Putting porous rocks in your potting mix is essential for adding aeration and improving drainage. However, some have suggested placing gravel at the bottom of your pot instead. But is this a sound practice?
As a general rule, NEVER place rocks in the bottom of a planter. It impedes drainage and exposes roots to a constantly saturated environment, causing rot. It also reduces adequate soil volume for the roots to expand, potentially stunting plant growth.
Below, I explain further why it’s a bad idea to put a rock layer at the bottom of your planter:
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How Does a Rock Layer in a Pot Prevent Drainage?
Imagine a sponge without the proverbial square pants. When it’s soaked and stands upright, residual water pools down at the bottom after the excess has already drained. But above it is a dry area. The fine line separating these two areas is called the ‘Perched Water Table’, which happens in your potting mix/soil.
When you water a plant, moisture travels down the soil via gravity and out of the pot’s drain holes. As this happens, the bottom becomes the saturated zone which acts as a reservoir. The plant can wick up water from this area into the unsaturated zone until the next watering session replenishes it.
Fascinating fact: if you put the same potting mix in multiple pots of different sizes and water each fully and allow them to drain, the ‘Perched Water Table’ line will be at the same height across the various container sizes. Crazy right? That means adding a rock layer at the bottom of your planter brings the wet soil zone/water table closer to the roots, possibly suffocating the roots and creating rot. Some plants can handle the extra water saturation, others just can’t, unless you know your plant can or you are willing to tinker, just avoid the rock layer.
While you may be thinking “Well I will just use a bigger pot so the roots aren’t below the water table”. By buying a bigger pot you’d spend more money on a pot and the rocks you put at the bottom so, unless your potted plant needs the weight to avoid getting blown over by the wind, just skip the rocks. Instead you could get the smaller, less heavy/expensive pot and use an appropriately draining potting mix. On top of saving you money, you’ll most likely get a healthier plant since premade potting mixes and potting mix recipes can easily be found online and other people have already done the tinkering to make sure it drains properly for your type of plant. Ex: succulent/cacti mix. So less money and time spent on pots, rocks, and overwatered plants that die on you, sounds good to me!
As for planters with no drain holes, the excess water has no place to go and will stagnate at the bottom of the rock layer. This creates a permanently moist environment that increases the chances of pathogens breeding in the standing water.
If you’re worried about chunks of soil draining out of the pot, place a coffee filter or landscape fabric over the drain hole. These items are porous enough to allow water to pass through but not the soil. Alternatively, as mentioned in the introduction, mix in porous rocks such as perlite or vermiculite (Amazon links) in the potting soil, and you’re good!
How Does a Rock Layer in a Pot Stunt a Plant’s Growth?
When you add a rock layer at the bottom, this leaves limited space to put soil in. Here are two consequences that may happen:
- The plant’s rootball will have rocks lodged in its composition, encouraging awkward and gnarly root growth. This is not necessarily a problem, but you’ll find it hard to remove these rocks during repotting, especially if your plant is rootbound.
- The plant’s roots can only grow in short, thin wisps due to the limited soil content in its environment. This will slow down if not stunt your plant’s development if it can’t grow strong and thick roots as it should.
These may not be a problem if the plant is in a larger pot. However, there is still the danger of exposing the roots below the ‘Perched Water Table’, i.e., the saturated zone. So unless you’re willing to do multiple experiments to find that fine line to prevent root rot, you’re better off filling the whole pot with soil.
It’s also worth noting that some people use this method to weigh down tall and narrow containers with topside-heavy plants like an indoor tree. This technique is viable, but you need to take certain precautions to avoid waterlogging the roots. Or you can just use a firm and heavy plant stand (Amazon link) instead.
Note: Drought-tolerant plants are an exception, you can plant these in porous materials like perlite, pumice, or leca. These are not like regular rocks as they can hold moisture while providing excellent drainage for the plant. Because of this a non porous rock layer shouldn’t be too much of an issue for succulents. We are at this point quickly approaching hydroponics if we were to grow non succulent plants in these mediums as there is no soil involved so we will leave that for another article. But if you would like to read more about it check out this article here.
What do you fill the bottom of a planter with?
Porous materials such as coffee filters, landscape fabric, or pantyhose are a safe alternative to line the bottom of a planter. Stone garden centers even sell a plastic mesh you can cut to size. All of these options allow more free-flow drainage and prevent soil from coming out.
All in all, unless you have cacti and succulents as your preferred plant babies, skip the whole rock/gravel filler in the bottom of your containers. In many cases you’d be doing more harm than good by exposing your plant’s roots to water for longer and giving it less room to grow; stick with plain potting mix instead. Happy planting!