Phlox: planting, growing and care

Description and Botany

The Phlox are herbaceous plants, annuals or perennials, that come almost exclusively from North America. They encompass just over 80 species. Most cultivated species, including Phlox paniculata, are native to the United States. There is also one species found in Siberia! In gardens, the most common are Phlox paniculata and Phlox subulata. These are easy to grow and hardy plants, as they originate from regions with a temperate climate. Today, there are said to be over 800 horticultural varieties! The vast majority are perennials, with the exception of Phlox drummondii, which is an annual.

In nature, different species of phlox can be found in various environments: sometimes in rocky areas (for the mat-forming phlox), near waterways (for the panicled phlox), and many species grow in woodlands. This explains why their uses in the garden will differ: Phlox subulata will thrive in rockeries, while others will prefer cooler, deeper, and more fertile soils…

Etymologically, the name phlox comes from the Greek and means “flame,” due to the bright colour of certain species. Phlox belong to the family Polemoniaceae, a relatively small family of just over 400 species, which includes cultivated plants such as Polemonium, with its beautiful blue flowers and finely cut foliage, as well as the cobaea, a climbing plant.

Phlox paniculata: botanical illustration

In terms of the general shape of the plant, phlox can have either an upright habit, like Phlox paniculata, or a mat-forming habit, like Phlox subulata. The latter can thus be used as ground cover! They can sometimes form large, colourful carpets… The height of phlox is therefore very variable: between 10-20 cm for the shortest, with a mat-forming habit… and up to 1.50 metres for the upright phlox (Phlox paniculata)! There are also intermediate varieties, such as Phlox carolina ‘Bill Baker’, which forms clumps 45-50 cm high. The cushion habit and fine foliage of Phlox subulata earn it the common name of moss phlox!

The use of phlox will therefore differ depending on their form: those with a compact habit will find their place at the front of borders or in rockeries, while upright varieties will integrate into the middle or back of a border or mixed border, behind shorter plants.

Depending on the species, flowering occurs between April and October. The panicled phlox flowers in summer, from July until September-October, while the flowering of Phlox subulata occurs in spring, in April and May. It lasts much less time.

Phlox then offer a very generous flowering! Those with a mat-forming habit are literally covered in flowers, hiding their foliage, which creates large pink or mauve carpets… As for the flowers of phlox paniculata, they are grouped in terminal inflorescences, rounded in shape, borne at the tips of upright stems. The flowering of phlox is generally very dense!

Phlox often offer flowers in soft, pastel shades: white, pale pink, mauve, or blue. But sometimes, they are much more pronounced: bright red, vivid pink, purple… The Phlox ‘Starfire’, with its red flowers, is among the most intense varieties! It also happens that the flowers are bicoloured. The centre of the flower (the eye) is often lighter or darker than the petal lobes. Sometimes, the flowers are striped, with coloured rays radiating from the centre, as seen in Phlox paniculata ‘Peppermint Twist’ or Phlox maculata ‘Natasha’. The variety ‘Sherbet Blend’ is very surprising with its flowers in anise green and pale pink! A truly unusual mix of colours… The Phlox divaricata offers a beautiful blue hue.

The flowers of most species measure between 1 and 3 centimetres in diameter. The corolla consists of five more or less fine petals, fused into an elongated tube. They give (especially when narrow) the flower a star-like shape. The Phlox drummondii from the ‘Popstars’ series offers impressive flowers due to their shape, with petals ending in long, fine points! The lobe of the petals is sometimes notched or bilabiate (Phlox bifida…). The tube of the corolla (petals) is surrounded by five fused sepals, and it contains five stamens. The Phlox ‘Blind Lion’ stands out for its extremely different flowering compared to other varieties! In reality, it doesn’t really resemble a phlox: it has very dark foliage and panicles composed of purple-black flower buds that do not open.

Many varieties have the advantage of offering fragrant flowers, particularly among Phlox paniculata and divaricata. This is what gives the poetic name to the variety ‘Clouds of Perfume’.

