A Comparative Study of Phlox Paniculata Cultivars Richard G
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Plant Evaluation Notes ISSUE 35, 2011 A Comparative Study of Phlox paniculata Cultivars Richard G. Hawke, Plant Evaluation Manager Mark Rudy Phlox paniculata 'Shortwood' arden phlox, a staple of summer United States, from New York to Georgia, Mary’), whimsical pink and white (‘Peppermint gardens since colonial times, are and west to Illinois and Arkansas. It has Twist’), and delicate pink and yellow some of the most recognizable been in cultivation in Europe since the (‘Sherbert Cocktail’). Each flower is composed and popular perennials today. Their telltale 1800s and many of the early cultivars of a long corolla tube and five petal lobes. magenta flowers enliven native landscapes originated in England and Germany. Phlox The individual blossoms are grouped in for many weeks from summer to fall, while a maculata, early or meadow phlox, is a many-flowered domed to elongated panicles myriad of cultivars in a rainbow of colors native of the eastern United States, too. at the ends of the stems and in the upper grace gardens everywhere. They owe their While similar in habit to garden phlox, its leaf axils. Flower size varies from ½ inch to prodigious nature as much to the oversized flowers are borne in elongated cylindrical almost 2 inches wide, whereas the panicles flower trusses that crown tall stems as to clusters earlier in the summer. The species typically range from 4 to 6 inches tall and 6 to the vivid pinks, purples, reds, and oranges name reflects its purple-maculated or spot- 8 inches wide. Flowers are fragrant, although of the flowers. Despite their well-known ted stems. Phlox ×arendsii, a hybrid be- the degree of fragrance is variable among susceptibility to powdery mildew, garden tween P. divaricata (blue phlox) and cultivars. Flowering occurs from early sum- phlox remain a perennial favorite prized for P. paniculata, boasts greater mildew mer to early autumn, with P. maculata and their dramatic, long-blooming floral show. resistance than garden phlox. Phlox P. ×arendsii typically blooming earlier than paniculata, P. maculata, and P. ×arendsii P. paniculata. Phlox paniculata, garden or summer phlox, are collectively referred to as border phlox. is one of 60 or so species of Phlox native Border phlox grow best in moist, well-drained to North America and is in the phlox family Flowers are without a doubt the main soils in full sun to light shade. Despite being (Polemoniaceae), which includes the name- ornamental attribute of the border phlox. hardy and adaptable to a wide range of sake Polemonium, or Jacob’s ladder. Phlox A pavonine palette of flower colors is geographic regions in USDA Zones 4–8, is derived from the Greek word for flame, available including shades of pink, salmon, border phlox perform best where summer and so named for the brilliantly colored orange, red, purple, lavender, blue, and nights are cool. Deadheading is recom- flowers. The specific epithet paniculata white. While the magenta-pink flowers of mended to promote continued bloom and refers to the rounded to pyramidal panicles wild Phlox paniculata are unmistakable, prevent seedlings, which are not true to in which the individual blossoms are held. modern cultivars offer a variety of distinctive type. While border phlox are generally Phlox paniculata is native to the eastern colors such as voluptuous cherry red (‘Miss easy to grow, a number of disease and pest 2 Plant Evaluation Notes issues can be troubling. High humidity and sun or partial shade, border phlox mix made it possible to evaluate many new warm temperatures bring on powdery well with Shasta daisies, balloon flowers, cultivars, thereby keeping our trial current. mildew, which is a common and often daylilies, astilbes, meadowsweets, and debilitating disease, particularly for Phlox yellow wax bells. And of course, border Five plants of each taxon were grown in paniculata. A number of other fungal leaf phlox are superb as cut flowers in fresh side-by-side plots for easy comparison of spot diseases may infect lower leaves but floral arrangements. ornamental traits and landscape perfor- are usually less damaging than mildew. mance. The evaluation garden was openly Additionally, spider mites, rabbits, and deer The Evaluation Study exposed to wind in all directions and can be significant pests. Two-spotted spider From 2001 through 2009, the Chicago Botanic received approximately ten hours of full sun mites infest the undersides of leaves, Garden (USDA Hardiness Zone 5b, AHS daily during the growing season, which especially