Phlox have long been a favourite border plant, the herbaceous perennial types providing upright stems topped with masses of often fragrant flowers in many colours. More recently shorter border types have been bred, removing the need to stake and generally providing greater resistance to powdery mildew. Then there are the less well-known, lower-growing types such as P. subulata and P. douglasii that suit the rock garden.
There are around 67 species and a wide variety of types — annuals or perennials, herbaceous or shrubby and evergreen or deciduous. Nearly all of the species come from the USA. The most well-known is P. paniculata, from which most of the tall border types are derived. Other tall herbaceous types include P. maculata and P. glaberimma.
Breeders have produced shorter varieties that are said to have greater resistance to powdery mildew, which can be a problem for phlox. Series to look out for include Cocktail, Flame, Volcano and Junior. There are also now many more varieties with variegated foliage, which extends the season of interest.
P. ‘Norah Leigh’ has a lovely leaf with irregular cream margins and subtle white flowers with a pink-to-lilac eye. A newer variegated type that is very popular is the more brash P. ‘Becky Lowe’, which has more yellowy/cream margins and salmon-pink flowers.
There are some lovely low-growing types that are less widely grown but known to the connoisseurs, especially rock garden and alpine enthusiasts. These include the vigorous species P. subulata and the less rampant P. douglassii, both of which have needle-like evergreen leaves and form a dense carpet of foliage then a mass of flowers that looks effective when spreading across a rock garden or at the edges of a path. There are also small cushion types such as P. caespitosa that look good planted in alpine troughs.
Generally, phlox can be grown in full sun or partial shade, with the tall border varieties liking a damp, fertile soil and the rock garden types a slightly more well-drained soil. The border types are best divided about every third or fourth year in early spring to maintain vigour and flower quality. Mulch them annually with well-rotted organic matter as well.
Phlox are susceptible to powdery mildew, grey mould and fungal leaf spots, so this can be frustrating. Watch out for pests such as stem eelworm on P. paniculata that causes swollen shoots and narrow distorted leaves. Destroy infected plants and do not replant in the same spot.
What the specialists say
Harriet Hardiman, director, Hayloft Plants, Worcestershire
"There are now so many different varieties of phlox but the paniculata varieties ‘Ice Cream’ and ‘Purple Elite’ are our current favourites. It is wonderful to watch passers-by turn for a second look to see the enormous flower heads — around three times larger than usual phlox — produced from June to September. Of course, the bigger the blooms, the more perfume is given off in clouds that waft around your garden welcoming passing bees and butterflies.
"Like most phlox they are very easy to grow, preferring part shade, slightly sheltered from any hot summer sunshine. They are also excellent as cut flowers so you can enjoy the sweet scent in your home as well as in your garden. Largely pest- and disease-free, they are unfussy about soil, whether grown in borders or containers. Despite being thought of as a cottage garden perennial, they also look fantastic in a more modern planting scheme."
Peter Reason, owner, Boundary Nursery, Huntingdon
"We specialise in the alpine types of phlox. They have some amazing bright colours, which are excellent for brightening up the spring, and are hardy and easy to grow. P. subulata forms a green carpet, creating good ground cover. If the weather is mild enough, flowers may be seen in February from varieties such as ‘Marjorie’ and ‘Scarlet Flame’. But some are scruffy through their non-flowering stage, so they need to be put alongside other plants.
"P. douglasii make much smaller and lower mounds. They can produce some stunning colours in spring but again the foliage can look tatty when not in flower. Another good variety is P. × procumbens. Like many phlox, it attracts pollinators — particularly bees. ‘Variegata’ has unusual pink, white and green variegated leaves, with pink flowers from mid spring to early summer.
"Phlox can get attacked by aphids when they put on their new growth in spring. They are fairly resilient, unless there are huge numbers. Eelworm can also be a problem, causing very stunted growth with yellowing foliage. One characteristic giveaway is when a plant is very stunted,with no real lush growth apart from one stem that is growing off to one side. The plant should be destroyed because eelworm is very difficult to eradicate with chemicals. Always choose lush green stock plants that have an even growth and no stubby yellow branches."
