How to Plant & Grow Garden Phlox (Phlox paniculata)


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Garden phlox plant with pink flowers
Garden phlox flowers come in many different colors, though wild varieties are usually pink or lavender. Niccolò Caranti, CC BY-SA 4.0, via Wikimedia Commons

Phlox paniculata is a stunning perennial with a lengthy blooming period. Considered a classic option for flowering borders, it is commonly known as garden phlox, summer phlox, border phlox, and perennial phlox. It is classified under the Polemoniaceae family which includes about 25 genera of popular ornamentals, distinguished from other flowering families by their fused carpels.

Phlox is known for thriving in many habitats and for having varied growth forms. Most species are native to North America, but they are now cultivated across the globe.

A valuable plant for its fragrant and vivid blooms, P. paniculata produces full inflorescences, grouped as panicles, from summer to fall. They range in color from white to red and all hues in between. Wild varieties tend to have pink or lavender flowers. Some cultivars produce bi-colored ones, with contrasting centers, stripes, or alternately colored lobes. When pollinated, these develop into dry seed capsules. They are borne on notably erect and branching stems, which can reach a mature height of 2 – 4 feet and a spread of 2 – 3 feet. Dark green, lanceolate leaves occur opposite one another and may turn black or die back in winter.  

Facts, Benefits & Uses of Garden Phlox

A green sweat bee resting on a garden phlox flower
Many pollinators, including this green sweat bee, are attracted to the colors and fragrance of garden phlox flowers. USFWS Midwest Region / No copyright

Garden phlox is a versatile species that can be cultivated as a ground cover or bush-like plant. Taller cultivars can be pruned to medium height and used to bring interest to bare spaces. Fairly low maintenance, this summer favorite brings high-impact colors, especially when it receives full sun. Unsurprisingly, it attracts all sorts of pollinators, including butterflies, moths, birds, and hummingbirds. They are drawn to both the colors and fragrance (caused by terpene emissions) of the delicate flowers.

Phlox stands should be situated close to a consistent water source or be watered regularly. Although they favor well-draining substrates, their roots are averse to extended periods of dryness. Thus, proximity to a pond or water garden should be beneficial. Once minimal requirements are met, their continuous growth and propagation are fairly simple. Dozens of cultivars have received the RHS Award of Garden Merit and are extremely popular with hybridizers.

P. paniculata isn’t just valued for its looks. Like many other vivid and fragrant plants, it has medicinal uses as well. Extracts from its leaves can supposedly treat boils or may serve as a laxative and treatment for stomach pains. As they are edible, the flowers are sometimes used to decorate cakes or fancy cocktails. Needless to say, they also look stunning in flower arrangements!

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Garden Phlox Fact Sheet:
PLANT TYPE
Herbaceous perennial
HARDINESS ZONES
USDA 4 – 8
LIGHT REQUIREMENTS
Full sun to light shade
BLOOM COLOR
White, pink, red, lavender, blue
BLOOM PERIOD
July to September
MAXIMUM GROWTH
4 feet
PLANTING DEPTH
Seeds just below surface; around 4 inches for cuttings
WATER QUALITY
pH 6.1 – 7.8

Garden Phlox Growth, Hardiness & Climate

Group of pink garden phlox plants
Mild conditions in USDA zones 4 – 8 and full sun exposure are best for garden phlox. F. D. Richards / CC BY-SA 2.0

In the wild, P. paniculata favours conditions in alpine regions, forests borders, woodlands, and prairies. Cultivars generally prefer full sun exposure, but may also thrive in a lightly shaded location. Sufficient ventilation is crucial for keeping the plant healthy, as pests can infest the shoots when they are too crowded. Individual plants will need to be spaced apart, if not trimmed back regularly. Branches can spread out and create dense stands.

Many preferences, such as soil pH and temperature, tend to vary according to cultivar. Mild conditions in USDA hardiness zones 4 – 8 are best for this species. A temperature range of 20 – 23˚C (68 – 73˚F) and around 6 hours of bright light each day should stimulate an abundant growth period, especially if the soil is kept moderately moist. To induce continuous branching and vegetative growth, the plant should be pruned regularly through the peak growth period. When daylight hours are shortened, it may enter a period of dormancy.


