University of Vermont
Department of Plant and Soil Science

Summer News Article
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PERENNIAL PLANT FEATURE: SPIDERWORTS

Dr. Leonard Perry, Horticulture Professor
University of Vermont

Spiderworts are a more attractive perennial than their name suggests.  They flower in early summer, in a range of colors depending on cultivar (cultivated variety).  With proper choices, and a little care, you can have plants free of disease and that don’t self seed.
    
The name, spiderwort, likely refers to the fact that sap released from the stems dries into web-like threads.  Another name, dayflower, refers to the fact that individual flowers only last for a day.  Yet, with many flowers in clusters on the ends of stems to 24 inches tall, blooming lasts for two or three weeks in late spring (South) or early summer (North).  Rebloom may occur later in summer or early fall.
   
Depending on cultivar (cultivated variety), blooms can be had in a range of colors including blues, violets, purple, pinks, and white.  Observe them closely, and you’ll see why some call them “trinity flower.”  Each bloom has three petals, three sepals (parts resembling petals), and six stamens.  Observe the stamens even closer, and their fuzzy stalks (“filaments”) with bright yellow anthers (the pollen carriers) on the tips appear like a drawing from Dr. Seuss.
   
The strap-shaped leaves are mainly green, but on some cultivars may be blue-green, chartreuse, or bright yellow.  Often the bright yellow ones revert to mostly green over a few years.  These leaves are quite dense early in the season, but tend to die back or become unsightly after bloom and when plants go dormant in summer heat.  They’ll last better in cool seasons, cool climates, and if kept watered.  Otherwise, if leaves become unsightly and deteriorate, cut them back to the ground, and new ones will arise with cooler temperatures later in the season.
   
Cutting stems back also will remove the untidy flowers, which tend to hang on a while after bloom, and prevent seeds forming.  If seeds are released, the subsequent unwanted seedlings can fill a bed—another trait that may turn some gardeners away from this plant.  If you only have one plant, you don’t need to worry about seeds, as plants are “self sterile”.  This means  that at least two plants are needed to cross pollinate and form seeds.
   
Height and width at bloom varies with the cultivar, but may range from one to two feet high, and two to three feet wide.  Spiderworts prefer full sun for best bloom and growth (eight or more hours of direct sun daily), but will tolerate part shade (four to eight hours of direct sun).  They’re hardy, surviving in USDA zone 4 (-20 to -30 degrees F), such as in most of northern New England. 
   
Spiderworts are good choices for feeding bees and butterflies.  Their overall appearance makes them good choices aesthetically for cottage gardens, woodland edges, watersides, and meadow plantings.  While they prefer a slightly moist soil, it should be well-drained and not get too dry.  Consider combining them in sunny gardens with perennial geraniums, bluestars, or ornamental grasses.  In part shade, consider them with hostas, astilbe, lungworts, and ferns.  Combining them with perennials works well to hide their leaves if they become unsightly, need to be cut back, or plants go dormant.
   
Although there are about 70 species of spiderworts, most cultivars you’ll find for sale are crosses among three—Virginia (virginiana), bluejacket (ohiensis), and zigzag spiderwort (subaspera).  These crosses are correctly lumped into the Andersoniana group, although you may find them listed incorrectly under a hybrid species (xandersoniana).  All are native to the eastern United States, even though they’re in the mainly tropical dayflower family along with such houseplants as Moses-in-a-boat and wandering Jew.     
   
These main spiderwort species were among the first plants introduced to Europe from the New World in the early 1600s.  The Virginia spiderwort was used medicinally by Native Americans to treat many ailments.  An interesting and odd fact about some species is that the fuzzy hairs on the flower filaments change from blue to pink upon exposure to low levels of nuclear radiation.
   
As part of the extensive perennial evaluation program of Richard Hawke at the Chicago Botanic Garden (www.chicagobotanic.org/research), 31 different spiderworts were grown and rated yearly for growth characteristics and resistance to leaf-spotting diseases.  Fourteen of these received top 4-star ratings and are listed in the website publication. 
   
While a good flower display on spiderworts consists of open flowers on at least 50 percent of flower clusters (inflorescences) at peak bloom, the cultivar ‘Red Cloud’ (rosy red flowers) was the best in the Chicago trials with 70 percent open flowers.  It produced flowers that lasted through the day too, unlike most cultivars that finish blooming around mid-day.
   
Others with consistently good blooming in the trials, as well as low disease ratings, were ‘Blue Stone’ (lavender blue flowers, yellow green leaves), ‘Concord Grape’ (purple flowers, blue-green leaves,early season bloom), ‘Perinne’s Pink’ (pink flowers, blue-green leaves), and ‘Zwanenburg Blue’ (purplish blue flowers, green leaves).
   
Leaf diseases don’t occur in all garden settings, or in all years.  To avoid plants getting these, avoid overhead watering, and choose least susceptible cultivars.  If leaves become diseased, cut them back if severely infected or as they die back after bloom and discard (don’t compost) leaves in the trash.
   

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