Many believe that placing a generous base layer of rocks and/or gravel at the very bottom of plant pots is a great trick. But does it really benefit container gardens? Or is it just an overblown gardening myth? Well, science can give you the answers you are looking for!
Generally, it is not recommended to place rocks at the bottom of plant pots. Doing otherwise will not improve drainage or air circulation in the soil. Contrary to popular belief, studies have shown that adding rocks at the bottom of planters can harm the plant by raising the saturation zone level and increasing the chances of root rot.
Rocks are a “not to go.” However, what is this saturated zone, and how can you increase its size so as to limit watering problems. Also, there is a case in which rocks at the bottom are actually important!
There is a widespread belief that adding some type of coarse material to the bottom of a pot, such as rocks or gravel, improves the drainage in the pot. But as explained by the University of Illinois, this is a myth. Due to the existence of the Saturated Zone (or Perched Water Table) in potting soil.
The Saturated Zone
When you water a potted plant, the force of gravity pushes the water downwards through the soil. However, when it goes down through the soil (from top to bottom), it reaches a point where some of the water stops going down and remains in the soil.
Why? Because the water sticks” to the soil a bit like water is “attracted” by a dry sponge. The point within the soil the water stops going down is the limit between the unsaturated (where water has flowed through) and saturated (where the water sits or “perches” in the soil) zone.
Remember: The saturated zone stays wet for a long time. Hence, you want to keep the roots away from it as it can suffocate them (if they are in water all the time, they cannot exchange gases with the outside).
Here is the real deal with the saturated zone.
Without going into boring physical details, the saturated zone is always the same height independently of the volume of the soil. That means if you put your herbs in a shallow pot, the saturated zone will use more volume of your container, compared to a tall pot (of the same volume). Hence, in a shallow pot, the roots of your herbs will be likely to be submerged in soggy soil.
What does that mean for your plant? This means that, as explained by other expert gardeners, adding rocks, gravel or any other material at the bottom of the pot uses its volume (where there are rocks there is no soil) reducing the volume of the unsaturated zone (as the saturated occupy the same height in the soil). This means that the always wet area is close to the surface, which, as shown clearly in the following video, reduces the amount of water that will drain from the soil.
As presented by North Carolina State University, putting rocks in the bottom of planters can create results opposite to what you were hoping to achieve. It raises the saturated zone closer to the roots of the plant, it is more likely that the roots will sit in this wet zone.
If this occurs, your plants are likely to get leggy, wilt, not have new growth, experience stunted, get rotten roots, and ultimately die.
There is only one situation where rocks should be used in planters: when you are growing the plant in a glass jar.
Jars do not have any drainage holes, so it is essential to create a natural drainage system. Rocks and gravel can help through the spaces through them that allow some water (remember, the saturated zone still applies) to leave the soil and sit at the bottom of the jar, away from the roots.
However, you will still have the effect of the saturated zone. So the rocks should be kept to a height of 1-2 inches (2-4cm) depending on the size of the jar. This height will make sure the roots of your plant are far enough away from the perched water table.
If this is the case, it is essential to sterilize the rocks first, as they can be carrying diseases, fungus, or pests that can harm your plant. They can be sterilized by soaking in a solution of 1-part bleach to 10 parts water and then allowing them to dry completely. Alternatively, if your rocks are smooth, they can be boiled calmly for ½ hour before use.
But be careful: craggy or porous rocks could hold gas and explode during boiling.
Were you looking for a way to increase the drainage of your indoor plants and herbs? In this case, forget about the rocks. Focus on: a pot with holes, a good-sized saucer, good quality potting mix, and perlite.
Whatever material the planter is made from, it needs to have at least one hole in the bottom, but four holes are ideal. As described by Washington State University, the holes play an important role in drainage but also are vital for root aeration.
Here’s a trick
If you want your plant or herb to be placed in a fancy pot (like this one on Amazon, very good-looking, and one of my favorites) that does not have holes, you can still go for it.
Just place your plant in a first planter with holes (does not need to be good-looking) then place the pot with holes inside the outer (good-looking) container.
A saucer is not only useful to catch water draining from the planter, but also it indicates when you should stop watering. Indeed, when the water starts dripping from the container to the saucer, this is a signal that you should stop watering.
Here is a tip from the experts
This expert gardener places rocks between the container and the saucer to prevent water pooling in the dish and to stay in contact with the soil.
According to this house plant professional with 30+ years of experience, the soil is the most crucial factor for drainage.
A coarse potting mix, full of large particles, will allow the moisture to drain through more effectively. You know when the soil has this feature by touching it when you plant your herb in the pot.
This higher drainage quality can also be achieved by adding perlite, a very lightweight mineral available from nurseries and online retailers.
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9 Signs of Bad Drainage and Aeration to Look for!
Here are common signs of poor drainage and insufficient aeration in potted plants:
- Yellow or very light green leaves
- Frequent leaf-dropping
- Collapsed flowers
- Brown roots
- Stunted growth
- Abnormally sticky soil
- Wilting of leaves and flowers
- Hard-packed growing media
- Growing media dries out too quickly
Before getting your hands dirty and making a lot of big changes in your container garden, is it even necessary?
Here’s the thing: Unless you can actually tell that there’s something wrong with your potted plants’ drainage and aeration, you’re most probably better off leaving your well-established plants alone. Otherwise, you run the risk of damaging and killing your plants.
So if your plants are not showing any of the aforementioned signs, just continue caring for them as you always have and stick to your routine!
Do note that the signs that I have listed above are indicative of “bad” referring to not only the lack of but also the unwanted excess of drainage and air circulation. You see, too little or too much of anything is never good!
If there is only one point you get from this article, let it be this – do not put rocks at the bottom of your planter. Sure, there will be many people out there that have done this for years and will advise you that you must. But rocks serve no useful purpose in planters, and as you have seen, only put them at risk of root rot. Hopefully, what you have learned here will make your indoor herbs and plants happy for many years to come.
The best-quality potting soil has a combination of physical and chemical properties. It should be able to retain water, provide proper aeration, and have the right amount of nutrients (coming from the correct type of compost). Check it below.
What to use for drainage in a pot? To increase the drainage of a pot, it is important to add coarse material to the soil. The best, recommended by many expert gardeners, is the addition of perlite (or cacti mix)
Can you put broken terra cotta pots or tiles at the bottom of plant pots? One can use broken terra cotta pots or tiles at the bottom of plant pots as planter fillers if there isn’t enough growing medium at hand. However, doing so can inhibit proper water drainage and soil aeration, and even raise the level of the saturation zone. Moreover, adding these can make pots heavy and hard to carry.
How will you know if potted plants are not well-drained? The easiest way to know that a potted plant is not well-drained is by checking the moisture content of its growing media. Home gardeners can easily do this using a moisture meter like this one on Amazon. Generally, if a potted plant is not well-drained then the meter will read “wet.”
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