Can You Grow Sunflowers in Containers?
As mentioned, dwarf varieties, those under 4 feet (1 m.) in height, lend themselves very well as container grown sunflowers. If you want to grow the really impressive 10 footers (3 m.), which is still doable, a larger container will be required.
About Potted Sunflowers
The size of the sunflower will dictate the size of the pot. Smaller varieties will do well grown as sunflowers in planters. Cultivars that grow to 2 feet (61 cm.) or less should be planted in a 10 to 12 inch (25-31 cm.) diameter planter while those that grow 4 feet (1 m.) or taller require a larger 3 to 5 gallon (11-19 L.) or even larger pot.
How to Grow Sunflowers in a Pot
Regardless of the variety, all sunflowers grown in containers should have drainage holes and be situated in an area that receives full sun. Sunflowers need well-draining soil that retains moisture. A good quality general purpose potting soil will work well. For larger pots, mix the potting medium with some vermiculite to lighten the weight of the pots. Add a layer of drainage material such as gravel, terracotta pot pieces, or polystyrene foam to the bottom of the pot and then add the potting medium, filling the container to about halfway. Plant the sunflower and fill in around the roots with additional soil, then water well. Be sure to keep an eye on the watering needs of sunflowers grown in containers. They will dry out more rapidly than those grown in the garden. A general rule of thumb is to provide an inch (2.5 cm.) of water per week depending upon weather conditions. Water the plants when the top inch (2.5 cm.) of soil feels dry to the touch. Fertilize the flowers with a high-nitrogen liquid plant fertilizer and then when a bloom begins to form, switch to a liquid fertilizer high in phosphorous.