How to Plant Garden Phlox

Phlox paniculata seeds and fruit
Garden phlox can be planted with seeds, but it’s easier to use cuttings or divisions. Nadiatalent, CC BY-SA 3.0, via Wikimedia Commons

P. paniculata can be planted using its seeds, vegetative cuttings, or rooted divisions. Seeds must be cold stratified before they are planted. They should be packed into a clean bag, mixed with sand, and lightly moistened before placing them in a refrigerator for about 1 – 2 months. They can then be sowed onto a sheltered setup with sterilized compost, and then covered with a thin layer of soil. Germination can take 1 – 2 weeks when temperatures range from 15 – 18˚C (59 – 64˚F). This process should ideally be done in the fall so that seedlings can be out-planted the following year.

Propagation using cuttings and divisions is much easier. Tip cuttings should be at least 4 cm long. To hasten root production, they can be treated with a rooting hormone and then placed in a setup that receives around 12 hours of light. If light levels are insufficient, delayed rooting may result in non-uniform cell production. Relative humidity and substrate temperature should also be boosted to obtain desirable growth rates.

Once seedlings or cuttings are large enough to withstand outdoor elements or have developed their first branches, they can be transferred to their permanent outdoor positions. Make sure to space them about 12 – 18 inches apart to leave room for their branches. Branching can be induced by “pinching” young plants. This is done by removing the growth just above a node with buds. Make sure you leave the buds fully intact while doing so.


How to Care for Garden Phlox

Spider mites on a plant
When conditions are too hot or dry, spider mites can cause big problems for Phlox paniculata. If the infestation is not severe, you should try to manually remove them. Toby Young, CC BY-SA 4.0, via Wikimedia Commons

Garden phlox can be quite challenging to maintain, especially in humid areas. Its shoots tend to be sensitive to powdery mildew. As this may be caused by moisture accumulating on the surfaces of crowded branches, it is advisable to avoid watering the crown of the plant. Instead, target the soil just underneath.

Spider mites are also a major pest problem for this species when conditions are too hot or too dry. Manually remove them using rubbing alcohol or a mild soap spray. If infestations are severe, the plant may have to be treated with an organic pesticide.

On a lighter note, spacing out individual plants and regularly pruning them can greatly reduce the chances of pest infestation. This also affords the branches more sunlight, leading to a higher abundance of flower heads. Once spent, these can be deadheaded to encourage a second blooming period. You’ll find that garden phlox is a truly generous plant when it comes to providing your garden with summer and fall interest.


How to Winter Garden Phlox

Towards the end of fall, once the flower heads are completely spent, you will need to prepare your phlox for winter conditions. If your area experiences mild snowfall, trimming back the branches may suffice. In areas that experience extended frosts and heavy snowfall, the entire shoot system may have to be cut back to just 1 – 2 inches above the soil.

Cover the base of the plant in mulch to protect the roots from freezing. This species is technically evergreen, so some cultivars may survive without being cut down. However, they are likely to attract pests and diseases if their leaves and shoots deteriorate in the cold.


Is Garden Phlox Invasive or Toxic?

P. paniculata is a well-behaved, non-toxic species that rarely competes with or smothers neighboring plants. The extensiveness of its spread can easily be controlled by regular plant maintenance and by deadheading flowers.

Unfortunately, it is often confused with a highly invasive species called Dame’s Rocket (Hesperis matronalis). The latter is extremely troublesome in woodland areas due to its self-seeding capacity. It is fairly easy to differentiate between the two if you’re able to peer closely at the blooms. Unlike P. paniculata, H. matronalis has 4-petaled flowers and alternately arranged leaves.


Is Garden Phlox Edible? Do Animals Eat it?

Among the Phlox species, P. paniculata is the only one that is considered edible. This beautiful plant can be placed in gardens that are frequented by pets and wildlife. Its flowers are often candied or used as a colorful garnish in salads, desserts, and soups. Their taste, which is said to be slightly spicy and sweet, is likened to that of sugar snap peas. Extract from the rest of the vegetative parts can be used to make tea for the treatment of indigestion.

The leaves and flowers of garden phlox are definitely a treat for many wild animals. Cottontail rabbits, white-tailed deer, and groundhogs will happily graze on the plant, and may, unfortunately, leave it free of blooms. You will have to protect your phlox bushes from these foragers by using natural repellents or by fencing in your garden.


Where to Buy Garden Phlox & Seeds? (UK & US)

Phlox paniculata can be purchased as bare root or potted plants from plant nurseries and garden stores throughout its native range. Due to its popularity and easily manageable spread, its seeds are sold in packets all over the world. Keep an eye out for stunning P. paniculata cultivars, which include ‘Candy Twist’, ‘David’, ‘Neon Flare Blue’, and ‘Younique Mauve’. Investing in a handful of cultivars is always worth it as the variation in floral color is an attraction in itself.

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