Answer: It helps to know the mature size of the plants you choose so you can space them accordingly. For instance, in a 14-18" container for a shady area, plant a tall growing large-leafed Coleus in the center, plant 3 Celosia around the Coleus, and plant impatiens along the sides. If desired, plant red or pink lobelia interspersed at the edges so that it cascades down the sides of the container.
For a sunny container, plant a group of three cosmos together at the back of the container, a few inches apart (chose the same or complimentary colors). Plant two or three California Poppy in front of the cosmos, to either side. Plant three Petunias at the front and side edges of the container to trail down the sides.
Or, intermix Salvia and Zinnia at the center of a large container, surrounded by mounding and trailing Nasturtium. Train the trailing Nasturtium branches over the sides as the plants grow. Trim as necessary to keep looking neat.
These are just a few suggestions. You can find more by doing a web search on container design. Enjoy!
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