Ornamental · Polemoniaceae
Garden Phlox
Phlox paniculata
Hand it sun, breathing room, and steady moisture, and garden phlox tops the border with color from midsummer onward.
Full sun
Every 3–7 days
USDA 4–8
2–4 ft · 0.6–1.2 m
Overview
Look closely at garden phlox and you see tall, upright stems carrying broad, domed heads of fragrant flowers, each panicle packed with small five-petaled blooms. This hardy perennial hails from the eastern United States, returning fuller every year once established. The florets open in pink, white, lavender, and red, and their scent and shape keep butterflies and other pollinators working the heads all season.
Part of what earns phlox its keep is when it flowers. The heads unfurl in midsummer to fall, filling the gap left as earlier perennials go over, and they hold their color for weeks on end. The clumps creep outward at a slow pace, so one plant thickens over time into a leafy, full stand tall enough to hold the back of a bed.
The single drawback is powdery mildew, drawn to the same close, motionless air that a tightly planted border tends to trap around the leaves. Set each plant apart, aim water at the roots and keep it off the foliage, and the disease largely stays clear. Reach for a mildew-resistant variety and it becomes something you scarcely have to think about.
Care
The short version: full sun to part shade, rich moist well-drained soil, and steady water through dry stretches. Leave each clump enough space for air to pass, wet the base rather than the leaves, and lift and divide it every few years to keep the flowering strong.
Light
Garden phlox blooms hardest in full sun, though partial shade and partial sun suit it well enough. Deep shade thins out the flower heads and leaves the stems lankier and quicker to catch mildew.
Water
Hold the soil evenly moist, watering every 3 to 7 days and stepping it up through a dry summer. Phlox cannot ride out drought, so a layer of mulch pins the moisture in place and keeps the roots cool below. Direct the water at the base rather than the leaves to stay ahead of powdery mildew.
Soil
Set it in moist, fertile, well-drained ground with plenty of organic matter, be it loam or a denser clay. It tolerates a broad pH span of 6.0 to 8.0, so most garden soil works once you dig in some compost.
Temperature
Garden phlox is thoroughly hardy, weathering winter lows to -29°F (-34°C) and summer highs to 95°F (35°C). Such a wide span suits a great many climates, which is what makes it so reliable in the border.
Humidity
Ordinary outdoor humidity does the job, so there's no target number to chase. Air movement counts for far more than the moisture around the leaves, since the still, packed conditions phlox dislikes are just what powdery mildew wants.
Feeding
Feed in spring with a balanced fertilizer or a generous helping of compost turned into the soil. A single seasonal feed into already fertile ground carries the plant right through its long run of bloom.
Pruning
Deadhead the faded flower heads to keep fresh blooms coming and to shut down stray self-seeding. Thin the crowded stems early on for cleaner airflow, then cut the whole plant to the ground in winter and rake off the debris so disease has nowhere to overwinter.
Repotting
Garden phlox lives in the ground, not a pot, yet it still asks to be divided. Lift and split each clump every 2 to 4 years in spring or fall; a crowded center throwing smaller blooms on weaker stems tells you it's overdue.
Size & spacing
Plants stand 2 to 4 feet (61 to 122 cm) tall and build clumps 2 to 3 feet (61 to 91 cm) across. Grant each one that full width of space so air moves freely and mildew finds less of a hold.
When to plant
Garden phlox is a perennial that comes back on its own each year, so the actual planting is a one-time task set for spring after the last frost or for early fall. After that it keeps to the calendar largely unaided, pushing up new growth in spring and flowering from midsummer into fall.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Sow: Late winter to early spring (species seed; cultivars do not come true from seed)
- Plant: Spring after last frost or early fall
- Harvest: Mid-summer to mid-fall (July to September)
Hardiness
USDA hardiness zones: 4–8
Temperature: down to -34°C
Troubleshooting
Nearly every problem with garden phlox turns up on the foliage, and powdery mildew tops that list by a long way. Spot the first signs and you can usually halt a problem before it wrecks the show. Find the match below, then act while there's still ground to gain.
Stalks buckling above a brown, mushy crown
Soil that stays sodden invites crown and root rot, which turns the base of the plant soft and brown and leaves the stems drooping over. The telling clue is a clump that flags and folds even while the ground around it sits wet. A clump this far gone cannot be rescued, so dig it out, correct the drainage, and cut back the watering before the rot travels further.
