Question
QUESTION: I have a new "zz" about 18" tall, the leaves of the tallest stalk are turning black one by one from the top down.  I brought it home 7 days ago potted it the next day into a small (6"?) pot - it was in a 4" plastic square.  Then I sat it in my large east facing windowsill.  It was in a north facing storefront foyer in the store - probably cooler but similar light.  Any idea how to stop this before I lose it?  Thanks.

ANSWER: Hi Shannon,

Exposure to cold can cause leaves to turn black, but that is unlikely in this case unless it got cold on the way home. Otherwise, root damage is the likely cause of blackened leaves, particularly the newest leaves.

Root rot caused by keeping the soil constantly moist damages roots and causes blackened leaves. However, you have not had the ZZ Plant long enough for the roots to have rotted. However, we don't know how the plant was cared for prior to your purchase, so root rot cannot be ruled out.

I never recommend repotting a newly acquired plant. Plants are already under stress in a new environment and repotting adds to that stress. In addition, repotting is not quite as simple as it appears. For example, removing some or all of the "old" soil will damage many of the fine root hairs that do most of the work. Using the wrong potting mix is another problem. Of course, I don't know just how you went about repotting, but that appears to be the most likely cause of the problem.

Let me know if there is any other information that might help me make a more definitive diagnosis.

I have written articles on ZZ Plants and repotting that I will email for free to you (or anyone else) who sends a request to me at [email protected].

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

If this information has been helpful, please click the Rate Volunteer bar below and enter a rating and nomination for me. I am a volunteer on this site so Ratings are the only compensation I receive for answering plant questions.

Need more information? Visit my website at www.HorticulturalHelp.com
or email me at [email protected] or call me at 917-887-8601 (EST)
 
Regards,
Will Creed, Interior Landscaper
Horticultural Help, NYC


---------- FOLLOW-UP ----------

QUESTION: Both great answers.  I carried it home - it was a 2 mile walk and about 30 degrees outside - would that be enough time to have caused this?  If not - maybe root damage.  Either way - should I remove the blackened leaves, cut back that stalk or let nature take it's course?  Thank you!!

Answer
Hi Shannon,

Thanks for the generous ratings. Much appreciated!

Unless you are an extremely fast walker, yes, more than 10 minutes of exposure to freezing temps is enough to cause cold damage to a tropical plant, such as your ZZ Plant. The leaves and stems suffer the most damage because they are more exposed to the cold than the roots that are somewhat insulated by the soil and pot.

Remove all of the damaged (discolored) leaf and stem tissue. If most or all of the leaves on a stem are damaged, then cut off the entire stem at its base.

As long as the roots were not damaged by the cold - and that is a possibility - then healthy new growth will eventually emerge. Keep in mind that the ZZ is a slow grower. Be sure to keep it in a moderately bright location and do not water it until the soil is dry at east halfway deep into the pot. Do not fertilize it or disturb the roots. And don't take it for any outdoor walks before the warm weather returns!

Good luck!

~Will