I have some potted plants (Philodendron selloum, Monstera deliciosa) that were propagations from larger plants. Their leaves are on average only about 4 inches long. They were propagated about a year ago. Is there a way for me to encourage larger leaf growth, or will that come with maturation?
The plants are in potting soil in pots that are a suitable size, and are in front of north facing windows, as that is all I have to offer.
Phosphorus is supposed to help increase leaf size in plants generally, I've read. It's also supposed to help plants mature, and increase internode length. Cold temperatures can decrease phosphorus availability.
A soil pH imbalance might reduce phosphorus availability.
I gave my rubber fig some basalt rockdust once, and it's been growing small leaves and dark spots ever since. I imagine the rockdust raised the pH, due to the calcium in it. It was fine before.
If you gave it something interesting, you might consider giving it new soil and not giving it that thing again.
Use potting soil designed for indoors (not for vegetables, outdoors, or seeds).
A plant that's not growing right can have a host of problems. Probably the most common in potted plants is drainage or sun/light issues.
If you are using clay pots with holes over a saucer, try putting some Legos between the bottom of the pot and the saucer. You don't want any of your soil sitting in standing water--ever.
Are your roots free? Did the transplant get put into too small of a pot?
Is there something in your soil to improve water management such as perlite, sand, coco coir, peat, clay pebbles...?
Can your plant get more sun?
Beyond that, look closely at your leaves; there may be signs of a nutritional problem.
If you want to "cheat": Humus can be turned into a water-soluble salt called humic acid. My favorite is Diamond Nectar by GH. It creates robust growth. Raise the CO2 level for a few weeks with a tank or a few blocks of dry ice. I'm presuming you're indoors with a south facing window. Within a month you should see substantial new growth.
Edit: You face an impossible task without more light. Even a table lamp with a grow light, light bulb would be fine. With a north-facing window...You have about as much lux as a 60W equivalent (6W energy use) grow light at 12 inches. You need to at least double that to have any chance of larger leaves. However, there's nothing inherently wrong with slow growth and small leaves. You could just leave it alone.
