Growing Phlox in Iowa

Selecting perennials for the home garden can be a bit intimidating as hundreds of plant species and innumerable cultivars are available.  Among those that deserve consideration are several species of Phlox.  

The word phlox is Greek, meaning flame, and refers to their brightly colored flowers.  There are over 60 phlox species native to North America. While many of these species are suitable for Midwestern gardens, below are a few species that are the most popular for Iowa Landscapes. 

Garden Phlox

Garden phlox (Phlox paniculata) is the most widely grown phlox species in home landscapes.  Plants range from 1 ½ foot tall to over 5 feet tall. Blooms first appear in mid-June in Iowa and may last for a month or more. Flower colors include lavender, white, orange/peach, pink, red, and many with light or darker centers or "eyes." This prairie native prefers sunny sites with moist, well-drained soils and good air circulation. Plants often tolerate part sun and watering may be necessary during hot, dry periods.  

Planting and Dividing Garden Phlox

Garden phlox can be divided in spring or late summer/early fall.  In spring, dig up the plants just as new growth begins to appear.  Divide each clump into sections, with each division containing at least 2 or 3 shoots and a portion of the root system.  Replant the divisions immediately.  Keep the newly divided perennials well-watered through spring and summer. 

When dividing in late summer/early fall (mid-August through September), dig up the entire plant, divide, and replant as you would for spring division.  Plants divided in late summer/early fall should be mulched in late fall (November).  A 4- to 6-inch layer of straw, pine needles, or similar material should prevent repeated freezing and thawing of the soil that could heave late summer/early fall planted divisions out of the ground and cause severe damage.  Remove the mulch in early spring (March).   

Powdery Mildew on Garden Phlox

Unfortunately, many cultivars of garden phlox are susceptible to powdery mildew.  Powdery mildew produces a grayish-white coating on the stems and leaves of infected plants.  Infected leaves turn yellow and eventually dry up and turn brown.  Severely infected plants are unsightly.  Good cultural practices, such as adequate plant spacing and removing infected plant debris in fall, can reduce the severity of powdery mildew.  While this disease can be managed with frequent and regular fungicide applications, this is often not desirable for the gardener because of the time, money, and chemicals involved. 

The best way to minimize powdery mildew problems is by selecting mildew-resistant cultivars.  Several trials have determined which selections are best for the landscape. Performance was based on many factors, including (but not limited to) flower production and incidence of powdery mildew.   Below are top-rated cultivars from the Chicago Botanic Garden and Mt. Cuba Center trials of garden phlox listed by height. This doesn't mean you shouldn't try other garden phlox cultivars. Some of the newest cultivars haven't been in production long enough for sufficient testing, and powdery mildew may arise (or maybe not!).

Recommended Cultivars of Garden Phlox

Cultivars are organized by powdery mildew resistance and then flower color.  New cultivars may not be listed as they were not available for trials. This does not suggest that newer cultivars are not worthy of consideration for planting.

** Highly Recommended.  
The entire list of trialed garden phlox, including those that performed poorly, can be found at the Chicago Botanic Garden website and the Mt. Cuba Center website.

Spotted Phlox (Phlox maculata)

Spotted phlox (Phlox maculata) is similar to garden phlox in appearance and cultural requirements.  However, there are several differences.  Spotted phlox is earlier flowering, has darker green leaves, conical flower heads, and better mildew resistance.  Plants are generally 2 to 4 feet tall.  Spotted phlox is native to Iowa.  It is most commonly seen along roadsides and prairie swales in northeast Iowa.  The species has mauve-pink flowers.  Recommended cultivars include:

• ‘Alpha’ (rose-pink flowers with darker eyes)
• 'Flower Power' (white with pink blush)
• ‘Natascha’ (pink and white striped flowers)
• ‘Omega’ (white flowers with pink eyes)
• ‘Rosalinde’ (purple-pink flowers)

Woodland Phlox (Phlox divaricata)

Another native Phlox species is woodland phlox (Phlox divaricata).  It is commonly found in moist, partially shaded woodland sites.  Woodland phlox produces loose clusters of fragrant, blue to-violet flowers in spring (April to June).  Plants are typically 12 to 15 inches tall.  Woodland phlox is an excellent plant for woodland gardens.  It can also be used in the front of perennial beds or planted in clumps amongst other low-growing, shade-tolerant perennials.  Rabbits love woodland phlox.  Plants may need to be protected with fencing during establishment.  Attractive cultivars include:

• ‘Blue Moon’ (lavender blue flowers)
• ‘Clouds of Perfume’ (lavender blue flowers)
• ‘Fuller’s White’ (grows 8 to 12 inches tall, white flowers)
• ‘Louisiana Blue’ (purple flowers)
• ‘May Breeze’ (white flowers)
• Phlox divaricata subsp.  laphamii ‘Chattahoochee’ (lavender blue flowers with dark purple centers)

Prairie Phlox (Phlox pilosa)

Prairie phlox (Phlox pilosa) is a native prairie wildflower.  It is commonly found in open woods, prairies, and meadows.  Prairie phlox blooms from April to June.  Flowers vary in color from pale pink to rose to violet.  Plants stand 12 to 24 inches tall.  Prairie phlox is excellent for sunny, dry locations, rock gardens, and natural areas. 

Moss Phlox (Phlox subulata)

A common sight in many home landscapes in spring is the brightly colored flowers of moss phlox (Phlox subulata), commonly called “creeping phlox.”  Moss phlox forms dense, carpet-like mats.  Plants are 4 to 6 inches tall.  Its foliage is narrow, stiff, and needle-like in appearance.  Flower colors include white, pink, red, blue, and purple.  Moss phlox is easy to grow.  It performs best in sunny areas and well-drained soils.  Shearing back plants after flowering promotes dense growth and some rebloom.  Moss phlox is useful for edging beds and as a groundcover for sunny slopes.  It also looks good planted amongst rocks or atop a wall. Excellent cultivars include:

• 'Amazing Grace’ (white flowers with magenta eyes)
• 'Candy Stripe' (pink and white striped flowers with darker eyes)
• ‘Drummond’s Pink’ (deep pink flowers with darker eyes)
• ‘Emerald Blue’ (lavender blue flowers)
• ‘Emerald Pink’ (bright pink flowers)
• 'Purple Beauty' (purple flowers with darker eyes)
• ‘Scarlet Flame’ (magenta-red flowers with darker eyes)
• 'White Delight’ (pure white flowers)

Creeping Phlox (Phlox stolonifera)

Another low-growing phlox is creeping phlox (Phlox stolonifera).  Plants grow 6 to 12 inches tall and bloom in spring.  Creeping phlox does best in moist, well-drained soils in partial shade. Creeping phlox is an excellent groundcover and edging plant.  Excellent cultivars include:

• ‘Blue Ridge’ (lavender-blue flowers)
• ‘Home Fires’ (pink flowers)
• ‘Pink Ridge’ (lavender-purple flowers)
• ‘Sherwood Purple’ (purplish blue flowers)