How to Grow Phlox

A perennial favorite. Phlox is a great choice if you want to add ground cover as well as floral color to your garden. The plant’s star shaped flowers appear in pleasingly fragrant clusters, drawing attention to the flower bed. Taller varieties of the plant are also a great way to introduce height and soft structure to a flower bed or border.

As well as being a reliable summer flowering plant, phlox is also pleasingly easy to care for. Here is everything you need to know.

The colorful, fragrant flower clusters add color and interest to a garden. Planting taller varieties helps to add structure and height to a flowerbed. 

Different Varieties of Phlox

There are 3 main varieties of phlox: tall, medium and low growing. The vast majority of cultivars are hardy in USDA zones 4 to 8.

Tall phlox varieties work best at the back of a bed or border, providing a colorful backdrop for other, smaller plants. Garden or Summer phlox is one of the tallest varieties. In ideal conditions it can reach 5 ft in height. Flowering from mid summer onwards Garden phlox does best in full or partial sun positions.

Many tall varieties are also resistant to powdery mildew. One such cultivar is the pleasingly attractive Jenna. Producing small, purple flowers, the plant is similar in appearance to the butterfly bush. David is another reliable disease resistant variety that produces attractive bright white flowers.

Phlox comes in a range of colors and sizes. This versatility has helped to make the plant a popular member of the flower garden.

Medium height varieties are a great way to fill gaps in your flower beds. Annual or Drummond’s phlox is one of the most popular annual varieties. This variety does best in partial sun positions where it can reach a height of about 2 ft.  Traditionally the Drummond’s cultivar struggled in warmer areas. However there are now heat tolerant varieties available. This means that gardeners in warmer zones can successfully grow the cultivar.

Finally, the low growing varieties are a great way to introduce colorful ground cover options to the garden. The most commonly grown is Creeping or Moss phlox. A great ground cover option, this variety spreads in mounds that are about 5 inches thick, carpeting the ground as it grows. In the spring flowers emerge, covering almost all of the plant’s foliage. This looks particularly effective if allowed to trail over a container, living wall or box planter. Creeping and moss varieties prefer partial or full sun positions.

The cultivar Emerald Blue produces attractive lilac-blue flowers, creating a waterfall effect. For something a little different try the white and pink flowering Candy Stripe.

Creeping varieties are a great ground cover option. They also look really effective if allowed to sprawl over the edge of a container or wall. 

Phlox Characteristics 

Garden phlox or Phlox paniculata is native in the United States from New York down to Iowa, Arkansas, Mississippi, and Georgia. It slowly escaped from gardens and naturalized into a host of areas outside of the natural range. If you look for it in Missouri, you’ll find it south of the Missouri River because this area boasts thickets, rich low woods, gravel bars by streams, alluvial banks, and bluff bases. 

It’s an upright perennial that grows in tight clumps that can get up to four-feet tall and up to three-feet wide. It has stiff stems with opposite, lightly veined, deep green elliptic leaves that are four to six-inches long. It grows tubular, fragrant purple-pink flowers with white florets that get densely packed onto very large, domed, tiered clusters from July to September. Each floret comes with a very long corolla tube with five petal-like, flat lobes. Hummingbirds and butterflies swarm to these flowers. 

A lot of people grow different cultivars of this plant, including lavender, white, rose, pink, red, and bi-colored. It’s usually a good idea to get cultivars that can resist damage due to powdery mildew. The genus name comes from the Greek word phlox. This means flame, and it references the flowers’ brilliant colors. 

How to Grow from Seed

You can purchase phlox as young plants ready for transplanting into the garden. You can also grow the plants from seed. Starting from seed may take longer but it is often more affordable. It also allows you to choose from a greater variety of plants.

Start seeds undercover early, about 6 weeks before the last frost. Fill small pots or containers with fresh potting soil. Plant one seed per container.

Lightly cover the seeds with soil, water and place in a propagator or plastic bag. This allows you to create a mini greenhouse, helping to encourage germination. A propagator with humidity vents is a great way to regulate the growing conditions, creating an ideal environment for seed germination.

In ideal conditions, germination takes between 4 and 5 weeks.

Following germination remove the containers from the propagator or plastic bag and apply a diluted dose of all-purpose fertilizer. Water well and place on a bright windowsill.

After the last frost has passed, harden off the plants before planting out.

How to Plant

The best time to transplant young plants is in the spring, after the threat of frost has passed. Remember to harden off plants before transplanting.

Phlox does best in rich, well-draining soil that is evenly moist. Light requirements vary depending on which variety you are planting. Most varieties prefer full or partial sun positions.

Before planting dig over the soil with a shovel, to a depth of about 15 inches. Mix in a 3 inch layer of compost. This helps to enrich the soil as well as improving drainage.

