The Phlox genus encompasses nearly 70 species, and includes annuals, perennials and shrubs. Most of the perennial varieties are upright, but a few species are mat forming. Most of the popular garden varieties derive from the species Phlox paniculata, an elegant, erect, herbaceous perennial with sword-shaped leaves. It produces loose, branching clusters of five-petalled flowers (or panicles, hence ‘paniculata’). These are typically white, pale lilac or darker purple and continue to appear throughout the summer and into the early autumn.
Phlox arrived in the UK as a garden plant from its native North America in the early 1800s and soon became a firm favourite for the summer herbaceous border. The plants not only produce an abundance of colour but also a delicious fragrance that evokes the very essence of a traditional English cottage garden. The flowers are pollen and nectar-rich, and they attract hordes of pollinators such as honey bees, bumble bees and butterflies.
The name ‘phlox’ comes from the Greek for flame, perhaps making reference to the wild species’ vivid flower colours.
This article focuses on caring for the upright, herbaceous, perennial phloxes.
Phlox, summer phlox, perennial phlox
Sun or light shade
Established plants will deal with temperatures below freezing
Rich, moisture retentive but well drained loam
Blood, fish and bone in spring and water soluble fertiliser in autumn
Division, soft cutting, root cutting
Slugs, eelworm, mildew and other fungal infections
A wide range of phlox cultivars is available from nurseries and garden centres. Many different colours have been produced, although the most common varieties are generally white, pink, blue or purple. Orange and reddish varieties have also been developed. There are also cultivars with two-coloured flowers, and a few with variegated foliage.
The following list includes some of the most popular, reliable and widely available cultivars. All have been given the RHS Award of Garden Merit following a range of horticultural trials over many years.
Phlox paniculata ‘Uspekh’
‘Uspekh’ is a herbaceous perennial, growing up to 1m high. It has the typical green, lance-shaped phlox leaves and scented, purple flowers with a white star at the centre during mid to late summer.
Phlox paniculata ‘Miss Ellie’
A bushy, upright herbaceous perennial growing up to 1m high, ‘Miss Ellie’ has dark, lance-shaped leaves and exuberant panicles of fluffy, rose-pink and fragrant flowers from mid to late summer.
Phlox paniculata ‘Prospero’
‘Prospero’ grows to 90cm tall. It has dark green leaves and terminal panicles of fragrant, white-eyed, light lilac flowers with pale-edges to the petals. It is earlier flowering than most, blooming from the beginning of summer.
Phlox paniculata ‘Danielle’
A shorter, robust and weather-resistant variety up to 70cm tall, ‘Danielle’ produces clean white flowers with a wonderful scent over an extended period throughout the summer.
Phlox paniculata ‘Prince of Orange’
‘Prince of Orange’ is an erect, herbaceous perennial up to 80cm tall. It produces panicles of very fragrant, vibrant, orange-red flowers in mid summer.
Phlox paniculata ‘White Admiral’
One of the more popular white varieties of phlox, ‘White Admiral’ is upright and grows up to 90cm in height. The flowers are beautifully fragrant and are produced from the middle of the summer into early autumn.
Phlox paniculata ‘Rosa Pastell’
‘Rosa Pastell’ grows up to 70cm high. The variety has dark stems and striking, loose heads of dark pink buds that open into highly scented, pink flowers with a darker eye throughout the summer.
Phlox paniculata ‘Blue Paradise’
This tall variety can grow up to 1.2m in height, and generally needs staking in an open border. The rounded heads of fragrant, dark-eyed violet-blue flowers appear in early summer, and bring a lovely early fragrance to the herbaceous border.
Phlox paniculata ‘Grey Lady’
‘Grey Lady’ is a bushy but upright perennial, growing up to a height of 1m. The blooms are a delicate shade of pale lavender-grey, with a lovely fragrance. Flowering takes place from early to mid summer.
Phlox paniculata ‘Miss Pepper’
‘Miss Pepper’ grows up to 1m high, but has sturdy and relatively wind-resistant stems. Its sweetly fragrant pink petals frame a dark pink eye, and are backed by contrasting purple sepals.
Phlox paniculata ‘Le Mahdi’
‘Le Mahdi’ is a clump-forming herbaceous perennial to 1m tall, with dark foliage and panicles of fragrant violet-blue flowers.
Phlox paniculata ‘Flamingo’
A great choice for a pink variety, ‘Flamingo’ has fragrant flowers with a cerise eye. This cultivar grows up to 1.1m high.