The flowering of phlox: Phlox paniculata ‘Peppermint Twist’, Phlox subulata (photo Anneli Salo), and Phlox drummondii ‘Popstars F1 Blue’

The foliage of phlox is quite ordinary. The leaves are entire, lanceolate for panicled phlox, and linear, much finer, for Phlox subulata. They are generally opposite, arranged two by two on the stems, but those at the top are sometimes alternate. The leaves vary greatly in size, from those of large panicled phlox that can reach up to 15 centimetres in length, to those of mat-forming varieties, like Phlox subulata, which do not exceed 2 centimetres long! The leaves are often ciliate along the edge of the lamina. They are relatively homogeneous and are distributed along the stem.

The leaves are green most of the time, but there are also some variegated varieties, such as ‘Becky Towe’: its leaves are green in the centre, and irregularly margined with yellow. Combined with its pink flowering, this variety is very bright and colourful! In panicled phlox, it is common for the foliage at the base to be unsightly, which is why it is often recommended to place other low perennials right in front to hide it.

The foliage of phlox: Phlox paniculata ‘Bright Eyes’, Phlox subulata ‘Scarlet Flame’, and Phlox paniculata ‘Becky Towe’

The leaves of Phlox paniculata are generally deciduous: the aerial parts of the plant (stems and leaves) dry out in autumn and disappear, to regrow in spring… But the foliage of moss phlox (Phlox subulata) is evergreen!

After flowering, phlox produce capsules that open by three valves when mature to release brown, oval seeds. The fruit is quite insignificant, as it does not have much decorative interest.

The fruits and seeds of Phlox paniculata (photo Nadiatalent)

The main varieties

Upright phlox for borders

Creeping phlox for rockeries

Annual phlox

These mainly include Phlox paniculata and maculata. They are intended for use in borders, behind shorter plants.

Phlox paniculata Blue Paradise

This is a variety that offers fragrant flowers, mauve in colour, more or less bluish. To be planted in a border or mixed-border!

• Flowering time August, September
• Height at maturity 90 cm

See the product sheet

Phlox paniculata Fujiyama

This phlox offers a lovely flowering of immaculate white, in rounded panicles borne at the top of the stems. It is fragrant! You can integrate it into a cottage garden, for example in association with blue flowerings.

• Flowering time August to October
• Height at maturity 90 cm

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Phlox paniculata Rosa Pastel

A very beautiful variety that bears bouquets of soft pink flowers, shaded by the centre of the flower in a deeper pink. It will find its place in a romantic border! It has been awarded the Award of Garden Merit by the Royal Horticultural Society (RHS).

• Flowering time August, September
• Height at maturity 75 cm

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Phlox paniculata David

This is a vigorous phlox that offers a beautiful white flowering. It is also one of the most resistant varieties to powdery mildew!

• Flowering time August, September
• Height at maturity 1,10 m

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Phlox paniculata Bright Eyes

This variety bears soft pink flowers, shaded by the centre of the flower in a much deeper pink! A phlox that can easily fit into a romantic-style garden. Moreover, it is not very susceptible to powdery mildew!

• Flowering time August to October
• Height at maturity 60 cm

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Phlox paniculata Orange Perfection

This phlox offers a brilliant, very bright flowering, consisting of orange-red flowers. It is perfect for bringing vitality to borders, and can also be used to compose bouquets!

• Flowering time August to October
• Height at maturity 80 cm

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Phlox maculata Natasha

This phlox has the particularity of offering a bicoloured flowering, with white and pink rays that start from the centre and extend outwards.

• Flowering time August, September
• Height at maturity 90 cm

See the product sheet

These include, among others, Phlox subulata, bifida and douglasii. They can form wide ground covers or small rounded cushions, covered in flowers in spring! They are perfect for planting in rockeries, on slopes or on low walls. They adapt well to stony and well-drained soils, and cope better with drought than the upright phlox.

