Darryl’s Analysis

Environment:
This plant has no chance of growing well from this far away from the window.  You should put the plant right in front of those windows, especially because you said you don’t get any direct sun coming through the windows – the strength of “indirect light” is inversely correlated to distance from the window.  If you use a light meter, you will see that the light levels drop off dramatically as you move away from the window – trust me when I say the best possible light your plants can get indoors is ***right in front*** of your largest windows.  You only need to block direct sun with a white sheer curtain if it will shine on the plant for an extended time (more than 3-4 hours).

Effort:
WHEN to water: you should be aiming for “partially dry” (around halfway between fully saturated and totally dry), not simply “once a week”.  Observe soil dryness rather than adhere to a schedule.

HOW to water: fully soaking and letting excess drain away.  One liter is probably sufficient for this size of pot but do so once you’ve moved the plant closer to the window and you are making the determination to water based on observing soil dryness.

Once light is improved, you should use fertilizer to get the best possible growth.  Suggestions here.

I can’t really tell what kind of soil this is but generally for Dieffenbachia, a mix of coco coir/peat moss with some perlite or bark chips will work.  This soil looks like it may be too water retentive.  Try to repot into a slightly bigger pot than you have currently and use the above mentioned soil mix.

Expectations:
Regarding older leaves yellowing – this is inevitable because the fact is leaves have a limited lifespan. They won’t stay green forever if given “proper care” – there is no way to absolutely stop the yellowing of leaves (nor is there a way to reverse it).  That’s why the goal of long term plant ownership is for new growth to be faster than older leaves dying – and the reason why adequate light is critical.  If you improve the light situation for this plant then it should grow more new leaves than it loses older ones (up to a certain point).

My Dieffenbachia is constantly losing its lowest leaves but because the light situation is conducive to growth and I’m watering/fertilizing accordingly, there is always new growth at the top.  See how the lower parts of the stem have markings where older leaves have fallen off – this is how plants grow.

If you want to learn a healthy, balanced approach to houseplant care, check out my online course or my book.