The tubers of the dahlias must be dug up before winter and overwintered frost-free. From spring onwards they are planted back into the garden to reveal their true beauty again. However, since the summer bloomers cannot tolerate icy temperatures, you have to wait until winter is actually over. Only in permanently frost-free soil can dahlias sprout without any problems and quickly develop into stately plants during the spring.
Location
A sunny location offers ideal conditions for dahlias. The soil should be sandy and slightly acidic. Clay soils are not recommended for dahlias as there is a risk of waterlogging.
Ideal planting time
If you want to plant dahlias, you should first decide whether they are tubers, young plants or cuttings. Tubers are usually planted directly in the garden bed. Alternatively, it is also possible to place them in a bucket or cold frame. In all cases the planting time is different. Dahlias should not be watered after planting unless the soil is very dry or when planting very late.
Planting tubers outdoors
After the rest period in winter, dahlias can be planted in the bed from the end of April to the beginning of May. It is important that the correct planting depth is maintained and that the soil is somewhat dry (do not plant in wet soil). If the probability of severe frosts is low, dahlias can go into the bed a little earlier. But quite apart from the spring frosts, there is also another reason against planting dahlias too early: in cold soil (especially when it is wet) sprouting is delayed, so that the tuber and pre-formed eyes suffer. Dahlias planted later have a better start and grow faster.
As a rule, the frosts that occur from the end of April are only short-term and superficial. Therefore, the tubers, which are five centimeters deep in the ground, do not need any additional protection. If you want to be on the safe side, you can place an upside down flower pot over the planting site and secure it with a few leaves all around. If the dahlias sprout, the pot must be removed as quickly as possible.
Tip:
Very small dahlia tubers should first be planted in pots and allowed to develop in the greenhouse (or a cool, bright room in the house). Then plant outdoors from mid-May.
Planting cuttings outdoors
Cuttings may only be planted outdoors after the Ice Saints, when there is practically no risk of frost. Since the cuttings initially have no tuber but only fine roots, they would not survive the freezing temperatures.
Planting in the cold frame or greenhouse
A greenhouse protects the freshly planted dahlias to a certain extent from cool temperatures. If the outside temperatures are only around freezing point to slightly below zero from mid to late March, they can be planted in a pot and planted in the ground in a cold frame if they are intended to be planted out in another bed later. Transplanting dahlias that have already spread their roots into the ground is difficult because they are often damaged when they are removed. If you want the dahlias to spend the whole summer in the cold frame, they can be planted directly in the ground.
Tip:
Soil fatigue rarely occurs with dahlias. Therefore, they can be planted on the same beds over and over again for years - provided that the soil is worked properly and there is enough fertilizer.
Advance
In order to give the dahlias a good head start, many gardeners plant them in flower pots early in the year and place them bright and cool but frost-free (garage window, cellar window, hallway or greenhouse) until the Ice Saints arrive. In this case, the tubers can be planted in flower pots or containers from the end of February to mid-March. It is very important that the dahlias are bright.
Planting depth
Larger dahlia tubers are inserted about a spade deep into the soil so that they are only covered with a few centimeters of soil. If they are planted too deeply, their shoots have to laboriously struggle to reach the light. If the layer of soil covering the tuber is too thin, it will not grow well and will not be protected against light late frosts. The rule is: Cover with about a hand's breadth of loose soil.
Planting spacing
If the dahlias are planted in their final location, it is important to maintain the correct planting distance so that the plants can develop well and do not compete with each other for light and soil.
- very tall varieties: 70 to 100 cm
- medium height varieties (pompom, ruff and ball dahlias): 60 to 70 cm
- low varieties (mignon, bed dahlias): 30 to 50 cm
Put tubers in water before planting?
Opinions differ on this question. The fact is that the tubers are often very dry after the wintering phase. To grow better, many gardeners recommend placing the tubers in room temperature water for a few hours or even days. Nothing contradicts this philosophy, as the weather at the end of April is relatively warm and rainy. However, if the garden soil is very moist - which is often the case in April - this procedure is not recommended. In the cold April soil there is a very high risk of rotting for the tuber.
Divide tubers
If the dahlias are older and the tubers are relatively large, they should be divided. Strong gardeners can break them apart by hand; it is easier with a knife. The interfaces do not need any special treatment. However, it makes sense to let them dry out slightly before putting them in the moist soil. Division is necessary because too many tuber parts interfere with each other when new, young tubers are formed. The shoots are then often weak and small. The tubers should only be divided before planting, otherwise they will dry out during the long overwintering phase. At least one eye must be visible on each root neck during division.
Tip:
If in doubt, the dahlia tubers can simply be placed in the light and warmth for a few days. Then they start to drift slowly.
Conclusion
While cuttings and early plants may only be planted in the bed from mid-May after the Ice Saints, well-developed tubers can be planted in the garden soil between the end of April and the beginning of May. To give the dahlias a head start, it is possible to grow them indoors or in the greenhouse from the end of February. Planting dahlias in the cold frame is recommended from the end of March, as long as there are no permanent frosts in the double-digit minus range.