Dig a hole large enough to comfortably hold the plant. If you are unsure a good rule of thumb is that the hole should be twice as large as the pot currently containing the plant.

Carefully remove the plant from the container and place in the hole. If the plant is difficult to remove, squeeze the side of the pot. This loosens the soil, helping you to slide the plant out of the container without damaging the root system.

Position the plant so that the top of the root ball is level with the surface of the soil. Fill in the hole around the plant. Gently firm down the soil, being careful not to compact it. Water well.

If you are planting a number of plants, space them 18 to 24 inches apart. The exact spacing depends on the growth habit and spread of the variety you are growing. To find the right spacing for your chosen variety consult the seed packet or plant information label.

Planting in Containers

Phlox also grows well in planters or containers. Low growing varieties such as creeping phlox look particularly effective if they are allowed to drape over the edge of a container.

Always plant in clean containers. This helps to prevent disease spreading around your garden. The containers should also have drainage holes in the bottom. Planting in self watering pots allows you to enjoy the flowers without the need to regularly water your plants.

Fill your containers with well draining, all purpose soil.

Plant as you would in the ground, spacing at least 6 inches apart. Don’t overcrowd the container. Crowded plants can struggle to flourish and are often stunted. They are also more prone to disease.

After planting apply water soluble or liquid fertilizer. This helps plants to acclimatize and promotes flowering.

Place the containers in a light, full sun position.

Phlox Care Tips

Once planted phlox is pleasingly easy to care for.

Taller varieties in exposed positions may require staking or some other form of support. This is best positioned when planting. Bamboo stakes, such as these by Hydrofarm, are a robust, natural way of supporting taller plants.

When to Water

Water the plants once a week during dry periods. Aim to keep the soil evenly moist.

When watering try to water only the base of the plant, keeping the foliage as dry as possible. This helps to prevent diseases from striking. An expandable hose is a great way to easily water larger gardens.

Applying Fertilizer

Apply a shovelful of compost or a light application of an evenly balanced fertilizer to the soil as you plant. This can be repeated just as the flowers begin to open. Fertilizing a third time, as the flowers fade, may encourage a final flowering.

An application of compost or balanced fertilizer helps to encourage healthy growth and a bountiful flowering habit. 

Mulching Your Plants

Apply a thin layer of compost and a layer of mulch, about 2 inches thick, around the plants every spring. This not only gives the plants a nutrient boost, it also helps the soil to conserve moisture.

Winter Care

Cut back the stems to just above soil level after the first harsh frost. Discard the foliage.

In colder regions, before the ground freezes completely, apply a protective layer of mulch to the base of the plant. Remember to remove the mulch in early spring, once the last frost has passed. This helps new growth to emerge.

When to Prune

Prune away spent flowers with garden scissors. This helps to keep plants neat and healthy as well as encouraging new flowers to emerge. Deadheading also prevents plants from reseeding and spreading unchecked throughout the garden.

If you want to encourage a bushier plant, with more flower heads, cut  the stems back by about a third in early summer.

In the fall, once flowering has finished, cut tall and medium sized plants back so that they are about 2 inches above the soil. This helps to prevent frost from killing the plants.

Divide taller plants every 2 or 3 years. Dividing plants helps to prevent beds or borders from becoming overcrowded. It also keeps the plants healthy and free from disease.

Deadheading Phlox 

Phlox can and will reseed itself, but deadheading it can stop a large amount of this process. Deadheading is a common practice by gardeners who want to confine a plant’s spread, but it could also cause any future seedlings you have to get too weedy and not bloom correctly. On the other end of the spectrum, practicing deadheading on your phlox allows the plants to focus on blooming while keeping the plant’s main crown healthy. 

Since there are both benefits and drawbacks of deadheading phlox, it’s a personal decision on whether or not you do it. Not doing so won’t necessarily encourage the plant to bloom again. 

How to Propagate Phlox Plants

Division is an easy way to propagate new phlox plants.

To divide, dig up existing plants in late summer or early spring. Carefully brush away any dirt from the root system.

Separate the root clump into even sections. Each section should have some roots and at least 3 shots. If this is difficult to do by hand use a shovel or sharp knife. A whetstone is a great way to keep garden tools sharp.

Replant divisions immediately.

Source: https://www.youtube.com/watch?v=nlzzJ0bszY0 

Phlox can also be propagated from seed. To do this, instead of deadheading, allow some of the flowers to die on the plant. As the petals fall, a seed pod becomes noticeable. Once the seed pod becomes brown and shrivelled, cut it from the plant.

Place the pods in a paper bag and allow to dry. To hasten the process, place the bag in a warm, airy room. Dry pods are easily broken apart.

