Even though delphinium plants are more suited to being planted in gardens, it is not impossible to grow them in containers. Perennials usually prefer open spaces because their root systems grow shallow and wide rather than deep. If the plant is adequately cared for and divided as soon as it gets too big, the delphinium can thrive in a container as well.
Step 1 - Prepare the Container
The container must be large enough to permit root development and retain moisture. Clean the container and cover the drain hole, either with the wire mesh or fine gravel, for drainage. Add a few inches of moist soil to cover the bottom of the container.
Step 2 - Add Potting Mix
Add the potting mix to the container and mold it to the sides to create a space for the plant. Use soil-less potting mix or use 1 part peat, 1 part Perlite, and 1 part organic soil for a homemade mix. The mix must be able to hold water and nutrients, but also provide good drainage.
Step 3 - Remove the Delphinium from its Container
Invert the nursery container and gently ease the delphinium out. Do not use tools to loosen the plant because this may damage the sensitive root ball. Loosen the root ball of any roots entangled around the plant and any matting. Note that the best time to transplant a delphinium is in early spring when it is dormant.
Step 4 - Plant
Place the delphinium in the container with the top of the roots just above the soil level and below the rim of the container. Fill the container with the remaining potting mix and gently press down to stabilize the plant.
Step 5 - Stabilize
Insert three stakes, forming a triangular shape, to provide support for the plant. Delphiniums tend to achieve considerable height and will require the support the stakes provide. Choose bamboo stakes covered in green plastic to make them as unobtrusive as possible.
Step 6 - Water and Fertilize
Water the plant thoroughly and apply liquid fertilizer. A 12:10:10 mix is ideal. Delphiniums are very hungry plants, so they must be fed approximately every 2 weeks. If the weather is hot and dry, they will require daily watering, but don't go overboard or the roots may rot.
Step 7 - Position
The delphinium will need access to direct sunlight for at least four to five hours daily, so position the plant near a window that allows in plenty of light. If you plan to place them on a porch or patio outdoors, the delphiniums will have to winter indoors because the container does not provide sufficient protection for the roots against the cold weather, which will inevitably lead to root damage. The container can also be insulated against the cold weather with bubble wrap if the plant cannot be moved indoors.
Delphiniums are beautiful annuals, and with proper care, can last up to five years in their containers. As with garden planting, the delphiniums will need to be divided every three years, maybe even sooner depending on how quickly they grow. You could also transplant them into larger containers.