Hi there! I'm having an issue with my Philodendron Selloum. About a month ago, I trimmed its excessively long aerial roots because they were spreading everywhere. I read on the internet that it's okay to do so and that it shouldn't harm the plant. However, since then, my plant has been getting worse, and I don't know what to do. All the leaves are getting slowly yellow. I tried using a 7-4-7 NPK fertilizer I have from the previous tenant, but it doesn't seem to have helped at all. I was watering it enough, once a week as usual. Do you have any suggestions on how I can help it, and is there anything I can do? I also noticed some cracks on its trunks. Thanks!

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It's normal for them to lose the lowest leaves periodically, especially when light levels drop in the winter. They absorb minuscule amounts of water through their aerial roots so that shouldn't be a worry.
I notice that the pot has an attached saucer and that it is large for the pot. Both indicate that it's been in there awhile and it may have a buildup of mineral salts in the soil. These can damage the fine root hairs that do all the water and nutrient absorption which leads to burnt leaf edges.
It's time for a good leaching. This is going to be difficult since the plant is so large, but can really help. Take it to the bathroom and put it in the tub or shower. Fill the pot with as much water as you can. Let it drain and tip it on the side with the drain hole in it until no more water comes out. Do this 2 more times. If you can scrape the crusty stuff (highly technical name) off the edge of the saucer that would be good. Then put it back in its home. If the soil level seems low you can add soil to fill the pot. It will grow new roots into the fresh soil.
There will still be some yellowing, but it should be accompanied by fresh new growth.

Happy Plant Parenting!
The cracks on the trunk are normal.

Maybe also time a bigger pot too?
FYI, P. selloum is a synonym for
Tree Philodendron (Philodendron bipinnatifidum)
Check out all the member photos!

Actually it's not really ok to trim off the adventitious roots. I'm not surprised your plant is rebelling now. The adventitious roots can and do absorb moisture through humidity and if they are allowed to grow back down into the soil they absorb water and nutrients once in the soil. Instead of cutting them off it's better to stick them back in the pot. Which for the size of your plant, is way too small

CalPolygardener said: to the site!
It's normal for them to lose the lowest leaves periodically, especially when light levels drop in the winter. They absorb minuscule amounts of water through their aerial roots so that shouldn't be a worry.
I notice that the pot has an attached saucer and that it is large for the pot. Both indicate that it's been in there awhile and it may have a buildup of mineral salts in the soil. These can damage the fine root hairs that do all the water and nutrient absorption which leads to burnt leaf edges.
It's time for a good leaching. This is going to be difficult since the plant is so large, but can really help. Take it to the bathroom and put it in the tub or shower. Fill the pot with as much water as you can. Let it drain and tip it on the side with the drain hole in it until no more water comes out. Do this 2 more times. If you can scrape the crusty stuff (highly technical name) off the edge of the saucer that would be good. Then put it back in its home. If the soil level seems low you can add soil to fill the pot. It will grow new roots into the fresh soil.
There will still be some yellowing, but it should be accompanied by fresh new growth.

Happy Plant Parenting!
The cracks on the trunk are normal.

Hiiii! Thank you for this amazing reply I was definitely looking for to calm my struggling soul (no, it's the plant that struggles)! I will try to do this and perhaps consider repotting to a larger "home"

Yes, I just noticed that the cracks were there before indeed Also, the amount of light here in the Netherlands is really low right now.

Calif_Sue said: Maybe also time a bigger pot too?
FYI, P. selloum is a synonym for
Tree Philodendron (Philodendron bipinnatifidum)
Check out all the member photos!

Yes, sounds like a good suggestion because this one seems to be a bit small already and the plant has been in there for at least 10 years I think.

Gina1960 said: Actually it's not really ok to trim off the adventitious roots. I'm not surprised your plant is rebelling now. The adventitious roots can and do absorb moisture through humidity and if they are allowed to grow back down into the soil they absorb water and nutrients once in the soil. Instead of cutting them off it's better to stick them back in the pot. Which for the size of your plant, is way too small

Thank you for your reply! Is there a preferred way to trim off the roots so they do not occupy the whole apartment eventually? I indeed trimmed all the roots (newbie) and that indeed seems like the reason. Plus the small pot and the need of leaching once in a while.

m_x_uk said: Yes, sounds like a good suggestion because this one seems to be a bit small already and the plant has been in there for at least 10 years I think.

10 years in that tiny pot?! I think it would be so happy to have more root room!

This is one I bought 6 years ago, it was purchased in a 1 gal pot. I took that photo 2 years ago. I have it outside on my porch by the front door.

Having adventitious roots on plants like Thaumatophyllums and Monsteras is just a fact of plant ownership that has to be dealt with and sucked up. The plants make these roots for their own good. They benefit the plant. They are not meant to benefit you, the human.
Whenever you take it upon yourself to bring something alive into your sphere of everyday life, whether it's a pet, or a plant, you need to be thinking about how to grow it or treat it like it would behave naturally. Not how it's most convenient for YOU.
The main purpose for the unruly roots on a plant like yours is that they anchor the plant and allow it to grow upright like a tree, which is your plant's normal growth habit. It makes those roots all along the trunk and sends them down into the ground to hold itself upright and keep from sprawling and falling over, like Sue's plant stays upright. Normally a plant the size of yours would occupy a radius on the ground of possibly 5-8 feet. Yet you have it crammed into a tiny pot for years where eventually it reaches a critical mass of not having its needs met and starts to decline.
It was maintaining as long as you were letting it be largely undisturbed. Then you changed the status quo, and now it's unhappy.
You need to rehouse this plant into a large tub. I'd suggest at least 15 gallons. 20+ would be even better. There needs to be adequate pot space for you to direct the adventitious roots back down into the soil so that they can function normally for the plant.
You really didn't grow it for 10 years to get it to this enviable size to let it decline now.
The grower should always try and meet the needs of the plant. Not expect the plant to conform you his/her wishes to be 'civilized' in an apartment or other space that the plant would not normally grow in