during hot, dry periods, causing Plant Heat Zone 5) evaluated 78 Phlox taxa averaged 164 days per year for the trial stippling as they suck out nutrients. Leaves in full-sun trials. The goal of the comparative period. The clay-loam soil was amended disfigured by spider mites usually appear trial was to identify outstanding border phlox with composted leaves and had a pH of 7.4 puckered and chlorotic. Beyond the for the Upper Midwest. The study concen- throughout the evaluation term. The site cosmetic aspect, spider mite infestations trated on cultivars of Phlox paniculata but was normally well drained, but at times the reduce the overall health of the plant. included P. ×arendsii cultivars as well as a soil retained moisture for short periods in number of unspecified hybrid cultivars. summer and winter. Weather conditions at Border phlox are often touted as the the test site are shown in Table 1 (below). backbone of the summer garden because In a previous trial from 1993 through 1996, of their tall stature and exuberant flowers. the Chicago Botanic Garden evaluated a Maintenance practices were kept to a In recent years breeding has reduced the variety of spring- and summer-blooming minimum, thereby allowing plants to thrive height of many of the new cultivars, but phlox. Ten of the 20 taxa evaluated were or fail under natural conditions. Water was border phlox still provide color in the heat cultivars of Phlox paniculata, including provided via overhead irrigation as needed of midsummer when many perennials have ‘Katherine’, which was the top-rated, and mulch consisting of shredded leaves slowed down. By careful selection, it is mildew-resistant garden phlox in the trial. and wood chips helped with water conser- possible to grow multiple border phlox for Results of the 1993–96 phlox trial can be vation and weed suppression. Moreover, almost continual bloom throughout the downloaded at www.chicagobotanic.org/ plants were not fertilized, winter mulched, summer. Partner tall border phlox with plantevaluation/. Feedback after the trial or chemically treated for insect or disease other garden amazons such as joe-pye confirmed that a more comprehensive problems. The test garden was surrounded weed (Eupatorium spp.), bee balm (Monar- trial of P. paniculata cultivars would by an electric fence to deter deer browsing. da didyma), and switch grass (Panicum vir- be worthwhile. Undertaking our second Plants were cut back in fall and debris was gatum). Whether tall or short, in full trial in stages over a nine-year period removed from the garden. Table 1: Weather Summary for 2001–09 2001 2002 2003 2004 2005 2006 2007 2008 2009 Lowest temperature °F (°C) -4(-20) -5(-21) -5(-21) -9(-23) -2(-19) -8(-22) -10(-23) -6(-21) -17(-27) Lowest temperature date 1/1 3/4 1/27 1/30 12/7 2/18 3/5 1/20 1/16 Highest temperature °F (°C) 98(37) 101(38) 98(37) 93(34) 100(38) 100(38) 96(35) 93(34) 96(35) Highest temperature date 6/29 7/21 8/22 6/6 6/24 7/31 7/9 7/17 8/9 Number of growing-season daysa 171 146 150 155 158 143 196 181 175 Number of days below 0°F (-18°C) 2 1 4 10 2 2 11 16 8 Number of days above 90°F (32°C) 19 30 15 5 24 15 20 6 7 Last frost date 4/19 5/21 5/4 5/3 5/4 5/7 4/16 4/30 4/18 First frost date 10/7 10/14 10/1 10/5 10/23 10/12 10/28 10/28 10/10 Annual rainfall in inches (cm)b 44.3(112.5) 33.6(85.3) 31.7(80.5) 35.5(90.2) 24.4(61.9) 42.5(107.9) 41.0(104.1) 49.5(125.7) 38.8(95.5) Annual snowfall in inches (cm)c 10.9(27.7) 37.6(95.5) 15.6(39.6) 27.2(69.1) 44.4(112.7) 23.4(59.4) 38.5(97.8) 78.5(199.4) 28.8(75.2) aAverage length of growing season is 161 days. bAverage rainfall is 35.8 inches (90.9 cm). cAverage snowfall is 38.1 inches (96.8 cm). Data was collected at the Chicago Botanic Garden weather station. Latitude: 41°51’N. Longitude: 87°37’W. Altitude: 578.74 ft. (176.4m) 3 Plant Evaluation Notes Performance Report Beginning in June 2001, plants were regu- larly observed for descriptive traits such as flower color, bloom period, foliage color, and plant habit. In addition, data was collected on disease and pest problems, winter injury, and habit quality and plant health issues related to and/or affected by cultural and environmental conditions. Table 2 (pages 4-5) shows the descriptive traits and final performance ratings for 75 of the 78 taxa in the trial. Although ‘Nicky’ and ‘Darwin’s Joyce’ finished the trial, they are considered synonyms of ‘Düsterlohe’ and ‘Norah Leigh’, respectively. All plants of ‘Jeana’ were inad- vertently removed in the spring of the third year and not replaced; thusly, not included in Table 2 with taxa that completed a full evaluation term.