No, Gravel in a Potted Plant Does Not Improve Drainage
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Placing gravel in the bottom of a potted plant under potting mix does not increase drainage as the water will form a waterlogged layer of dirt vs. being able to drain from the finer potting mix materials to the rough gravel. The saturated layer of dirt, soil, or potting mix can trap in bacteria, fungus, and even lead to root rot causing health issues for your potted plants.
In addition to creating a non-draining layer of dirt, the gravel may shift around and block the holes in the bottom of the pot that are meant to release water. When water cannot be drained, you further increase the chances of plant health issues from overwatering and saturation.
With that said, there have been multiple tests done by organizations like the NIH which tested drainage materials and layers including gravel and coarse sand. Their findings showed that some materials like coarse sand may have an impact, but the more efficient method of increasing drainage or water retention is the actual potting mix materials themselves.
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Why People Think Gravel in Potting Plants Increases Drainage
The idea that gravel will increase drainage in potted plants likely comes from its use in outdoor gardens where gravel can and will help with drainage. In an outdoor setting gravel can allow for water to pass through its air pockets, allows for a more free flow of water so it doesn’t pool and waterlog plant roots, and may help to prevent soil from getting compacted so the right amount of moisture can soak in.
Gravel has another effect, moisture retention. Similar to mulch, the gravel will work as a barrier preventing sunlight and wind from drying up or blowing away moisture so the soil retains moisture for a longer period of time. This is why it is popular in dry climates like the desert. But gravel's ability to drain excess water is limited to outdoor spaces and large areas where the water can continue to flow.
In a pot, the gravel is below the potting mix vs. above the soil when it is used in an outdoor garden. In outdoor spaces, water can continue flowing and move away from the plants. In a pot, it has to go downwards and out, which may not happen because of the compacting of the potting mix from the initial flow of water to the gravel layer.
It can be used in some species like succulents and cacti on top of the potting mix as a way to retain water, but that is a rare exception and it is more about the style and decor rather than function like outdoors.
Potting Mixes for Drainage
Although you do not want to use gravel in your potted plants, you can find potting mixes designed for drainage and aeration as an alternative to gravel. Perlite for example is a volcanic rock that looks and feels a bit like gravel, but it is much smaller and won’t create the same saturated layer that traps water.
Coarse sand can have large air pockets due to the large particle sizes running from 2.0 to 4.75, that help improve the flow of water and give roots room to breathe and grow. Coconut Coir is another material you’ll find in “chunky” potting mixes like our Coco Bag that can both help with drainage and retain moisture. This is perfect for houseplants like Ficus and Hoya that need moisture retention but can be susceptible to root rot. The same goes for Orchid Bark (pine bark) which retains moisture while increasing aeration and drainage for healthy root systems to grow.
Gravel will not increase drainage for potted plants, but it may trap water in and lead to health issues. Instead of gravel, use chunky materials like coconut coir or orchid bark, and find potting mixes with materials like perlite that increase aeration and drainage so your houseplants can thrive.
