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Divide phlox while it still looks healthy. The most common mistake home gardeners make when growing phlox is waiting until the plant shows signs of failing or over-crowding before they divide it. As a rule of thumb, allow the diameter of the plant cluster to grow to the same width as the plant's height before dividing it. Divide the phlox sooner if the plant becomes over-crowded, or if the center of the plant fails to bloom and produces less foliage. Divide phlox in early spring, late summer or early fall. Phlox division is most successful when done in the early spring before new growth begins, or in the late summer or early fall when flowering is done. If you want to leave a piece of the plant in its original location, you can plant 1 of the divisions back into the original spot later. Dig a trench around the outside of the entire plant cluster, severing the plant's roots with the blade of your garden shovel as you go. Chop the roots under the plant by circling back around the trench again with your shovel. Push the shovel blade into the ground, angling it to reach under the plant's root base. Push down on the handle to lift the roots. Continue around the phlox until the cluster lifts from the ground. This gives you a better look at the roots and plant crowns. The crowns are visible at the top of the root base. Make sure each new division has at least 1 or 2 healthy plant crowns and an ample root base to sustain the plant. There are several ways to divide the cluster: Break pieces off from the outside of the cluster using your fingers to separate the roots and your hands to snap the plant crowns apart. Keep breaking divisions off until the entire plant has been divided. Use a serrated kitchen knife to cut sections off the cluster you are unable to divide the plant by hand.
Determine when to divide phlox. Remove the entire cluster of phlox from the ground. Wash the soil from the roots with a garden hose. Make multiple small divisions from the cluster. Cut the cluster into 2 to 4 smaller clusters by chopping it with the garden shovel.