When the pods are dry, open them up and remove the seeds. Place the seeds in a paper envelope. Remember to label the envelope with the date as well as the name of the seed. Older seeds are less viable and harder to germinate than young seeds.

Store the seeds in a dark, well ventilated place until you are ready to sow.

Winterizing Phlox 

If you’re someone who lives in a colder region where the ground freezes during the winter, you can take steps to winterize this plant. You can apply a layer of mulch around the plant’s roots before the ground freezes. You can also prune your phlox for winter by cutting it back once the flowers start to fade. You can prune the plant late in the summer through the fall months to avoid the plant reseeding. 

Common Pests and Diseases

Powdery mildew is a common problem. Correctly spacing the plants, and keeping the foliage as dry as possible when watering, helps to prevent the issue. Cutting back affected stems after flowering has finished helps to prevent the disease from spreading.

If powdery mildew continues to be a problem a number of varieties are now mildew resistant.

Other common issues include rust and various leaf diseases. Most of these can be prevented by adopting good growing practices such as correctly spacing the plants and keeping the foliage as dry as possible when watering.

Aphids and spider mites can target plants. Regularly check plants for signs of infestations.

Wash away pests with a blast from a hosepipe. More persistent, or larger, infestations can be treated with an application of neem oil. You can also make your own insecticidal soap. This is an effective way to treat infestations without using potentially harmful chemicals.

Bright and reliable, phlox is a great way to add color and structure to a floral garden. The plant’s extended flowering period, often flowering for the entire summer, means it is also a popular plant for pollinators. 

How to Use Phlox in Your Garden 

There are several ways that you can incorporate phlox into your garden or containers without having it take over or overshadow your other plants. They’re especially helpful to have in your garden during the midsummer months when the early bloomers are starting to fade for the season. 

Consider adding phlox to your garden in any spaces along the sides or back where you could use a welcome touch of color well into September. They look striking when you incorporate them into cottage gardens, mass plantings, pollinator patches, meadow gardens, containers, and in cut flower gardens. 

You do have to pay attention to the type of phlox you have because it can grow tall enough to overshadow other plants. If you do go with taller varieties of phlox, plant it in the back of the garden or in the middle of a border. Place lower-growing cultivars in front of it to create a very full and lush look. You can also mix it with flower shrubs, and abelia is a good choice. 

Phlox Frequently Asked Questions

Even though phlox is a relatively low-maintenance flower, it’s common to have questions about growing and caring for it. We’ve picked out some of the most common ones and answered them for you below. 

1. When can you expect phlox flowers to bloom?

There are a range of phlox flowers, and most of them will bloom at different times during the year. Taller phlox cultivars that can easily grow from 10-inches to 4-feet high usually bloom throughout the summer months. There are several cultivars that bloom during the early spring months, but the Intensia and Astoria varieties tend to have the most intense blooms. You can expect flowers from them to appear later in the fall months, and they can even last through mild winters. 

2. Are phlox sun or shade plants?

Many people pick out this plant because it’s a very low-maintenance choice for growing conditions. It tends to do best when you plant it in sunny, cool climates with a very well-watered soil medium. However, they can do well in partial shade to full sun. You’ll want your phlox to get a minimum of six hours of direct sunlight every day in cooler climates. In hot and dry climates, make sure that they go into a partially shaded area so they don’t burn. 

3. How quickly can your phlox plants spread?

This is a very reliable perennial plant that does very well when you plant it as a ground cover because it produces masses of flowers when it blooms. However, the plant isn’t an aggressive spreader like mint or bamboo. Under normal conditions, phlox will grow at a slow or moderate rate. So, the chances of it spreading quickly and taking over are very slim. 

4. Will phlox come back each year?

Yes. Phlox is a very low-maintenance and beginner-friendly perennial that will come back each year as long as you get the growing conditions correct when you originally plant it. 

5. Do phlox plants bloom throughout the summer? 

There are dozens of phlox varieties that bloom according to different schedules. Some will bloom in the spring months and go through the summer, and some won’t bloom until the end of summer but they’ll go well into the fall months. 

6. What should you do with the plant after it flowers?

Since this plant is a perennial, it’ll come back year after year. It’s best to cut the plant’s foliage short right after the first frost hits because they’ll turn black if you don’t remove them from the plant. You can also apply a layer of mulch around the plant’s base to protect it from the winter chill. 

7. Is phlox pet-safe?

If you have dogs, cats, or other pets, you can safely plant phlox around them without any issues. Neither the seeds nor the plant are dangerous. Even if your pet manages to eat some of the plant, they shouldn’t experience any discomfort. 

A popular perennial, phlox is a mainstay of pollinator, meadow and cottage planting schemes. The flowers also make great cut flowers. One of the few perennials that comes in a wide range of colors, the plant’s extended flowering period has helped to make phlox a mainstay of the summer garden.