An upright herbaceous perennial widely used in flower borders, the garden phlox (Phlox paniculata) bears its dome-like flower clusters in midsummer to early fall. Prospering in a moist, fertile soil in a sunny location, garden phlox is native to the eastern United States. This species of phlox physically looks much different that other types of phlox that are more prostrate or cushion-like in form, such as the woodland phlox (Phlox divaricata), annual phlox (Phlox drummondii) or creeping phlox (Phlox subulata). All phlox species do share a common flower structure.
Emerging in spring, the upright stems of the garden phlox elongate and mature to a height between 3 and 4 feet tall. While many smooth stems arise from the fibrous root clump, they collectively create a plant that is 24 to 40 inches wide. Some varieties of phlox developed by horticulturists are smaller in size, perhaps growing only 18 to 24 inches tall and equally as wide.
The leaves are long and narrow, a blend of an oval shape and lance. They measure between 4 and 7 inches in length and no wider than 1 1/2 inches. The medium to dark green leaf blade has a prominent midvein. Leaves occur in opposite pairs on the smooth stems. It isn't unusual for some leaves in the pair to be missing, making the leaf arrangement look more alternating.
Anytime in midsummer the stem tips bear a cluster of hundreds of blossoms, collectively described as a cyme. An individual flower measures about 1/2 to 1 inch across; it is tube-necked with a wide, flat face of five petals. It emits a delicate fragrance and attracts butterflies for pollination. Wild types of the garden phlox produce pink to lavender flowers, although years of breeding finds modern selections displaying white, red, salmon and bi-colored flowers. The overall size of a flower head (the cyme) ranges from 4 to 8 inches in length and width. Cutting off the cyme after the flowers wane often results in side branching on the stem tip and another production of flowers in late summer to early fall.
Pollinated flowers develop into small, dry fruits called capsules. As the plant enters autumn, these capsules split open and drop seeds to the ground below. They germinate the next spring to develop into new plants. Seedlings are highly variable in their characteristics such as plant height as well as size, shape and color of flowers.
When suffering in a drought, garden phlox foliage often wilts or is held at a downward angle from the stems. Lower leaves are often yellow or dry to brown and drop off prematurely from the plant. In autumn as nighttime temperatures get close to freezing, the foliage also collectively turns yellow with occasional flashes of orange. Frost kills the above-ground leaves and stems, making them turn brown. The stems persist into winter until snow or natural decay collapses them.