Poor Performing Impatiens

(Question)

I’m a simple gardener happy to plant impatiens in my shady back yard. Several years ago I thought I would “improve” my clay soil by introducing free city compost (available in areas around Toronto) to lighten the soil and add what I hoped to be nutrients. I’ve never been able to grow impatiens since. Previously the impatiens would start to grow almost immediately after planting and reach heights of 12″. Now they grow very little and after 3 or 4 weeks, one by one, they appear to be zapped by a ray gun. They just wither away and die. I’ve had to replant them 2 or 3 times each season. I’m heartbroken. My colourful back border has never looked worse. This year I need to get it right. Please help.

(Answer)

Dear gardener,

I am sorry to hear about your problem. Although I cannot be sure without checking your plants, it sounds to me that your impatiens have been affected by the impatiens Downy Mildew fungi.  The disease symptoms include: slow plant growth and vigor, leaves turn yellow, decay rapidly and drop, bare stems an shed flowers. The underside of the leaves has a fuzzy white mold.

This fungus has been moving North since discovered in 2004 in the southern US and, particularly affected the 2011 and 2012 growing season. The warmer winters, particularly that of 2011/12, allowed the spores to survive in our zone. The disease affects all varieties of Impatiens walleriana and it does not respond to any chemical controls.

The disease was most likely brought home with the plants themselves as not many greenhouse growers were aware of it until recently. Another possibility is that the spores were brought by strong wind, particularly, as that is the way it has been moving north. I am not convinced that the city compost has anything to do with it as the high temperatures at which it is composted generally kill any pathogens.

Unfortunately, the spores are probably in your back border soil now. Fortunately, it does not infect other plants and there are some cultural practices you can try this year:

  • Do not plant impatiens on that bed. Instead, you may want to try begonias (shade resistant). If it is a sunnier bed, you can try many other colourful annuals.
  • Continue amending your beds with soil. That is a very good practice as the healthier your soil, the healthier your plants will be.
  • When cleaning the garden, planting or raking the beds, leave the back bed for last. This will avoid spores presently in the soil getting attached to your tools and going elsewhere in the garden.
  • You can try New Guinea Impatiens (Impatiens hawkeri)  as they do not seem to be affected by the fungi (so far).

Scientists are not sure how long can the spores survive ‘normal’ winters. Please refer to the following document for further identification and information:

https://www.ladybug.uconn.edu/factsheets/documents/impatiens.htmlhttps://www.ladybug.uconn.edu/factsheets/documents/impatiens.html

Hope this helps.