. Cut off the shoot right by the base.
Disease and Vermin
- Killer snails are the Dahlia's worst nightmare. In some cases, it helps to remove the bottom leaves. The stem is bitter, and the snails do not like that. Sheep wool around the stem also helps to keep the snails away.
- Earwigs also like the flower heads. Place some moist corrugated cardboard on the ground, that they will crawl into during the day and then remove the cardboard.
- Thrips (small insects) and plant lice might be a problem at the start of the season. Use a lice insecticide.
- Mosaic disease is a virus that discolours the leaves or makes them curl up. It can’t be cured.
- Mildew can occur if the plants don't get enough air around them. Mildew is a bacterium that forms small cauliflower-like shoots right above ground. Cut the plant all the way down to avoid infecting the other plants.
In 1964, the RHS divided Dahlia into eleven groups based on the shape of the flower. In every group, there are colourful flowers. The eleven groups are:
- Single-flowered Dahlias: The flower has a single row of eight petals, which may overlap. The centre is often yellow. (Height approx. 1m)
- Anemone-flowered Dahlias: The flowers often look a bit like stuffed anemones. (Height approx. 75cm)
- Collerette Dahlias: The flowers have a single row of eight petals, like the wild species. The centre is often yellow and framed by smaller petals as a collar. (Height approx. 1m)
- Waterlily Dahlias: Constitute a smaller group. The flowers have two rows of petals with a yellow centre. Similar to the collarette Dahlias.
- Decorative Dahlias: The flower has flat but involute petals and can be found in many colours.
- Ball Dahlias: The flower has involute petals and has a round appearance. However, the top can be a little flat. The flower head can have a size of approx. 10-15cm.
- Pompon Dahlias: The flowers are firm, filled, and round; almost as round as a pompon. The flower can have one or two colours.
- Cactus Dahlias: big flowers with long pointy petals. Most sorts are uniform. (Height up to 1,5m)
- Semi-cactus Dahlias: Looks a lot like the cactus Dahlias; however, the petals are a little wider and thicker.
- Miscellaneous Dahlias: Small plants that can vary between 20-50cm and with only 2-3cm big flowers.
- Fimbriated Dahlias: Sorts you can't really categorize. The petals can have many shapes and forms.
Om Lars Lund
Danish horticulturist and journalist
Lars Lund has for many years engaged in the garden and greenhouse. Lars has published many books about greenhouses, and he has participated in many Danish horticultural TV shows. He is a walking garden encyclopaedia, and he has answers for most basic cultivation questions – also the more ambitious ones.
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