Question
I am having a problem with gnats on my diffenbachia.  I have tried retail insect spray and have let my plants dry to the point of yellowing and wilting.  Must water but still have gnats!  I have seriously let these plants dry several inches down in the dirt and nothing seems to work.  Any suggestions?

Thanks for your help.

Answer
Hi Deborah,

Fungus gnats are almost always preceded by repotting into a larger pot and the gnat larvae are introduced by the new soil added. In addition, most folks mistakenly add soil on top of the original rootball when they repot. I realize that you have not indicated that this is what you did, but my experience tells me you probably did.

If so, remove and discard all of the loose soil you added on top of the original rootball. It is in this extra soil on top that most of the larvae reside so that will eliminate much of the problem. Then, allow the top of the remaining rootball to dry at least one-quarter of the way deep into the pot. Dieffenbachias can withstand drought quite well - better than gnat larvae.

The leaf yellowing may actually be because of root rot that usually occurs after unnecessary repotting.

If my assumptions about your Dieffenbachia are not accurate, please post or email a photo to me at my address below so I can properly assess the problem. The photo should include the entire plant, including its pot.

I have written articles on indoor plant pests, repotting and on Dieffenbachia care that I will email for free to you (or anyone else) who emails a request to me at [email protected]. I have also written an indoor plant care book in a PDF format that I can sell you if you contact me at my email address.

Please let me know if any of this is unclear or if you have any additional questions.

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Regards,
Will Creed, Interior Landscaper
Horticultural Help, NYC

Visit my website at: A link to HorticulturalHelp.com