Lurking on the undersides of leaves, a shade gardener’s nightmare slyly spreads a white, fuzzy coating that can go unnoticed. If you look on a warm, dry day, you may find absolutely nothing. On rainy days, that white fuzz emerges and releases millions of tiny spores to spread the dread to other nearby impatiens. Then, one day during late spring—especially one with very cool, damp weather—you look and all you have left are sickly yellow sticks where your beautiful impatiens border used to be. Your impatiens have been attacked by Plasmopara obducens—the impatiens downy mildew—and they will die.
What is It?
If you have never heard of this dastardly fungal disease, join the club. It only came to my attention during a random luncheon conversation with some Massachusetts garden club colleagues. Julie Hess, horticulturist at the Missouri Botanical Garden, began seeing it at about that same time, so we are all just coming into the know. First reported in greenhouses on the East Coast in 2004, it has now been confirmed all over the U.S., Canada and Europe.
Impatiens downy mildew is particularly aggressive in beds where impatiens are used every year. The ‘resting’ spores that form in the dying stems are released into the soil over winter and then infect the next year’s new crop of plants. Then, this nasty organism produces short-lived spores that spread easily with splashing water and cause new infections on nearby plants. The widely popular Impatiens walleriana is the main victim. The new plants that you buy each year may have been infected in the grower’s greenhouse and carried home from the store, spreading the infection. By running irrigation at night or watering too often, the spores have the ideal damp conditions under which they may spread.
There is no reasonable treatment to save a plant that is infected, like cutting back the diseased parts or treating with fungicide. Julie suggests to cut your losses, cull the entire plant (roots and all), and move on to something else for that shady bed. Use disposable rubber gloves while handling infected plants and do not touch healthy ones. Do not compost diseased plants. Toss them out in sealed plastic bags with the household garbage.
Just as farmers have rotated crops for centuries to help maintain the health and balance of their soil, so, too, must the ornamental gardener now think in the same way. Impatiens have always been the go-to color plant for season-long bloom in the shade, but their excessive use has brought us to this point. If you already have this problem in your yard, do not use impatiens for four or five years in an attempt to rid the soil of the ‘resting’ spores. Use this as a new planting opportunity and try something different.
Colorful Annuals and Tender Plants for Shade
Begonias are just as common as impatiens for shady beds and borders. Go a little upscale with your new planting plan and bring in the bright red leaves of the kingly Begonia rex ‘Stained Glass.’ Try the large camellia-flowered tuberous begonias or the splashy, brilliant orange sprays of B.boliviensis ‘Bonfire.’
Torenia, the wishbone flower, also is great in the shade. ‘Summer Wave’ is a high performance hybrid series that comes in ‘Blue,’ ‘Amethyst’ and ‘Large Violet.’ The ‘Catalina’ series goes beyond the blue shades and adds bicolors in yellow, white and pink.
The ‘Shadow Dancers’ fuchsia series has been selected for shady area hanging baskets and offers many floral color combinations. Julie’s favorite is Fuchsia 'Koralle,' the honeysuckle fuchsia, adored by humans and hummingbirds alike for the sweet nectar in the edible flowers. As with almost all fuchsias in Missouri, bring this one in before frost.
Caladiums are the most colorful plants for the deep shade. Slow to start, they thrive on our summer heat. Start early indoors or purchase pots already sprouted. ‘Pink Symphony’ looks like a pink silk handkerchief with a few delicate green veins and white touches. ‘Candy Land’ also has a narrow spade leaf, but in white, with green edges and pink spots. ‘White Christmas,’ a classic with white background and striking green veins and edges, is very visible in shade, as is ‘Stardust,’ which has a medium green frame with a clean white starburst centered to the stem.
Choices for Partial Shade
Try the colored foliage of all of the new coleus selections. Coleus ‘Sedona’ is the color of a warm, Arizona sunset, with shades of the local red rock in sun and shadow. ‘Velvet Mocha’ has a similar, but simpler, color pattern on a narrow, lacier leaf. ‘LifeLime’ is another brilliant member of the ‘ColorBlaze’ series and offers a bright, clear chartreuse leaf with occasional small maroon dots.
The Browallia ‘Endless’ series with ‘Flirtation’ white, ‘Celebration’ pale lavender and ‘Illumination’ rich blue all perform well as basket or box plants that will spill over the edges or flow across beds. They offer a very long bloom season and should perform well until frost.
Perilla is often recommended for both culinary and ornamental use and will grow under almost any conditions, but be warned—it is a weed! Don’t use it unless you plan to eat a lot of it to keep it under control or ‘dead-head’ it so there are no seeds.
Choices for Shade
I love the soft silver leaves and pale pink flowers of Lamium maculatum ‘Pink Chablis.’ This spotted deadnettle is heat-tolerant, drought-tolerant and deer-resistant.
Try some of the purple, bronze or chartreuse-leaved Heuchera or Heucherella varieties that are on the market now. Heuchera ‘Southern Comfort’ is a favorite that does well in heat and humidity, with lush mounds of large, fuzzy, peachy-bronze foliage that persist through the winter and get larger every year. With Heucherella, Julie has had good results with the ‘Falls’ series—‘Redstone,’ ‘Yellowstone’ and ‘Sunrise’ Falls. These will send out little runners in groundcover fashion instead of just increasing in diameter year after year.
Tradescantia ‘Sweet Kate’ has chartreuse stems and leaves, blue flowers and is quite an eye catcher in a shady spot. One of Julie’s favorites to brighten up a shady spot is Polyganatum odoratum ‘Variegata,’ the variegated Solomon’s seal. It competes well in the root zones of trees and is excellent for cutting.
One of the best shade perennials for flowering is the hardy begonia, Begonia grandis. It’s just a little slow out of the gate at the beginning of the season, but it will bloom until the day it freezes.
Julie’s Success-in-the-Shade Selections
Here are some selections Julie Hess has used successfully in the shade:
• Begonia ‘Dragon Wing’ Series
• Begonia ‘BabyWing’ Series
• Alocasias and Colocasias – elephant ears
• Tradescantia. Julie recommends T. ‘Blue Sue’, T. ‘Kartuz Giant’, and T. ‘Green Inch Plant.’ with bright chartreuse foliage.
• Eustomarussellianum, also known as lisianthus, prairie gentian or Texas bluebells, will flower very well in partial shade.
• Salvia guaranitica ‘Black and Blue,’ a hummingbird favorite, also will do well in partial shade.
So, our beloved impatiens may be under attack, but it is a great reason to start exploring new plant choices for the shade. Just make sure to leave a little room for the hammock.