Usually, when a plant is getting leggy, it’s showing signs that it is etiolated. The most common reason this happens is when your plant is getting insufficient sunlight, causing it to stretch out in search of good light. Some signs of etiolation are a “leggy” appearance with longer stems, paler leaves due to lack of chlorophyll, and less condensed leaves. Not to worry! This won’t really harm your plant in the long run if care is adjusted and simply acclimating it to a brighter spot should help. These plants can maintain in lower lighting, but indirect bright light will help them thrive. Bright indirect light can be found in places next to an east-facing window or a few feet back from an unobstructed southern or western window. If the southern or western window has something like a sheer curtain, or natural shade from a tree or building outside, the plant can be placed a little closer.
Over time, roots that sit too long in waterlogged soil will suffocate the plant at the root, preventing proper oxygen and nutrient uptake. This will initially cause leaves to turn yellow and weaken stems, which can lead to the plant stretching or drooping. Drenched soil can eventually cause the roots to rot which will kill the plant outright.
You want to water when the top 50-75% of soil is dry. Water thoroughly, and be sure to empty the saucer of any excess water to prevent root rot.
Check the plant’s roots to see if they might be too root-bound. Look for roots creeping up along the top of the soil or growing through the drainage holes at the bottom of the pot. Another sign that your plant is root bound is if when watering your plants, the water rushes through the pot and out the drainage hole. This shows the roots are taking up too much room in the current pot and there is not a high enough soil-to-root ratio. This inhibits your plant from taking up the proper amount of water and nutrients it needs.
Dieffenbachia appreciates conditions between 65 to 75° F and an ambient humidity of 60%, and sudden shifts above or below this range can shock the plant and cause the leaves to turn yellow or drop off. Try to avoid keeping this plant in overly hot rooms or near an AC vent in the summer, or anywhere near drafty and cold windows and doors in the winter.