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Hi, Liz, Yes, you should
Hi, Liz, Yes, you should stake it. Insert a cane and and tie it in as it grows. And you are correct: You need to pinch out the growing tips when your dahlias get to a height of 16 inches or so to encourage branching. Deadhead your blooms as they die off. According to the experts at RHS, "For giant blooms, restrict the number of flowering stems to three to five per plant; for smaller blooms allow seven to 10 flowering stems per plant. To produce a long-flowering display and strong stems, remove the two pairs of flower buds developing in the leaf axils below the terminal bud."