Fix:
- Remove and destroy severely affected plants
- Improve soil drainage
- Avoid overwatering and waterlogged sites
Leaves veiled in a powdery gray-white bloom
That chalky white dusting on the leaf surface is powdery mildew, the trouble garden phlox is famous for, and it grabs hold wherever the air goes still and the foliage stays wet. Open up the clump by thinning stems so a breeze can pass through, wet the roots and not the leaves, and pull off any leaf already caked with it. Start with a resistant cultivar such as David or Jeana and the whole problem mostly never begins.
Fix:
- Thin stems to improve air circulation
- Avoid overhead watering and water at the base
- Remove and destroy infected foliage
- Cut plants to the ground in winter and clear debris
Foliage flecked and dulled to bronze beneath fine silk threads
When the weather turns hot and parched, spider mites settle onto the leaf undersides and rasp away at them until the green fades to a dull, dusty bronze flecked with pinprick spots. A sharp spray of water knocks the colony loose, and keeping the ground moist stops the plant from drying out and inviting more. Any that cling on give way to insecticidal soap or horticultural oil.
Fix:
- Spray foliage with a strong stream of water to dislodge mites
- Keep plants well watered during heat
- Apply insecticidal soap or horticultural oil if needed
Toxicity
Garden phlox is safe to grow around everyone in the household. The ASPCA records it as non-toxic, with no harmful compound in any part of the plant, so pets, livestock, and children can all share the garden with it freely. A stray taste of leaf or flower does no harm.
Safe for
People · Cats · Dogs · Horses · Rabbits · Livestock · Birds
Cats None
Cats are in no danger around garden phlox, which the ASPCA records with no toxic principles at all. Should one bite into a leaf or a petal, nothing bad follows.
Dogs None
A dog can roam the same bed as phlox with nothing to fear. A chewed stem or a mouthful of bloom holds no toxin to make one sick.
Horses Safe
Nothing in phlox carries any threat to horses, so a mouthful cropped while grazing does no damage.
Rabbits Safe
Rabbits will nibble phlox foliage now and then, and it leaves them unharmed. Not one part of the plant holds a toxin for them.
Livestock Safe
Cattle, goats, and sheep are fine grazing near garden phlox, because no part of it is poisonous to them.
Birds Safe
Phlox holds no hazard for birds; its blooms invite pollinators in rather than pose any risk to a visitor.
People Safe
There's no poisoning risk from garden phlox for people, and that goes for children drawn to pick at the bright flower heads too.
Safe to grow where pets have access; no special handling required for toxicity.
Varieties
Garden phlox runs to hundreds of cultivars, selected for color, fragrance, and resistance to mildew. Here are a few worth knowing by name.
David
Prized for its snow-white panicles and a long-standing knack for brushing off powdery mildew.
Jeana
Packs tight heads of lavender-pink flowers, stands up strongly to mildew, and draws pollinators like few others.
Eva Cullum
Bears clear pink petals ringed around a deeper red eye at the heart of every flower.
Companions
Garden phlox pairs well with other sun-loving perennials that share its taste for moist, fertile soil and add their own form to the summer border.
How to propagate
Garden phlox multiplies with little fuss, and the method you pick follows what you're after. Division and cuttings each reproduce a named variety exactly, while seed hands you the wild species and an unpredictable spread of colors.
DivisionEasy
Spring or fall · One growing season
Splitting the clump is the easiest route to more phlox and keeps the parent in good health. In spring or fall, dig up an established clump, pull it apart into sections, and set them back in; each piece takes root across a single growing season. Repeat it every 2 to 4 years and the middle of the clump never hollows out and dies.
Stem cuttingsModerate
Late spring to summer · Several weeks to root
A softwood cutting reproduces a named cultivar down to the detail, something seed can never manage. Snip pieces of fresh new growth in late spring or summer; they need several weeks to strike roots before they're ready to pot on.
SeedModerate
Late winter to spring · One season or more to flowering
Seed sown in late winter or spring will give you the wild species, but count on a season or more before it flowers. Cultivars won't reproduce true from seed, so gather it only if any mix of colors that shows up is fine by you.
Myths
Every part of every phlox is dangerous for cats and dogs to eat.
Phlox sits on the ASPCA non-toxic list for cats and dogs alike, with no harmful compound on record. Any pet that samples a leaf or a bloom ought to be perfectly fine.
The seedlings of a named phlox match the parent's colors and form exactly.
Seed from a named cultivar seldom yields a plant that matches its parent, so the offspring wander off in color. Snip off the spent panicles to curb stray self-seeding, and propagate the varieties you want by division or cuttings instead.