Some months ago I noticed my philodendron, which I inherited from my grandmother in 1981, was losing leaves at an alarming rate. It was in a huge pot sitting in water. I tried drying it out, but it still appeared sick. I just moved it from a northern window to a southern window, which gets more sun. Now it is near a healthy younger plant on the left. I am afraid to water it and afraid to repot. Please help. Here it is in photo on the right. And another when I just moved it.
Sitting in water probably caused the soil to stay wet all of the time and that in turn caused the roots to rot. If it were my plant. I would take it out of its pot, get rid of all of the soggy soil (especially any clinging to the roots), and let it sit out bare root over- night. In the morning, feel the roots and see if they appear wet or dry. Re-pot your plant into a container that is just an inch or two larger than the root ball. There must be drip holes in the bottom of the pot! This will allow the soil to dry out quickly when you do water and prevent further root damage. If the roots were wet when you felt them, do not water for about 2 weeks. If they were dry, you can start watering again. Always allow the top 50% of the soil to dry out before watering. Never allow the plant to sit in water.
These plants are considered poisonous and should be kept away from pets and children. Read more about common houseplants that are poisonous in Don’t Feed Me To Your Cat! A Guide to Poisonous Houseplants
You can read more about philodendrons in the Popular Houseplant section of the website.