Ask the Clark Gardener

Dear Fellow Gardeners:


December 2012. How were your impatiens?

For all who enjoy Clark Botanic Garden, its landscape may look dramatically different next year because of an insidious disease potentially affecting thousands of its plants. What Iím talking about is downy mildew, caused by Plasmopara obducens. We at Clark noticed the beginning inroads of this plague last summer (2011) and started learning from visitors that this scourge was also affecting their home gardens. This year, the devastation caused by P. obdurens is receiving more publicity and recognition. P. obdurens is an oomycete which has zoospores which can swim in water, and it can spread quickly. That last sentence, whatever it means, should instill fear in even the most blasé of gardeners.

How do you know if this oomycete, or water mold, is sweeping through your garden in its march of death? Well, recall how the common garden impatiens (Impatiens walleriana) performed in your garden this year. Did you see unseasonal yellowing of the leaves? Did the leaves curl downward? You might have seen white-colored areas on the underside of leaves, which were aggregations of P. obdurens spores. Did your impatiens appear stunted? Dropped their leaves? Collapsed? Are you confident enough to be sure that your impatiens were sick from other than miserable care by you? Then your garden, too, probably has been invaded by P. obdurens.

This scourge is so serious that a recent article in one of Long Islandís newspapers was titled “The end for impatiens on LI?”. What has exacerbated the problem is that the spores of downy mildew can survive the winter in the soil. Growers are working to find and recommend reasonable substitutes for Impatiens walleriana for the 2013 growing season.

But hold on, you say. Youíve visited Clark Garden pretty frequently (and enjoyed it immensely), but you donít recall that impatiens played a major part in the composition of the flower beds that you saw on your visits. What is this about the mildew potentially affecting thousands of Clarkís plants? Well, you may have observed many plants growing wild, particularly in the more shaded areas in Clark, with yellow to orange flowers, between 2 to 5 feet tall, with watery stems, and blooming through most of the summer. These plants are impatiens and, yes, Clark grows thousands of them every year, unplanned and unplanted. We are not talking Impatiens walleriana here, but Impatiens capensis, more commonly known as jewelweed. I have not noticed any effects from P. obdurens on our jewelweed, but from what Iíve read, this mildew can devastate this type of impatiens too.

If Clarkís jewelweed gets clobbered by the mildew, Iíll miss it. Itís a weederís weed. Its flowers actually are pretty. Someone once asked me if we grew it on purpose. It looks decent for a long season, so one doesnít have to rush to pull it out. And itís very easy to pull out, roots and all. With a concentrated but unstrenuous effort, one can clear huge areas of the weed in a relatively short time. Thatís personally satisfying. And played right, one can impress and astound others with oneís apparent industriousness while exerting oneself relatively little for the end result. In addition, the watery juices from the stems are said to relieve itching from poison ivy and insects.

Still, there is so much of this weed at Clark that with everything else that needs to be done, we usually canít get it all pulled out by the time it goes to seed or even by the end of the season. Given the jewelweeds productivity in the summer of 2012, I expect that it will be back in force in 2013, mildew or no mildew. This leads me to make a public service offer. If you suffer from impatiens withdrawal next summer, Iím considering having jewelweed collection days in 2013. After paying a modest fee, visitors would be able to collect their own jewelweed plants to bring home to their own gardens. Hey, a similar concept works for strawberries and pumpkins and such, why not for jewelweed plants? Anyone interested? (There would be a limit of 1,000 jewelweed plants per visitor.)

And, just a reminder, if you have a comment, horticultural question, or general question about Clark Botanic Garden, remember to send them to me at CBGardener2@gmail.com.

The Clark Gardener

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