Garden phlox (Phlox paniculata) is a perennial that reaches 80 to 120 cm and flowers from July to September, once the early summer perennials have finished. It is regularly confused with annual phlox, a bedding annual and an entirely different plant, because both are sold under the single word phlox. Garden phlox comes back year after year, and what decides how the clump looks in August is the way you handle powdery mildew.
Garden phlox, annual phlox and moss phlox are three different plants
Three plants reach the shelves under the name phlox. They belong to the same genus but are used in completely different ways, and if you buy the wrong one it ends up in the wrong place.
| Species | Lifespan | Height | Flowering | Site and soil |
|---|---|---|---|---|
| Garden phlox (Phlox paniculata) | Perennial | 80-120 cm | July to September | Sun to partial shade, fertile soil that holds moisture |
| Annual phlox (Phlox drummondii) | Annual | 10-40 cm | July to September | Sun, free draining soil |
| Moss phlox (Phlox subulata) | Perennial ground cover | 10-15 cm | May to June | Sun, dry and free draining |
Annual phlox is sown from seed every year and used in pots, window boxes and as an edging plant. The whole sowing routine is set out in our article on growing annual phlox, and you buy the seed as annual phlox White Beauty. Moss phlox flowers over a short period in spring and spreads as a mat across rockeries and wall tops. Garden phlox is the only one of the three that grows tall enough to stand in the middle of a border.
Site, soil and planting distance
Garden phlox wants sun or partial shade. In full sun the stems stand firmer. In partial shade the dark cultivars hold their colour better through the hottest weeks. The soil should be fertile and hold moisture without staying wet, since root rot is one of the plant's problems in heavy clay.
A mature plant spreads 60 to 90 cm. Set the plants at least 50 cm apart, preferably 60 cm. That distance is the cheapest insurance against powdery mildew. Lay a 3 to 5 cm layer of compost or grass clippings around the plant after planting and the soil stays cool and moist for longer. More plants for the same border are among our perennials, and the basics of planting are set out in the article on growing perennials.
Powdery mildew decides how the clump looks in August
Powdery mildew is a fungus that settles as a grey mealy film on the leaves, usually working upwards from the base from late July. The plant does not die, but the lower half of the stems turns grey brown just as flowering peaks. Spraying does little once the attack is visible, and most of what works is done back in May.
Thin the shoots in May: An established clump throws more shoots than it can carry. Cut about a third of them off at ground level, the weakest first, so that 5 to 6 strong stems are left per plant. Air then reaches right into the crown and the leaves dry off faster after rain.
Water generously at the root: Aim the water at the soil so that it reaches down through the whole root zone. A plant that is allowed to dry out again and again between waterings becomes more susceptible to mildew, so even soil moisture is what makes the difference. Free water on the leaves does not feed the fungus. It is high humidity in still air that favours spore production, and if you need to rinse the foliage you do it in the morning so the leaves dry before evening.
Keep the spacing: Do not plant closer than 50 cm, and avoid spots right against a wall or a dense hedge where the air stands still.
Choose the cultivar for resistance: The difference between a susceptible and a tolerant cultivar is greater than anything you achieve through care. See the list below.
Clear up after the first frost: Cut the stems down to about 5 cm above the ground and rake away the fallen leaves. The fungus overwinters in the plant debris and infects next year's shoots.
Four cultivars with good resistance
The decision rule is simple: choose the cultivar for resistance first and colour second. A healthy white cultivar looks better in August than a pink one standing grey mottled from knee height down.
David: White and one of the most resistant cultivars in American trials.
Robert Poore: Magenta purple and often rated the most tolerant of the lot.
Delta Snow: White and scented. The cultivar placed among the best performing garden phlox in the American trial where mildew was what separated the cultivars.
Eva Cullum: Pink, for you who want colour and still want to escape the worst attack.
How to extend flowering with the Chelsea chop
The Chelsea chop is the name for shortening perennials at the end of May, at about the time of the English flower show in Chelsea. On garden phlox you use it on half the clump only, and flowering then becomes both longer and sturdier.
Pick out half: At the end of May or during the first days of June you mark out about half the shoots, spread evenly across the clump.
Take off a third: Shorten the chosen shoots by a third of their length and cut just above a pair of leaves.
Leave the rest alone: The uncut shoots handle the early flowering and are not to be touched.
The uncut shoots flower as usual in July. The cut ones flower 2 to 3 weeks later on shorter and thicker stems that do not lie flat after a rainstorm. Instead of three intense weeks, flowering stretches into September.
Watering, feeding and deadheading through the summer
Garden phlox is thirsty in July and August and drops its lower leaves as soon as it is allowed to dry out. Water generously once or twice a week during dry spells instead of a little every day, so that the moisture reaches down to the roots. Give compost or an organic fertiliser in spring when the shoots are around 10 cm tall.
Cut off the flower heads as they go over. The plant sets side shoots with smaller heads that extend the season, and you avoid the problem in the next section.
Divide the clump every three to four years
After three to four years the middle of the clump thins out and the flower heads get smaller. Dig up the whole clump in early spring or in autumn, split it with a spade or a knife into pieces with 4 to 5 shoots and healthy roots, and throw the woody middle away. Replant the divisions straight away and water them in thoroughly.
Named cultivars are propagated by division or root cuttings. Root cuttings are taken in late autumn: dig out a few thick pieces of root, cut them into lengths of about 5 cm and lay them flat in a box of sandy soil that is kept cold and frost free.
Self sown plants do not come true to the cultivar
Garden phlox sows itself if you leave the flower heads on. The seedlings do not inherit the mother plant's colour, because named cultivars do not come true from seed. A white cultivar gives seedlings that mostly revert towards a plain lilac pink base form, and those plants are often more vigorous than the cultivar you bought.
After a few seasons the self sown plants may have pushed the original out. Many people then think the cultivar has changed colour, but these are new plants that have taken over the spot. If you want to keep your cultivar, cut the heads off within a couple of weeks of them fading, before the seed has ripened.
Evening scent, butterflies and flowers to cut
The flower heads are scented, and the scent is clearest as evening comes. That is when the moths visit the flowers, while butterflies, bumblebees and bees work the day shift. More plants that draw insects in are among our seeds for pollinators.
Garden phlox also works in a vase. Cut the stems early in the morning when about a third of the flowers in the head have opened, strip off the leaves that would sit below the water line and put them straight into water. More on harvest and vase life is set out in the article on growing cut flowers, and cultivars to sow yourself are among our seeds that are good for cutting.
Author: Emma Vogiatzi
Fact-checked by Erik Hoekstra
Last updated 2026-09-01