Phlox subulata White Delight

This phlox bears pure white flowers in spring, giving it a bright and elegant appearance! It will find its place in a border or in a rockery.

• Flowering time May to July
• Height at maturity 15 cm

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Phlox subulata Candy Stripes

This is a bicoloured variety that forms carpets of a very soft hue: the flowers are star-shaped, composed of five pink petals edged in white.

• Flowering time May to July
• Height at maturity 10 cm

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Phlox bifida 'Minima Colvin'

This phlox forms cushions that are covered in spring with white flowers, consisting of petals whose tips split in two (bifid petals).

• Flowering time June, July
• Height at maturity 15 cm

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Phlox douglasii Waterloo

This phlox forms carpets composed of a multitude of deep pink flowers. It offers dense and evergreen foliage.

• Flowering time June, July
• Height at maturity 20 cm

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These are Drummond’s phlox, Phlox drummondii, a species to be sown in spring directly in place.

Phlox drummondii Moody Blues Seeds - Drummonds phlox

This phlox bears pretty flowers that come in different shades of blue, with the centre of the flower always in a deeper shade. Preferably place it at the front of a border.

• Flowering time July to September
• Height at maturity 25 cm

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Phlox drummondii Sugar Stars

A variety with superb flowers, with petals marked in blue-mauve on the outside and white at the base, creating a star at the centre of the flower.

• Flowering time August to October
• Height at maturity 50 cm

See the product sheet

Planting perennial phlox

Where to plant?

The majority of phlox thrive in sun or light shade. Sunny exposures will guarantee you a generous flowering, which is often a bit more fragrant! However, their exposure also depends on your geographical location: if you live in the northern half of France, plant them in the sun, but if you live in the Mediterranean region, they will appreciate a slightly shaded position during the hottest hours of the afternoon. Some species are more suited for planting in shade, for example in woodland, such as Phlox stolonifera.

Phlox do not like calcareous soils; they prefer slightly acidic or neutral conditions. They appreciate fresh, well-draining substrates, where water does not stagnate in winter. Also, avoid confined environments, which could promote fungal diseases.

Paniculate phlox prefer soils rich in humus, fertile, both light and deep. They will thrive in soil that remains cool in summer! They are quite suitable for planting in borders.

Small creeping phlox, such as Phlox subulata or douglasii, will prefer to be planted in dry, rocky, well-drained soil. They are perfect for rockeries and low walls! They also tolerate sea spray well.

When to plant phlox?

Install phlox in your garden preferably at the beginning of spring, around April, but you can also do it in autumn.

How to plant?

We recommend maintaining a distance of 30 to 40 cm between the plants.

1. Prepare the ground by digging a planting hole about twice the size of the root ball. Add well-decomposed compost, as phlox appreciate soils rich in organic matter.
2. Position the phlox in the planting hole.
3. Replace the soil all around and lightly firm it down.
4. Water.

We suggest installing a mulch to maintain soil freshness.

Continue to carry out regular watering in the weeks following planting.

Sowing annual phlox

Unlike perennial species, annual phlox should be planted in the garden by sowing directly in the soil. This mainly refers to Phlox drummondii. It does not like to be transplanted: it is better to sow it directly in its final location. You can do this in spring, around May.

1. Prepare the seedbed: add some well-decomposed compost, work the soil by breaking up large clumps and removing weeds.
2. Sow the seeds.
3. Cover with a thin layer of potting soil. You can lightly firm it down.
4. Water gently.

Keep the soil moist by watering regularly. When the young plants start to grow, thin them out by removing some of the shoots, leaving one plant every 20 cm.

Caring for Phlox

Water paniculate phlox regularly, especially during summer… They prefer soils that remain cool, as in the wild they grow near rivers. If they suffer from drought, there is a risk that the flowering will diminish and the foliage will become unsightly. Apply a mulch to help maintain soil moisture. This will allow you to space out your watering more. Subulate phlox are more drought-resistant. They do not like excess moisture.

