Dahlias really can't stand frost, so you can only plant them in the garden after the last night frost. That is usually beginning or mid-May (depending on the area where you live). To give the tubers a head start, you can already pull the tubers indoors in March/April, which means that you plant them in a pot (potting them up). By then the roots have grown nicely and the first shoots of the plant will already be visible when you plant them out in the garden after mid-May. The big advantage is that the dahlias will flower at least a couple of weeks earlier. Moreover, they are better protected against snails, which always love the youngest leaf shoots.
Step by step propagate or pot dahlias
1 Take a large plastic pot (approx. 20-30 cm in diameter) with holes in the bottom. It is better not to use a terracotta pot, as the potting soil dries out quickly in it. Fill the pot with potting compost to about 10 cm below the rim.
2 Depending on the size of the pot, place one or more dahlia bulbs on the potting soil with the old stem pointing upwards.
3 Fill the pot with potting soil and press it lightly, give a dash of water. Keep the soil slightly moist but never too wet, otherwise the tubers may rot. Place the pot in a light, frost-free spot in the house (e.g. windowsill or attic), in a greenhouse, shed or in a cold container. They grow best in a light place, at a temperature of 15 to 20 degrees.
4. If the dahlias grow too tall, move them to a lighter and/or colder spot. When the plant is 30-50 cm high, cut out the middle shoot to 1 cm above the 3rd leaf pair. The plant will then branch more widely into a sturdy, bushy plant that produces more flowers. You can, of course, also do this so-called 'topping' later when the plants are in the full ground.
4. By the time the risk of night frost has passed (in May) you can plant them out in the garden. Place the tubers in a sunny spot. Protect young shoots if there are still late night frosts. Dig a large hole, carefully remove the dahlia from the pot and plant out in the ground at the same depth. Cover with a few centimetres of soil.
Let the summer come and you will see that the dahlias bloom already in mid-summer. And they will continue to flower until the first night frost!
ps: Some tubers show their green sprouts faster than others, sometimes there is a difference of 5 weeks. So don't panic right away, but a bit of love & patience ;-)