Alternatives to Garden Impatiens Because of Downy Mildew Disease

Author: 
Suzanne Klick, Ag Tech Lead
Garden impatiens
Many greenhouse growers are deciding to not grow garden impatiens or significantly reduce their production due to the occurrence of downy mildew

Garden impatiens have been a big favorite with gardeners for planting in shady areas in their yards.  Garden impatiens have been a major crop for greenhouse growers to sell retail to garden centers and wholesale to landscapers.  Unfortunately, there is a downy mildew disease now infecting them.  Maryland had its first reported case of downy mildew on garden impatiens in early June 2012. All varieties of garden impatiens (seed or cutting propagated) are susceptible to downy mildew. New Guinea impatiens (Impatiens x hawkeri) are reported to be tolerant of the disease. SunPatiens, a hybrid impatiens developed by Sakata Seeds, is reported to be resistant. The impatiens downy mildew pathogen (Plasmopara obducens) does not infect any hosts other than impatiens. 

It is difficult to find options that easily replace garden impatiens which is a plant that grows so well in shady areas, produces blooms all summer, and offers such a wide variety of flower color choices.  Look for alternative plants that can be used in various combinations to fill in shady areas where garden impatiens have always done well. Note how much sun an area gets in a day and at what time of day it is most intense in order to make good plant choices.

With the problem of downy mildew on garden impatiens, what can you grow and plant instead? Below are some suggestions.

New Guinea Impatiens
New Guinea impatiens do better in areas with morning sun and afternoon shade.  In full shade they tend not to produce as many flowers and can become leggy.  Although New Guinea impatiens do not quite fill the niche of full shade areas like garden impatiens do, they do provide a variety of flower and foliage color.  New Guinea Impatiens Divine™ series is a recent NGI commercially available from seed.  Some growers are choosing this option when increasing their NGI production because it is less expensive than New Guinea impatiens from cuttings which makes it a good option for landscape companies.  Colors are white, cherry red, lavender, orange, pink scarlet and red violet.  Leaf colors vary as well.  Plants reach a height of 10 - 14 inches and spread 12 - 14 inches.  The SunPatiens series are good for growing in full sun, partial shade and heat.  There are three series of SunPatiens with different growing habits: one with a spreading habit, one with a compact and shorter habit and one with plants that are fast growing with a more upright and vase-like habit.  They produce flowers from spring to frost.

            
               New Guinea Impatiens Painted Paradise® Pink Improved

           
              New Guinea Impatiens Divine™ Mystic Pink

Caladiums
Caladiums provide good contrasting color in part shade and shade areas, but wait to plant later than mid April.  The soil temperature should be at least 65 °F, so plant around the first of May in southern MD (DC would be earlier by 2 - 3 weeks).  Planting in soil that is too cold can cause the tubers to rot before they sprout.

            
            Caladiums in woodland setting

Torenia
Torenia flower choices include blue, lavender, burgundy, magenta, white and yellow. Torenia has a compact habit and blooms consistently throughout the season in shady locations. 

             
            Torenia 'Kauai Magenta'

Sweet Potato Vine
Sweet potato vine can be used in areas with sun and part shade.  The foliage will provide color in various shades of green, red, and purple.  There are newer varieties like ‘Sweet Georgia Heart Purple’ with foliage that has a deep, rich purple color.  Cultivars like ‘Illusion Emerald Lace’ (green foliage) and ‘Illusion Midnight Lace’ (purple foliage) have a more mounding habit.  'Blackie' is another purple foliage cultivar that can be planted.  'Marguerite' is one with bright green foliage.

             
             'Blackie' and 'Marguerite'


Coleus

Coleus is another option for shade.  The foliage of coleus adds a wide range of color to a garden from green to gold to pink to red to purple. Be sure to look for shade tolerant cultivars and not the newer sun-loving ones. Karen Russ, HGIC Information Specialist, and Bob Polomski, Extension Consumer Horticulturist, Clemson University, have written an article with a list of sun tolerant and shade tolerant cultivars.  It is available at  http://www.clemson.edu/extension/hgic/plants/landscape/flowers/hgic1162.html

            
                 Coleus ‘Red Ruffles’ is a shade tolerant variety

             
                 Coleus Stained Glassworks TM ‘Trailing Plum’ is a sun
                 tolerant variety

Lobularia maritima (Sweet Alyssum)

For white, pink, or purple flowers, Lobularia maritima (sweet alyssum), is an option for part shade areas.  It is heat and drought tolerant.  There are sterile varieties available that are more vigorous and bloom over a longer period.  Non-sterile varieties tend to go to seed in June.  A good practice is to shear plants after first bloom to encourage a second flush.

              


Streptocarpella

Plant streptocarpella in shade to part shade.  It has a mounding habit and will bloom from planting to frost.  This plant is often used as a houseplant so consider planting it in a container or hanging basket.

 

Streptocarpella 'Concord Blue"


 Begonias

Wax begonias (photo) and tuberous begonias do well in light shade areas.  They prefer moist, well-drained soil.  Begonias continue blooming until the first frost.  For a less compact form, consider angel wing types of begonias.

 
A few more options:

Iresine is planted for its foliage which can be shades of pink, red, green, and purple.  It would be an option for a part sun area.  Flowers are inconspicuous.

Plectranthus is usually grown for its foliage and used as a houseplant.  Plectranthus ‘Mona Lavender’ is a selection that produces many lavender blooms and can be grown in part shade.

Browalia speciosa produces blue flowers through the summer and can be planted in sun to shade.

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