There are too many gardening tricks out there that just don’t work or even worse can hurt your plants! As lovers of the garden, we feel it is our duty to right these wrongs and help you to better understand your garden and plants.
Myth Debunked: Adding Gravel or Styrofoam to the Bottom of Pots Improves Drainage.
A gardening myth that is very commonly encountered is the suggestion of putting gravel or Styrofoam chips at the bottom of a pot to improve drainage. This practice has the opposite effect and makes pots drain more slowly! Though this seems counter-intuitive, this effect is due to how water moves through soil.
Water moves through the connected air spaces in soil (pores in the soil) by gravity. This process is called “percolation”. Water moves slowly from a fine-textured layer into a coarse textured layer, due to the physics of how water moves in small pores in soil. Basically, soil needs to be fully saturated with water to percolate into a coarser textured area than it is held in.
You can use a square kitchen sponge to get an idea of how this works. Saturate a sponge in water and hold it so the long surface is horizontal. Allow the water to drip from the sponge until it stops. Then turn the sponge so that the long surface is vertical. Notice that additional water begins to drip from the sponge. This is due to the effects of gravity on water in the pores of the sponge. When the long end is vertical, a longer area of connected pores filled with water is affected by gravity, causing additional water to drain.
Instead of using gravel at the bottom of a pot, put a layer of screen or several layers of newspaper at the bottom of a pot to cover the drainage holes. This will ensure that pots drain well but don’t lose soil from the drainage holes. For plants that need particularly well-drained soil, add additional perlite to the potting mix. This creates additional large pores, and helps to ensure that the mix is well-aerated and drains well.
Allen R. Pyle is the lead horticulturalist at Jung Seed Co. and has been a professional horticulturist for over 20 years, with decades of experience in gardening and landscaping. Allen has degrees in both horticulture and entomology from Michigan State University. He has extensive experience in plant propagation, pest management, growing perennials, and organic gardening. His knowledge spans a wide range of plants, including edibles, ornamentals, herbs, weeds, and native species. Allen is passionate about plants and gardening and is always happy to share his knowledge and expertise with others. He regularly speaks and writes on plant-related topics for both professional and amateur gardening audiences. Allen is also certified in Permaculture design.