It sweetens the soil and stops anything getting smelly. It’s a similar material to the ingredient in odour-eaters that you get for your feet, to absorb unwanted smells to keep the bottle fresh.
Quick tip, place the container before you fill it – once the soil is in there it will become much heavier to move around. Avoid bright sunlight if you can, a semi-shaded windowsill or dresser top is ideal.
Plant varieties like ferns, Pilea, Peperomias, or Hypoestes all tolerate and appreciate humid environments, meaning they are ideal for planting in bottle gardens.
Once they are planted in, watered well, and fitted with a cork in the top, it will create its very own micro-climate. This works by the container recycling its own moisture as it evaporates and hits the body of the glass, then as it cools the water falls back into the soil, completing the cycle.