Question
My daughter gave me a very large Christmas cactus. The plastic pot's drainage system was evidently not working correctly, as it would go weeks without needing water. Now the soil has developed a very strong, unpleasant odor of mildew. What should I do to save the plant and get rid of that odor? Thank you.

Answer
Hi Val,

I don't want to mislead you and have you think that I have a sure-fire cure for this problem. Holiday cactus roots do not tolerate constant wetness for extended periods of time because their roots soon start to rot. Extensive root rot is usually fatal. What we don't know is how extensive the root rot may be.

Remove the plant from this defective pot. If all of the visible roots look soft and mushy, then I don't think your plant will survive. Likewise if you see mildew spread throughout the soil, it is unlikely to recover. Otherwise, leave the soil intact and move the plant to a similar sized, dry terra cotta pot. Terra cotta will help the soil dry out sooner. When the soil dries, the mildew and its odor should also disappear. A light sprinkle of horticultural lime on the surface of the soil may also help with the odor.

Provide lots of very bright, indirect light on a north or east windowsill and keep it as warm as possible. The goal is to let the soil and roots dry out naturally as quickly as possible. Others may advise you to replace the wet soil with dry soil. While that may seem like a good idea it is not because the process of replacing the soil will damage most of the already fragile root hairs that do most of the work.

Do not water your plant again until the top half of the soil, has dried out. Then resume normal watering.

This may not work, but it is all that you can do. Replacing soil and fertilizing will not help.

Good luck and please let me know if any of this is unclear or if you have any additional questions. If you would like to e-mail me some photos, I may be able to provide some additional insights.

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Regards,
Will Creed, Interior Landscaper
Horticultural Help, NYC

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