It is advisable to remove faded flowers, as this will encourage the appearance of new buds, and it is likely to prolong the flowering period. The taller varieties (Phlox paniculata) may require staking.

As phlox enjoy rich soils, we also recommend adding some well-decomposed compost at their base in autumn, possibly incorporating it superficially into the soil by scratching. This will promote abundant flowering! You can also apply an organic fertiliser in spring.

Divide paniculate phlox approximately every four years to rejuvenate the clumps. Indeed, after a few years, these clumps become exhausted and their flowering becomes less abundant… This will allow you to replant them elsewhere in your garden or to give some to your friends and family!

The tall species with an upright habit, such as Phlox paniculata, should be cut back after flowering. Each year, at the end of autumn, trim the stems to 5 or 10 cm above the ground. You can also prune phlox during the growing season to maintain a more compact, bushy habit.

Diseases and Pests

Phlox are sturdy perennials that live long in our gardens. It is quite common for them to be affected by powdery mildew in summer. This fungal disease, which is not a real nuisance for growing phlox, manifests as a white fluff on the leaves. The most sensitive seem to be Phlox paniculata, while Phlox divaricata and maculata are more resistant. However, even among Phlox paniculata, some varieties are more resistant than others. Among them, we can mention ‘David’, ‘Eva Cullum’, ‘Bright Eyes’, ‘Peppermint Twist’, or ‘Robert Poore’. To prevent powdery mildew, it is better to avoid planting too densely, allowing air to circulate, and to water by directing the stream at the base of the plant, without wetting the foliage. Once the disease is observed, you can remove the affected leaves to limit its development and treat using sulphur.

Phlox can also be affected by septoria, which results in brown spots on the leaves, as well as by verticillium wilt, which causes wilting of the foliage and leads to the plant’s decline. However, these two diseases are less common in phlox than powdery mildew.

They can also be attacked by nematodes (usually in summer): these are microscopic worms that enter the stems and cause the plant to decline. The stems twist, become swollen and brittle, flowering is poorer, and the foliage distorts. You can remove the affected plants to prevent nematodes from reaching those that are still healthy.

Regarding other pests, phlox can be attacked by leaf miners, and slugs and snails sometimes nibble on the still tender young shoots.

Multiplication: sowing, propagation by cuttings

Phlox can be multiplied by sowing, propagation by cuttings, and division. For perennial species, we recommend division, while sowing is more suitable for annual phlox. Phlox divaricata tends to layer spontaneously when its stems touch the ground, and Phlox stolonifera spreads through its stolons. For these two species, it is therefore easy to take fragments to propagate the plant.

Sowing

You can collect seeds from phlox to resow in spring.

Sow annual species (notably Phlox drummondii) directly in place around May.

1. Prepare the soil to create a fine seedbed, free of clods, large stones, or weeds, and take the opportunity to add some compost.
2. Sow by spreading the seeds on the surface.
3. Cover them with a thin layer of potting soil.
4. Water gently.

Continue to water regularly. Thinning will be necessary later.

Sow paniculate phlox in March, under cover.

1. Fill a pot with potting soil and lightly firm it down.
2. Sow the seeds, spacing them 2 to 3 cm apart.
3. Cover them with a thin layer of substrate.
4. Firm down and water.
5. Place the pot under a cold frame.

Ensure that the potting soil remains moist until germination. You can plant them in the garden around May.

A sowing of Phlox paniculata (photo Nadiatalent), the young shoots of Phlox paniculata ‘Tatjana’ (photo Bff), and Phlox paniculata ‘White Eye Flame’

Tuft Division

You can divide Phlox paniculata and maculata approximately every four years. This has the advantage of regenerating the clumps. They become more vigorous and have a more generous flowering. We recommend doing this in early spring, around March-April, or in autumn.

