Answer: Dahlias need warm temperatures, lots of sun, rich soil and plenty of moisture to grow well, and as yours grow you will probably find that your plants are too crowded. Luckily they are easy to transplant. Realistically you might be able to grow about 6 seed dahlias well in that size pot, in the garden they would be planted about ten inches apart. You will also find that they seem to find their stride once the weather is warm. Fertilize them with a fertilizer intended for blooming plants, be sure to water them regularly, and since it can take them a number of months to reach blooming size, have a little more patience!
When you remove faded blooms, take the entire flower including the "bump" at the base of the petals where seeds would form. To make the plant look tidier, I usually nip off the bare flower stem just above the top set of leaves, too.
Finally, dahlias normally branch and fill in on their own as they mature. That mounded look occurs as they blend together and each tries to maximize the light it receives. Good luck with your dahlias!
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