Phlox paniculata ‘Mother of Pearl’
Growing to 75cm tall, ‘Mother of Pearl’ has fragrant, white flowers infused with delicate tinges of a blush-pink colour.
Phlox paniculata ‘David’
A great choice for the white garden, ‘David’ has a bushy and upright form and reaches a height of a metre, or even more in good conditions. It has a cluster of very fragrant, pure white flowers from mid to late summer.
Phlox paniculata ‘Grenadine Dream’
A shorter, sturdy and self-supporting variety, ‘Grenadine Dream’ grows up to 60cm high. The dark green leaves provide a contrasting backdrop to the pleasantly scented, reddish-purple flowers from mid to late summer.
Phlox paniculata ‘Monica Lynden-Bell’
‘Monica Lynden-Bell’ is another shorter variety, reaching up to 80cm in height. The fragrant flowers are a delicate pale pink with a pearlescent quality.
Phlox × arendsii ‘Miss Mary’ (Spring Pearl Series)
The hybrid ‘Miss Mary’ grows up to 1m in height, and has more oval-shaped leaves than most garden phloxes. It produces an abundance of clusters of scented, cerise flowers throughout the summer.
Phlox is very easy to grow and requires little attention, though plants can take a couple of years to establish fully and reach their maximum potential.
They should be planted in a sunny or partially sunny location. Space them out well, at least 30cm apart and away from other plants, to ensure good ventilation, as they are vulnerable to mildew.
Prepare the ground for planting by digging in some garden compost or other fertile organic matter. Ensure the compost in the pot matches the level of the surrounding soil when preparing the hole and planting out.
Phlox are hungry plants, so apply a 5cm to 10cm dressing of garden compost or well-rotted manure each spring.
Most types of phlox have sturdy stems and are self-supporting, but some of the taller varieties will need staking with canes, hazel sticks or herbaceous perennial loops in windy gardens.
Once flowering stems have finished, they can be cut off to keep them looking tidy. The whole plant can be cut down to soil level in the autumn.
Some enthusiasts recommend a spring move of phlox plants every four or five years, as they can deplete the soil of certain nutrients, and to help prevent the build-up of disease. During the move, the plants can be divided, using the outer part of the clump with healthy-looking shoots and discarding the old, brown, woody centre.
Phlox prefers a position in full sun, though it will still grow well in a little light shade.
Do not water phlox late in the evening or at night as this can facilitate the establishment of powdery mildew. Water the soil around the plants well in the morning to allow any splashes on the leaves time to dry out before the conditions become more humid around nightfall.
The plant needs very good, rich, well-drained soil. When planting, add garden compost or well-rotted manure to enrich the soil.
Plants and flowering can be enhanced through providing them with a sprinkling of blood, fish and bone each year before the spring mulching with rich organic matter.
A dose of water-soluble fertiliser in the autumn will also keep the plants vigorous.
While not generally grown in containers, phlox can make a colourful addition to a mixed summer patio planter, though the plants will still need air circulation and space if mildew is to be kept at bay – not always easy in a crowded planter. The container needs to be well drained, large and deep in order to accommodate the plant and its roots. Use a good quality potting compost with 25% perlite to improve drainage.
After planting, water the container generously to settle the soil around the roots, trying to avoid wetting the foliage. The compost will need to be kept moist, always watering early in the day. Deadheading will keep fresh flowers coming through the summer.
If mildew does affect the plant, it is best cut down to soil level. Plants that remain healthy should be cut down to 5cm in the autumn, and they will re-sprout next spring.
Looks good with
Phlox is traditionally grown in herbaceous borders with other summer-flowering perennials such as lupins, Penstemons, Alchemilla mollis, Campanulas, roses, Heleniums, Echinaceas, Stachys, Achilleas, Delphiniums and hollyhocks.
Herbaceous perennials are generally grown in uneven-numbered groups for best effect, assuming there is room in the border. Groups of five or seven are most effective. Borders were traditionally laid out with the taller plants at the back, mid-size types such as phlox in the middle and shorter plants at the front. Some contemporary gardeners are rebelling against this tradition with less regimented designs, for example using taller but visually permeable plants such as grasses nearer the front of the border.
Deadhead the plant and take out any damaged or untidy stems through the summer to encourage fresh growth and more flowers.
