How to Grow Phlox in a UK Garden

Phlox is one of the loveliest summer perennials you can grow in a UK garden. Big, scented heads of flowers open from July to September, in shades from pure white to deep magenta. The scent alone earns it a place in any cottage border.

You’ll find plenty of varieties in garden centres, and the RHS plant finder lists many more. Here’s how to pick a good one, plant it in the right spot, and keep powdery mildew away.

What types of phlox can I grow in the UK?

There are two main types: tall border phlox (Phlox paniculata) and low creeping phlox (Phlox subulata). Both are fully hardy, but they do quite different jobs in the garden.

Border phlox comes in lots of soft pinks and purples, perfect for a cottage garden.

Border phlox is the classic cottage garden plant. It grows 60-120cm tall with big domed flower heads from July to September, and many are sweetly scented. It’s a star of summer flower borders and great for cutting.

Creeping phlox is a low alpine, just 10-15cm tall. It flowers earlier, in April and May, and covers rockeries and path edges in starry flowers. It spreads to about 50cm and copes with dry spells once settled. That makes it a handy ground cover plant for sunny banks and wall tops.

Woodland phlox (Phlox divaricata) is a third choice for light shade. It grows 25-35cm tall and has soft blue or lavender flowers in May. It’s harder to find, so look in specialist nurseries.

What are the best phlox varieties for UK gardens?

Phlox variety comparison table

White and pale varieties

David is the go-to white. It’s tall, scented and one of the best for shrugging off mildew. Bright Eyes has pale pink flowers, each with a little red eye in the middle. It’s a lovely soft choice for pastel schemes and flowers well through July and August.

Bold colour varieties

Starfire has the deepest colour of the lot, with rich magenta-red flowers on dark stems. Its spring leaves have a burgundy tint. It’s a bit more prone to mildew, so thin the stems in May and give it room to breathe.

Where should I plant phlox?

Pick a sunny spot with moist, well-drained soil and a bit of breeze. Get the position right and phlox gives you far fewer problems.

Sun and shelter

Aim for at least 6 hours of sun a day. In less light, stems stretch, flowers shrink and mildew gets in more easily. A south or west-facing border is ideal.

Tall stems can snap in strong wind. In an exposed garden, choose a compact variety like Flame Pink, or push in pea sticks or grow-through supports in May.

Soil requirements

Phlox likes rich soil that stays moist but never sits wet. That’s the main thing to get right.

On heavy clay, dig in garden compost and some coarse grit before planting. On sandy soil, add well-rotted manure or compost so it holds more water. Phlox sulks on thin, dry chalk and on very acid soil.

How do I plant phlox?

Plant in spring (March to April) or autumn (September to October), in soil you’ve enriched with compost. Spring is best on heavy clay. Autumn suits lighter ground.

Spring planting gives the roots time to settle in before summer.

Step-by-step planting

1. Dig a hole twice as wide as the pot and the same depth
2. Mix a spadeful of garden compost into the bottom
3. On clay, add a handful of coarse grit
4. Take the plant out of its pot and gently tease out any circling roots
5. Set it in the hole with the crown level with the soil
6. Fill in, firm gently and water well
7. Mulch with 5cm of bark or compost, keeping it off the stems
8. Water weekly for the first 6 weeks if it’s dry

Spacing

Space border phlox 45cm apart. That gap lets air move between the clumps, which is your best defence against mildew. Closer planting fills out faster but brings more disease.

In mixed borders, put phlox in the middle or towards the back. It sits nicely behind shorter plants like catmint, salvias and hardy geraniums.

How do I stop phlox getting powdery mildew?

Give the plants space, thin the stems and keep the roots moist. Mildew is the white, dusty coating you see on leaves in late summer. It spoils the look but rarely kills the plant.

The three rules for mildew prevention

1. Space and thin. Plant 45cm apart, then in May thin each clump to 5 or 6 strong stems when they’re 15-20cm tall. More air around the stems means less mildew.

2. Water at the base, not overhead. Wet leaves on a still evening are just what mildew likes. Use a can or a soaker hose aimed at the soil, not a sprinkler.

3. Don’t let it dry out. Plants stressed by dry roots in summer get mildew more easily. Water deeply in dry spells and keep a mulch on.

If mildew does turn up, pick off the worst leaves and bin them rather than composting them. Choosing a resistant variety such as David is the easiest fix of all.

