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Cut a stem at least 4-5 inches long that doesn't have any flowers or buds on it. Trim off the base leaves. Trim half of the top leaf off. A clear, glass container is best, as it allows you to see the roots once they start to form.

Summary:
Prepare your hydrangea cutting by removing excess leaves from the stem. Set cutting in a vase or glass filled with water. Wait for roots to appear. Change the water in the vase frequently to prevent molding. Plant the cutting once roots appear.