1. Choose a sufficiently large clump, several years old, and dig it up.
2. Divide the stump into several parts, prioritising those on the outside.
3. Replant the fragments immediately in the desired location, after preparing the ground. You can replace one fragment in the old location and the rest in new spots in the garden, or give them away!
4. Water generously.

Propagation by Cuttings

You can multiply Phlox subulata by propagation by cuttings. It is advisable to disinfect the tools beforehand to avoid spreading diseases.

Phlox paniculata propagates easily by root cuttings, around December.

1. Prepare a tray by placing a mixture of potting soil and sand. Then moisten the substrate.
2. Dig up a phlox stump, being careful not to damage the roots.
3. Remove the soil to expose a root (you can rinse it with water if necessary), and cut it into segments four or five centimetres long, each bearing a few small roots. Make clean, sharp cuts.
4. Place them in the tray horizontally.
5. Cover them with a few centimetres of substrate.
6. Keep your cuttings sheltered from frost.

The substrate should remain cool but not overly moist. You can plant your cuttings in the ground around April.

You can also take stem cuttings in autumn, after flowering:

1. Take segments about ten centimetres long.
2. Remove the basal leaves, keeping only those at the top of the stem.
3. Dip the base of the stem in plant hormone.
4. Plant in a pot filled with previously moistened potting soil.
5. Firm down all around.
6. Place a plastic bag over the pot to create a humid atmosphere for the cutting.

Associating Phlox in the garden

Phlox paniculatus find their place in borders or mixed borders, alongside other summer-flowering perennials. As they are quite tall, we recommend planting them behind shorter plants… Especially since the lower leaves of phlox paniculatus are not very attractive! The ideal solution is to hide the base by placing shorter flowers right in front, such as hardy geraniums. For a delicate, somewhat romantic atmosphere, feel free to plant them alongside roses, sages, or Calamintha grandiflora! Phlox also pair very well with sages, whose flowers are often blue or mauve… You can easily find them in similar shades! You can also place them alongside Polemoniums, carnations (Dianthus deltoides), ageratum, or campanulas…

An idea for a combination: Rose ‘Gertrude Jekyll’, Calamintha grandiflora, Salvia nemorosa, and Phlox cardina ‘Bill Baker’ (photo Friedrich Strauss – MAP)

As phlox often offer blooms in pastel shades, you can pair them with other pink or white flowers. The brightest varieties can be included in colourful borders, or bring vibrancy back to a somewhat pale border! You can create beautiful contrasts by pairing purple phlox with orange flowers, two colours that enhance each other for an intense and dynamic effect!

Don’t hesitate to mix different varieties of phlox to enjoy a longer flowering period!

Phlox subulata are excellent rock garden plants. You can create a dry border on a mound by adding large stones and a well-draining substrate. Plant rock phlox alongside wall bellflower (Campanula portenschlagiana), saxifrages, Cymbalaria muralis, Arenaria montana, or Aster alpinus! Add some small geraniums, such as Geranium cinereum or Geranium renardii! They can also be combined with the decorative foliage of Acaena!

You can also use Phlox subulata to flower an old stone wall! Or place them at the front of a border where the soil is well-draining. Take advantage of them to colour a somewhat dry area of the garden that is difficult to vegetate. They will create a stunning pink or mauve carpet in spring!

Did you know?

• Edible flowers

Flowers of phlox paniculata are edible: you can add them to fruit salads or use them to colour your dishes! They have a sweet and spicy flavour.

• A Japanese Festival to Celebrate the Flowering of Phlox!

In Japan, the Fuji Shibazakura Festival takes place every year in April-May, where thousands of visitors come to admire the carpets of Phlox subulata, planted in a 2.4-hectare park, with a view of Mount Fuji in the background! The phlox create large, colourful flowerbeds around Lake Motosu, in shades of soft pink, dark pink, and white. In Japanese, these moss phlox are called Shibazakura, which could be translated as “short grass meadow cherry tree”, due to their creeping habit and abundant pink spring flowering!

The Fuji Shibazakura Festival