Answer: While I believe your grandmother's plants flourished, I suspect it had more to do with tender loving care and lots of observation as to their water, lighting, and other needs as opposed to the mayonnaise. I think someone who spent that much time and care on their plants probably took pretty good care of their basic needs for water, light and nutrients as well as doing special things like repotting as needed, grooming them, and "extra special care" such as the mayonnaise treatment. Plants should be gently washed or dusted on a regular basis to keep their leaves clean -- this keeps the "pores" clear and also allows maximum light to reach their surface. The oil in the mayonnaise would add a bit of shine, too. In my experience though the plants do just fine if not better without such additives and also look more "natural". I hope this answers your question.
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