I've had these two hanging philodendrons for several months. All is fine until recently one has been slowly yellowing and shedding.

My first theory is too much sunlight? It has been getting HOT lately. As you can see, both plants are close to a window. I made sure the plants hang in a corner, so it's not directly in front of the window, It is a south-west facing window. The other window is smaller and a few feet away. That one is a south-east facing window.

My second theory is overwatering? I use a moisture meter and water all plants only when the meter is in the red (DRY)

Possible Solution:
I water more frequently. When the meter is in the middle (MOIST)

I close the blinds for the nearest window because the room can still gets light with just the furthest window open..

Correction: my second theory is that I am UNDERWATERING and I should water more often.

Is it only on the oldest leaves? What are you fertilizing with?

Philodendrons don't like to dry out.

sooby said:Is it only on the oldest leaves? What are you fertilizing with?

Same question as Sooby. Philos are very hungry plants. If it is mainly prevalent on the larger/older leaves, it could very well be a nutrition issue. It may be denying/pulling nutrients from the mature leaves to provide nutrients for new growth.

IDK about that, I never fertilize Philos.

There can be several things going on- how long since last repotting? A long time judging by the vines. Probably some soil problems complicating the issue.

My vote FWIW- when you water, take the whole thing to a sink, water until it runs out the bottom, tip the saucer too, then rehang. Now use the meter and see how it is after a week. or two.
Not sunburn. As WillC as has said many times, people usually overestimate the amount of light an indoor plant gets. Open the blinds.
Also check those stems with bad leaves- maybe just certain stems?

Bottom line, you can keep draping the newer stems over the old, pick off yellow leaves as they appear, and get a lot of mileage out of a plant like these

In the photos provided, I see very few yellow leaves and I'm wondering if we are trying to a solve a problem that doesn't exist. Philodendrons will develop some yellow leaves occasionally, especially as they age and if they are allowed to grow freely and never pruned.

Your hanging plants are too high and too far from the windows for there to be too much light for them. If you keep the shades down during the day, they will eventually suffer from inadequate light. Unless room temps are above 90 degrees F, then that is not a problem.

I don't trust moisture meters as they are notoriously inaccurate and can be misleading, But if your Philos are healthy and you have been using the meter to gauge your watering, then I would not change anything.

Well said, Will.

I'm worried about this part though, "I use a moisture meter and water all plants only when the meter is in the red (DRY)"

Philodendrons don't like to be dry, though, as stated, the moisture meters are not known to be very accurate, depending on the composition of the soil, and what the end of the probe hits when you stick it in. Soil may or may not actually be "red zone dry." If you hold a pot and feel how heavy it is, that's going to be much more reliable. If it's still as heavy as when you just watered, it's not dry. If it feels lighter, it has dried significantly.

Are you taking these to a shower or sink to water? If not, you may not be adding enough moisture when you do water to thoroughly moisten all of the soil. It's very difficult to do that without having much more run-off water than an attached drip saucer can hold.

One of the reasons I asked if it was only the oldest leaves is because that could just be normal aging (or nitrogen or other mobile nutrient deficiency), however the OP did say that the leaves have been shedding which might be why there aren't a lot of yellow ones showing.

Agree, the oldest leaves are the first to go if there's any hardship @ the roots. If conditions remain within acceptable range, individual Philo leaves remain on some plants indefinitely, for decades at least. Heart-leaf vine (P. hederaceum/scandens/oxycardium) is one of them.