Why you should NEVER add a drainage layer to indoor plants
I’m calling fake news on this one. Don't add a drainage layer to your indoor plants! No, this isn't a click-bait title that really turns out to be the opposite. Even the name is a myth. It does NOT increase drainage. You’d be better calling it a ‘root rot layer’.
It turns out the tradition of adding a drainage layer – typically bark or pebbles - to the bottom of your pot is good IF that pot’s going outdoors. It's great for big pots outside, where the roots are way up at the top and the water’s way down at the bottom. It helps containers from drying out too fast, holds on to more water, and shifts that water up closer to the roots.
But for our indoor plants? Adding a drainage layer is the opposite of what our plants need. The name makes it sound like such a good thing, but gravity and capillary action don’t agree.
What's going on inside the pot when you add a drainage layer?
Adding a drainage layer creates a perched water table due to the balance between gravity pulling water down versus capillary action wicking water up. Where those two forces balance sets the height of the perched water table in a pot. When you add a drainage layer, that shifts that perched water table UP closer to roots.
Capillary action is the reason some water gets retained, otherwise it would all drain out when we water our plants. A good potting mix will have the right balance of draining well, but still absorbing and holding onto enough water to give roots the amount they need, with enough air space to avoid roots ending up water-logged (which can also cause the dreaded root rot).
The reason bottom watering works is because of capillary action. Depending on what's in your potting mix, capillary action can literally pull water upwards against the force of gravity pulling water down.
Some mixes have higher wicking ability than others so are better suited to bottom watering and wick watering, like one of my favourites Bio Leaf African Violet mix. Some mixes hold higher levels of water for moisture-loving plants that hate fully drying out, like Peace Lilies, Begonias, African Violets and Ferns. They hold on to more water when top-watered, and take up more water, faster when bottom watering.
But if you try bottom watering with a super-chunky potting mix with low water retention (like a mix high in orchid bark), you'll have to either top water much more heavily, or when bottom watering, soak for longer and with the water level higher up the side of the pot, in order to fully saturate those chunks of bark. Plus it'll tend to dry out faster and need watering again more often.
That's also why you get such different answers about how long you should soak a plant when bottom watering. A good rule of thumb is to soak until the top layer of potting mix is saturated. If your potting mix has such low water retention that gravity wins instead of capillary action, you'll need to raise the water level around the pot to help the water get up to the top layer, or finish bottom watering with a top water.
Why drainage layers decrease drainage
The reason a drainage layer actually increases water retention is because it creates a different layer in the pot. When water is moving down through potting mix and meets a 'layer of change' it moves less freely from the finer layer (potting mix), into the coarser layer (pebbles or bark etc used for the drainage layer).
This is when capillary action wins over gravity, and instead of water continuing to move down through the pot, it builds up in the finer layer above. You might think the excess has drained out because the pot's stopped dripping, but in fact the water has stopped draining out and started to build-up above the drainage layer, instead of flowing out of the pot.
Only once the potting mix layer above is fully saturated, will gravity win and water will start to move down again into the coarser drainage layer. Fully saturated potting mix is the last thing you want to be happening up around the roots!
The better way to increase drainage
The simple secret to increasing drainage (other than never adding a drainage layer again), is to increase aeration evenly throughout the entire pot. You want more air space, everywhere.
It’s much better to DIY or buy yourself an airy, free-draining mix and add that right to the bottom of the pot. That will keep water flowing down and out. I used to DIY my potting mixes, but my go-to now are the Bio Leaf blends. Their Bio Leaf Aroid & Hoya mix in particular is my ultimate free-draining blend for my Philodendron, Hoyas of course, Pothos, Monstera and so many more.
But if you are more of a DIY fan, and worried your existing mix hasn't got enough drainage or air space and may be too high in water retention, adding perlite or pumice are popular solutions, so is fern fibre or some chunky horticultural orchid bark (which is different from garden bark used for mulch).
If you’re like me, I also used to add a drainage layer because it stopped soil coming out the pot’s holes every time I watered. A chunkier mix (I stay away from soil now) takes care of that happening, but you can also DIY or buy drainage mesh that you just add to the bottom of the pot before the potting mix.
Have you been adding a drainage layer to your indoor plant's pots? Share this with a plant-buddy and blow their mind. It definitely surprised me when I first found out!
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