David Anderson, general manager, Keston Garden Centre, The Garden Centre Group
"Phlox in bloom are a sight to behold, with masses of small, star-shaped, colourful flowers blanketing the plants. At The Garden Centre Group we sell several varieties throughout spring and summer.
"There are several types, the most common of which are spring-blooming creeping phlox and summer-blooming tall phlox. The creeping phlox is sold in the alpine section and in my opinion it’s far better than any of the popular varieties of aubrietia. It looks great as under planting, on rockeries and in border edges.
"However, undoubtedly the stars of the show are the impressive summer-blooming varieties bred from P. paniculata. We normally stock the shorter Flame series. At my centre we merchandise this highly scented, colourful plant in blocks of colour on hexagonal tables near the entrance, so that when customers approach they are overwhelmed by the intoxicating fragrance.
"Another good merchandising practice is using combination planting with foliage perennials around them. Heucheras and hostas enjoy similar conditions. Choose heucheras with patterned leaves such as ‘Regina’ or with one-colour foliage in unusual colours like ‘Caramel’ to complement the subtle tones of the phlox."
Species and varieties
P. carolina is an herbaceous perennial type that is quite tall at around 1.2m high. It has long leaves and purple or pink flowers in the summertime. Height: 1.2m.
P. carolina ‘Bill Baker’ Award of Garden Merit (AGM) (H5) has long, lance-shaped leaves and clusters of pink flowers in early summer. Height and spread: 50cm.
P. divaricata subsp. laphamii — wild sweet william or woodland phlox — is a spreading, semi-evergreen perennial. It has hairy
and sticky stems, lance to elliptic shaped leaves and loose clusters of fragrant, tubular, blue/violet flowers. Suits planting in woodland areas. Can form large colonies over time as the leafy shoots spread along the ground, rooting at the nodes.
P. divaricata subsp. laphamii ‘Chattachoochee’ AGM (H4) is a low-growing, semi-evergreen perennial with purplish stems, lance-shaped leaves and terminal clusters of slightly fragrant, purple-eyed, lavender/blue flowers. Height: 15cm.
P. douglasii, or the tufted phlox, is a native of the northwestern USA. It is very different to the general garden phlox, being very low-growing and forming cushions of needle-like dark-green leaves. The plants’ flowers may be purple, pink, pale lavender or magenta red. Prefers sun and can be grown in a variety of different soils. Height: 10cm.
P. douglasii ‘Crackerjack’ AGM (H5) is a popular variety with intense pink flowers from April to June and again in September. Height: 5cm. Spread: 25cm.
P. maculata is an upright, herbaceous perennial type with slender, hairy stems and clusters of lavender/rose, lilac/pink or white flowers. The stems often have burgundy spots. Height: 50-90cm.
P. paniculata is an American species from which most of our border phlox are derived. In its native habitat it can reach 1.8m, but garden cultivars are usually around 1.2m. The flowers of the species are usually pink. Prefers partial shade in dampish soil. There are hundreds of varieties. ‘Starfire’ (H7) and ‘Prince of Orange’ (H7) are among those with an AGM.
P. subulata, or moss phlox, is another low-growing, evergreen perennial type with cushions of needle-like, hairy leaves. Native to eastern and central USA, it produces small five-petaled flowers in rose, mauve, blue, white or pink. Prefers sun and tolerates a range of soils and coastal climates. Height: 15cm. Spread: 50cm.
P. subulata ‘McDaniel’s Cushion’ AGM (H5) is probably the most popular variety of the moss phlox. It forms a compact mat of fresh green linear leaves and produces large, very strong pink flowers from late spring. Height: 10cm.
P. × procumbens ‘Variegata’ is a robust plant that forms low cushions of green and cream variegated foliage, suffused with pink. Also produces vivid rose-pink flowers in May and June. Height: 10cm.
Thank you to Floramedia, which supplied the images for this article from its photo library