Cut the stems down in late autumn to tidy up the plants and carefully collect and remove any fallen and dead leaves. All of this material should be burned or thrown away. It should not be added to the compost heap to reduce the risk of re-introducing eelworm and mildew to the border in the following year.
While some types of phlox such as Phlox drummondii are available as seeds, the more common Phlox paniculata cultivars are bought as bare root or container grown plants.
Propagation of the cultivars is easiest by division or root cuttings.
Established clumps can be divided in early spring every four or five years or so. Separate the clump into sections, discard the old, woody centre, and plant the sections into the border straight away, watering in well but not waterlogging.
Sometimes it is possible simply to pull rooted pieces from the edges of clumps without digging up the whole plant.
Root cuttings are taken during early spring or in November. Phlox has rather fine roots that are a bit more fiddly to take cuttings from, but the technique is generally successful. Only take cuttings from vigorous and healthy plants to avoid spreading or perpetuating disease, especially viruses.
Root cuttings are a good way of overcoming eelworm problems without losing plants completely, as the eelworms rarely infect the plant below soil level, so new plants grown from root cuttings will generally be unaffected.
To take root cuttings, dig up the plant with as much of the root-ball intact as possible. Wash off the soil, and use a sharp, sterilised knife to cut off 5cm lengths of good, healthy roots. Whilst it is normally important to remember which way up the cutting was on the plant, any confusion can be avoided with phlox root cuttings by laying them flat in rows onto a 50:50 mix of potting compost and horticultural grit in a seed tray. Cover them shallowly with compost. The new roots emerge from one end of the root cutting and the shoots from the other.
Keep the cuttings moist but not over-wet, moving the tray to a well-lit position once the green shoots emerge. Placing the tray on a warm surface will hasten growth, but it is likely to be mid-summer before roots are sufficiently established for the cuttings to be grown on.
Common Phlox Problems
Slugs can be a problem with fresh spring growth.
Split stems or distorted, misshaped leaves are likely to be signs of stem and leaf eelworm infestation. There is no cure, so affected plants should be dug up and destroyed. Replacement phlox plants should be grown elsewhere in the garden, and good garden hygiene imposed, removing all dead leaves. Affected plants can be used to take root cuttings before destroying them if desired.
Phlox is very susceptible to fungal infections such as leaf spot and powdery mildew. Neither is terminal, and infection can be avoided or reduced through good care of the plants, ensuring they do not suffer water stress, providing adequate ventilation, and not watering late in the day. The best way to prevent mildew is to keep the soil rich by mulching generously in the spring.
Extra attention is needed in the early summer when the days are warm and the nights cool. Mildew flourishes in conditions of fluctuating temperatures, and the plant is more susceptible when the foliage is wet or if it is suffering drought.
Some cultivars have greater mildew resistance, and horticultural trials in the United States suggest that Phlox paniculata ‘Jeana’ is one of the best types to grow in badly affected gardens. The species Phlox amplifolia also performed very well, though it is not widely available in the UK.
Q The phlox smells lovely at the far end of my border – will it do well in the vase if I use it in an arrangement for the table?
Phlox is good for indoor arrangements, but the flowers will need to be looked after to stay their best. Do not use flowers cut from plants that are clearly affected by mildew, and cut any leaves from the stem before placing the flower in the vase. Provide some ferny foliage to set off the texture and colour of the flowers.
Q My phlox has been growing in a sunny border with some Achilleas for five years. While it grows tall and looks healthy, it has never flowered. There is no sign of mildew or leaf distortion. What is the problem?
If the plant seems green, lush and generally healthy, it may well be that the nitrogen to phosphorus ratio in the soil is skewed, favouring the production of foliage over flowers. Many herbaceous perennials are shy flowerers when they are over-provided with nitrogen and under-provided with phosphorus. Review your feeding regime, and in the first instance give the plant a good dressing of bone meal. Going forward, use a high potassium fertiliser such as sulphate of potash or tomato feed in late summer.
Q What types of phlox are best for my wildlife garden?
Most types of Phlox paniculata provide plenty of pollen and nectar for pollinating insects. Research in the US assessed 137 species and varieties of phlox for butterfly appeal and found that the cultivar Phlox paniculata ‘Jeana’ was particularly attractive. The flowers of ‘Jeana’ are amongst the smallest of all phloxes, though they are nevertheless attractive to gardeners as well as insects. The sprays of candy-pink flowers are sweetly scented, and grow on tall stems up to 1.2m tall.