What are the best companion plants for phlox?

Pair phlox with other summer perennials that like sun and moist soil. The soft, rounded flower heads look lovely in cottage-style borders.

White, pink and lilac phlox grown together make an easy cottage garden mix.

Recommended companion planting

Plant catmint at the front to hide the bare lower stems of tall phlox. Behind, grasses like miscanthus add movement and a soft backdrop.

Phlox is also a top pick for butterfly-friendly borders. The flat flower heads make easy landing pads, and red admirals, painted ladies and small tortoiseshells visit all through August.

How do I care for phlox through the year?

Water in dry spells, feed once or twice a year and deadhead to keep the flowers coming. Once you know the timings, it’s an easy plant to look after.

Deadheading encourages side shoots to flower and stops unwanted seedlings.

Month-by-month care calendar

Deadheading technique

Snip off the faded flower head just above the first pair of strong side buds. These will flower in a few weeks, stretching the show into September and sometimes October. Cut the whole stem only when all the side shoots have finished.

Feeding

Phlox is quite hungry. Scatter a general fertiliser such as Growmore (70g per square metre) in April as shoots appear. From mid-July, give a liquid tomato feed every 2 weeks. Homemade comfrey and nettle feeds work well too.

Division

Divide clumps every 3-4 years in March or early April. Lift the plant, cut it into pieces with a spade or knife, and replant the healthy outer bits. Throw away the woody middle. Each piece needs 3-5 good shoots and some roots. Water in well and keep moist for 6 weeks.

Can I grow phlox from seed or cuttings?

Yes, root cuttings in winter and soft shoot cuttings in spring both work well. Seed is possible, but named varieties don’t come true, so the seedlings vary in colour and height.

Root cuttings (November to January)

This is the best way to copy a named variety. Expose the roots on one side of the plant and cut some thicker ones into 5cm pieces. Phlox roots are thin, so lay the pieces flat on a pot of gritty compost and cover them lightly. Keep them in a cold frame over winter. Shoots appear in spring.

Basal cuttings (April to May)

Take 8-10cm cuttings from new shoots at the base of the plant in spring. Cut just below a leaf joint and remove the lowest leaves. Push them into small pots of half perlite, half multipurpose compost. Keep in a propagator or a covered tray at 15-18°C. Roots form in 3-4 weeks.

Division (March to April)

Division is the simplest way to get more plants. Follow the steps in the care section above, and replant the pieces straight away. They’ll flower the same summer.

Frequently asked questions

When is the best time to plant phlox UK?

Plant phlox in spring (March to April) or autumn (September to October). Spring is best on heavy clay, as the roots settle in before summer. Autumn suits lighter soils. Pot-grown plants can go in any time the ground isn’t frozen or waterlogged.

Does phlox come back every year?

Yes, phlox is a hardy perennial that returns every year. The stems die back in autumn and fresh shoots appear in March and April. Divide clumps every 3-4 years, or the middle goes woody and flowers less.

Why does my phlox get powdery mildew?

Mildew loves still, humid air around crowded stems. Space plants 45cm apart, thin each clump to 5 or 6 stems in May, and water the soil rather than the leaves. A dry spell at the roots makes it worse too, so water well in summer. Mildew-resistant varieties such as David help a lot.

Can I grow phlox in shade UK?

Border phlox flowers best in full sun. It copes with light dappled shade, but you get fewer flowers, leggier stems and more mildew. For a shadier spot, try woodland phlox (Phlox divaricata) instead. Creeping phlox wants sun too.

How tall does phlox grow?

Border phlox grows 60 to 120cm tall, depending on variety. Compact types such as Flame Pink stay at 40-50cm. Tall ones like Blue Paradise can reach 120cm and may need staking in windy gardens. Creeping phlox is only 10-15cm tall and spreads to about 50cm.

How do I stop phlox from spreading?

Border phlox spreads slowly and isn’t invasive. The clump simply gets wider. Divide it every 3-4 years to keep it in check, and deadhead to stop it seeding about. Creeping phlox spreads faster but is easy to trim back after it flowers in May.

What grows well with phlox in a border?

Other sun-loving summer perennials make great partners. Try echinacea, rudbeckia, Japanese anemones and ornamental grasses for colour from July to October. Catmint and hardy geraniums fill the front of the border nicely.