Rocks at the bottom of your planter? A good idea of aeration? Not exactly, and here why.
Hence, should you put rocks at the bottom of a planter? No, it is not recommended because the bottom of the soil in planters is the saturated zone. This is an area that does not drain. Putting rocks at the bottom of the pot pushes the saturated zone closer to the surface of the soil, potentially reaching the roots. This will leave the root wet for longer, increasing the risk 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 wide-spread belief that adding some type of coarse material in 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 into 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 this: 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 the real deal with the saturated zone.
Without going into boring physical details, the saturated zone is always the same height independently from 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 not 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, of course, do not have any drainage holes, and 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:
1. A pot with holes
3. Good quality potting mix
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 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 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 light-weight mineral available from nursery and online retailers (you can have a look here on Amazon).
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 within the soil. The best, recommended by many expert gardeners, is the addition of perlite (or cacti mix)
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