Dahlias, like most plants, are most likely to thrive if their unique characteristics are taken into account during planting. Fortunately, these flowers are not particularly finicky, so a few simple tips are all that will be necessary to succeed. The key factors to take into account when planting dahlias are the air and ground temperature, the characteristics of the soil, the availability of light and water, and garden spacing.
Before planting can begin, it is vital that any possibility of the ground freezing over has passed. In the moderate climes of the northern hemisphere, for example, the month of May is a safe bet. More important than the date, however, are the current weather conditions. A frost can kill the bulbs before they even get a chance to sprout—much less bloom—so it is very important to hold off planting dahlias until the ground reaches a temperature of at least 55 degrees.
When the ground temperature is warm enough and planting is imminent, it is time to consider the type of soil that is available. Dahlias will not necessarily die in soil that is less than ideal, but they are certainly more likely to thrive in soil that suits their needs. The best soil for planting dahlias is generally somewhat light and sandy, which helps facilitate drainage and prevents water from pooling around the plants. Dense soil can be mixed with some sand, lighter potting soil mix, or peat moss to help lighten it. Dahlias also prefer soil mixtures that are slightly acidic; soil testing kits can be purchased at most hardware stores, and will help determine acidity.
Before planting dahlias, it is wise to consider how much light and water will be available to the plants as they develop. Brighter locations are best for dahlias, as they generally need several hours a day of full sunlight. Asses the availability of water as well; if the weather does not allow for enough natural rainfall it will be necessary to manually water the dahlia plants because dahlias need long, thorough watering a few times a week. There is one exception to this: planting dahlias as tubers requires soil and garden conditions that are somewhat dry, because over-watering the bulbs before they sprout can cause them to rot.
Spacing the tubers depends somewhat on whether the mature plants will be encouraged to grow tall or wide. Typically, it is a good idea to leave at least one foot of empty space between plants to provide ample room for spreading as they grow. If the dahlias will be particularly large and wide, it is preferable to leave more space between the plants to prevent overcrowding.