Short answer
A soft, mushy stem or base on a ZZ plant is almost always caused by root and rhizome rot due to overwatering. To fix this, stop watering immediately, remove the plant from its pot to inspect the rhizomes, and cut away any black, mushy sections before repotting in fresh, well-draining soil.
Seeing softness at the base of your Zamioculcas zamiifolia is a serious concern, as it signals that the plant's thick, water-storing rhizomes are decaying. Because ZZ plants are desert-adapted succulents, they are highly intolerant of 'wet feet' and will succumb quickly if the soil does not dry out completely between waterings.
The ZZ plant stores massive amounts of water in its bulbous rhizomes, meaning the soil must be bone-dry before you add more moisture. If the soil remains damp for too long, oxygen is excluded from the root zone, leading to rapid decay of the tissues.
Check: Check the soil depth with a moisture meter or wooden skewer; if the soil is damp even two inches down, the plant is being watered too frequently.
Fix: Cease watering, remove the plant to inspect the rhizomes, trim away any rotted (slimy or black) parts, and repot into a mix of 50% succulent soil and 50% perlite.
Even with correct watering, a pot without drainage holes acts as a reservoir, trapping moisture at the base where the rhizomes sit. This creates a perpetual state of waterlogging that inevitably causes soft stems.
Check: Lift the nursery pot or check the bottom of the decorative pot to see if water is pooling at the base.
Fix: Repot the ZZ plant immediately into a container with large drainage holes and ensure the saucer is emptied after every irrigation.
Standard potting mixes often hold too much moisture for a succulent like the ZZ plant, staying wet for weeks at a time. This lack of aeration suffocates the roots and promotes the growth of pathogens that thrive in low-oxygen environments.
Check: Observe if the soil surface stays dark and moist for more than 4 weeks after watering, or if it feels dense and muddy to the touch.
Fix: Repot using a gritty, fast-draining cactus or succulent blend that ensures water passes through the container quickly.
When to worry
If the mushiness has reached the main stems above the soil line or the base smells foul, the rot is advanced; you must take emergency action by unpotting to save any healthy rhizome segments.
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Diagnose my ZZ PlantIf all rhizomes are mushy and soft to the touch, the plant is likely unsalvageable. However, if you find even one firm, light-colored section, you can remove the rot, dust the cut with cinnamon, and repot it in fresh, dry soil.
Yes, cut off any stems that have become soft or mushy, as they are already dead and will only spread decay to the healthy parts of the plant.
On average, a ZZ plant only needs water every 3 to 4 weeks. Always check that the soil is 100% dry throughout the pot before adding more water.
Consistent with guidance from: University of Florida IFAS Extension, Royal Horticultural Society.