Answer: The dahlias should grow about 22 inches wide, so when planting two dahlias next to each other you would space them 22 inches apart so they just barely meet when full grown. You measure from the center of one plant to the center of the next plant. When you plant something else next to the dahlia, allow 11 inches of width on that side for the dahlia, plus half the expected width of the neighboring plant. You can plant a little farther or a little closer, depending on how tightly spaced/spread out you like your plants to be. I hope this helps.
Q&A Library Searching Tips