Perennial
How to grow phlox
Phlox paniculata
Border phlox (Phlox paniculata) is a fully hardy perennial that flowers from July to September in sun or light shade. Plant in March to April or September to October, 45 to 60 cm apart, in fertile soil that stays moist in summer. Moisture is what decides success: a phlox that dries out at the root drops its lower leaves and gets powdery mildew. Mulch and feed in March, thin the shoots in April, and divide every three to four years.
- Hardiness
- Fully hardy
- Position
- Sun to light shade
- Soil
- Fertile, moisture-retentive
- Ideal pH
- 6.0 to 7.0
- Water
- Moderate to high
- Feeding
- Moderate feeder
Sowing and planting phlox
WhenPlant Mar to Apr or Sep to Oct; flowers Jul to Sep
Starting off
Plant pot-grown plants Mar to Apr or Sep to Oct. Divide every 3 to 4 years in Mar or Oct, discarding the woody centre. Seed does not come true.
Spacing and planting
Plant 45 to 60 cm apart in groups of three; set the crown at soil level. Not a container plant beyond a 40 cm tub for dwarf types.
Varieties worth growing
- 'David'
- Pure white, scented, about 1 m tall, and the variety with the best resistance to powdery mildew.
- 'Bright Eyes'
- Pale pink with a crimson eye on a compact plant of around 80 cm; a reliable old variety.
- 'Blue Paradise'
- The closest phlox to true blue, deepest in colour at dusk and on cool days; strongly scented.
- 'Starfire'
- Deep cherry red flowers over dark, bronze-tinted foliage; the strongest red among border phlox.
- 'Franz Schubert'
- Lilac with a paler edge and a darker eye, sturdy stems to 1 m that rarely need support.
Phlox month by month
Timings are for central England. Allow two weeks later in the north and on cold, heavy soil.
- January
Take root cuttings from a lifted clump, laid flat in trays of compost in a cold frame.
- February
Nothing to do. Order plants.
- March
Plant and divide. Feed, then mulch 5 cm deep over moist soil.
- April
Thin each clump to its five or six strongest shoots.
- May
Put in grow-through supports for tall varieties. Cut the front stems back by a third late in the month for later, shorter flowers.
- June
Light top-dress as buds form, watered in. Start weekly soaking if it is dry.
- July
Flowering begins. Water at the base, not over the leaves.
- August
Deadhead the main head back to the side shoots for a second, smaller show.
- September
Plant new phlox while the soil is warm.
- October
Plant and divide, discarding the woody centre of old clumps.
- November
Cut all stems to the ground and clear them away.
- December
Nothing to do in the border. Root cuttings can be taken from now.
Feeding phlox
A moderate feeder that responds more to moisture and organic matter than to fertiliser. One balanced feed under the spring mulch and a light top-dressing of the same fertiliser at bud stage is the whole plan.
Moderate, rich, moist soil and a spring feed; mulch to keep roots cool.
Feeding plan
Planting
5 g per litre of root ball, dusted onto the roots: 10 g for a plant from a 2 litre pot. Once only.
March
About 100 g/m², the label rate. A clump at 50 cm spacing covers roughly 0.25 m², so 25 g, two level tablespoons, round each clump, lightly forked in.
March, straight after feeding
Garden compost
A 5 cm mulch over damp soil, kept just clear of the shoots.
June, as flower buds form
50 g/m², half the label rate of about 100 g/m²: about 12 g, a level tablespoon, scattered round each clump and watered in. Once. Nothing after that.
Signs to watch
Lower leaves yellow and scorch in a dry Jul
Water stress, not nutrition
Mulch and soak weekly
Leaves yellow between the veins on chalk
Magnesium short
Epsom salts
Weak stems and small heads on an old clump
Congestion
Divide in Mar
Pests, diseases and problems
White powder over leaves and stems from July, leaves then brown and shrivel
Powdery mildew, worst on crowded stems and dry roots
Thin to five or six shoots per clump in April, mulch, soak the roots weekly in dry weather, and cut affected stems to the ground after flowering and bin them. Grow 'David' where it strikes every year.
Stems stunted, swollen and split; upper leaves narrowed to threads or puckered
Phlox eelworm living in the stems and leaves
Dig up and bin affected plants and grow no phlox in that spot for three years. The eelworm does not enter the roots, so healthy new plants can be raised from root cuttings taken in winter.
New shoots eaten off at ground level in March and April
Slugs and snails
Torch patrols on mild damp nights, nematodes from late March on moist soil, and keep mulch pulled back from the emerging shoots.
Dark brown spots with pale centres on lower leaves, which yellow and fall
Fungal leaf spot, spread by rain splash in wet summers
Pick off spotted leaves, thin the stems for air flow, water at the base rather than over the top, and clear all the old stems in November.
Pruning
Pruning and care
Thin shoots to 5 to 6 per clump in Apr for stronger stems and airflow. Deadhead the main head to the side shoots for more flower in Aug. Stake tall types with a ring of canes or a grow-through grid in May. Cut down in Nov; divide every 3 to 4 years in Mar and mulch in spring to keep roots moist.
Good to knowThin stems; keep watered in dry spells.
Growing phlox in pots
Border phlox is not a good pot plant. It has a wide, fibrous root system that wants cool, constantly moist soil, and in a container it dries out, sheds its lower leaves and mildews. If you have no border, a compact variety will manage for two or three years in a tub at least 40 cm across filled with loam-based compost, stood where it gets shade in the afternoon and watered daily in summer. Top-dress in March with the 4-4-4 at 2 g per litre of compost, about 50 g for a 25 litre tub.
Phlox: common questions
How do I stop powdery mildew on phlox?
Keep the roots moist and the stems thinned. Mildew takes hold on phlox that is dry at the root and crowded above, so mulch in March, thin each clump to five or six shoots in April, and soak the soil weekly in dry spells. Divide old clumps, and plant a resistant variety such as 'David'.
Should I cut back phlox after flowering?
Yes, in two stages. In August cut the faded main head back to the side shoots just below it, which often gives a second flush. In November cut every stem to the ground and clear it away, because mildew and leaf spot overwinter on the old growth.
When should I divide phlox?
Divide border phlox every three to four years in March or October. Lift the clump, split it with a spade or two forks, replant the vigorous outer pieces with the crown at soil level, and throw away the woody centre. Water the divisions through their first summer.
Sources
Growing information on this page was checked against these references in November 2026. Feeding rates are Dr Forest label rates.