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Using Rocks at The Bottom of Plant Pots: Drainage Explained

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Increasing drainage in plant pots is key in preventing overwatered soil and the ill effects it can have on plants. Placing rocks at the bottom of the pot may seem like a good solution but it can do more harm than good.

When rocks are placed at the bottom of a plant pot, the saturated zone becomes adjacent to the soil surface and reaches the roots. The saturated zone exists in the soil at the base of a planter and does not drain. As a result, the roots will remain wet for an extended period resulting in root rot.

Although there are downsides to having rocks at the bottom of planters, there are also some cases where it can actually be beneficial.

This article will give you a better understanding of what to expect when using rocks in plant pots, the types of rocks you can use, as well as how to use them.

Do Rocks Affect Soil Drainage? 

Using rocks at the bottom of plant pot

For a long time, gardeners used to add a layer of gravel before placing the houseplants in pots, based on expert advice. 

However, according to the University of Illinois, placing rocks at the pot’s bottom for drainage improvement is considered a false belief because of the presence of the saturated zone.

See our post on how long should soil remain wet after watering.

I use this Miracle-Gro Potting Mix from amazon, which has the perfect mix and ensures my plants stay healthy long after planting.

The Saturated Zone:

Without getting into any boring details, let’s get straight into it. In case of rainfall or watering the houseplants, water moves downward due to the force of gravity through the soil. 

As a result, all roots get wet, and excess water drains away. 

On the contrary, water percolates through the soil but gets sideward across some different layers like rocks. 

It elevates the saturated zone closer to the roots. Consequently, roots can’t exchange gases with the outside due to prolonged staying in this wet zone, resulting in the ultimate death of the plants.

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When Would you Need Rocks at the Bottom of a Plant Pot?

There are some situations where rocks at the bottom of the plant container are helpful. Now, what are these conditions? Let’s look into it. 

When the Drainage Holes are Too Big

Placing larger than required holes at the base of a planter often happens, and as a result, soil can run out of the pot with the water whenever the plant is watered.

Additionally, when water passes through the soil with larger than normal drainage holes, rocks at the bottom of a planter will limit extra soil washing.

As a result of water running freely out of soil, the runoff water can sometimes emerge yellow or brown from the pot depending on the soil composition.

Using a plant saucer can contain the mess and prevent unsightly staining of floors, but this will not stop the problem.

Placing rocks at the bottom of a planter when repotting, will reduce the size of the drainage hole which would prevent excess soil from washing out any time the plant is watered.

In this situation, rocks are placed flat on the bottom of the pot just to cover the holes. They do not take up much real estate and do not significantly affect the volume of soil in the pot or the saturated water table.

To Weigh Down the Pot

Placing rocks or pebbles at the base of the plant container before filling soil in seems to be a good idea from this perspective. 

Rocks are heavy and having them at the base of a pot will tend to provide that much-needed anchorage that will help the plant stay upright during heavy winds.

Moreover, the rocks will weigh down the plant pot keeping it from tipping over.

Using rocks in this way works well with taller plant pots with a small diameter or base.

But what do you do when a plant is tipped over. Yes, it causes stress on the plant which can be dealt with using the methods described in this post.

Additionally, you can see our article on the pros and cons of placing stones in soil.

What Type of Rocks Can You Use?

Well, different rocks are used as substrate at the bottom of the planter, including: 

  • Lava rocks form a place for excess water to pool away from the plant’s roots. They provide additional benefits by being porous, absorbing, and releasing excess water slowly.
  • Limestone is a common type of sedimentary carbonate rock containing minerals other than calcium carbonate.
  • Slate-like rock is an original type of sedimentary rock made of clay and volcanic ash. 
  • Cinder Blocks are pyroclastic materials that are fragments of solidified lava. They are identical to pumice stones. 

Disadvantages of Putting Rocks at The Bottom of Plants Pots:

As we have already discussed above, some of the conditions where putting rocks at the pot’s bottom is beneficial. 

There are a few disadvantages of using them as well including:

Less Available Soil: 

Using rocks at the bottom of pots limits the amount of soil placed in the plant pot by occupying more space in the container. 

Less soil means less nutrients available for the roots to absorb. Rock at the bottom of a planter will take up space where soil could have been occupied.

The consequence is that less soil provides less opportunity for plants to obtain the full amount of nutrients. 

Moreover, these rocks provide no nutritional benefits to the soil as they are inorganic and can’t decompose. This provides them no reason to help in plant growth.

Does not Increase Aeration:

According to Willis, water can’t move easily from one media layer to another, even from pot’s soil to a denser medium like gravel or rocks. 

This will result in the accumulation of water at the soil’s bottom until it becomes saturated with no space left for air to circulate.  

Lack of proper aeration makes the plant roots more stressed, subjecting them to more plant diseases and death.

Does not help Soil Moisture:

One of the most significant functions of soil regarding plants is storing water and providing water and minerals that are important for plant survival. 

Water moves toward the bottom of the container because gravity and soil can hold water better than rocks or gravel. 

If you partially filled your plant pot with rocks or pebbles, it means the soil you would be placed on it is in lesser quantity. 

Less soil means less water retention, which is not enough for fulfilling plant needs.

Want to know how to dry overwatered soil? See 7 easy ways to quickly dry soil.

Effect on pH of the Soil:

Acidic soil is usually preferred by most plants and trees for their growth. This is where the soil acidity exists between the 6.0 to 7.0 range.

In this range, the nutrients in the soil can easily dissociate into the water and become available for the plant to use.

Whenever the soil pH rises above the 7.0 mark and increasing (alkaline environment), the harder it gets to obtain minerals and nutrients from the soil.

Adding rocks at the bottom of a planter can create an alkaline environment, producing a threat to plants whereby nutrients and minerals are less available for the plants.

This results in many nutrient deficiencies for the plant and can lead to death if not corrected in the early stages.

How do Rocks Affect the Soil’s Water Table?

A perched water table is formed at the bottom of the container’s soil, where the drainage level is present, although it is open at the bottom. 

This saturated water level is known as the soil’s water table. It is a separating barrier between saturated (where water “perches” or gathers in soil) and unsaturated (where water has flowed through) zone.

In the unsaturated zone, the plant’s roots can grow well. The height of the saturated zone remains the same, independent of the volume of soil. 

This means that the addition of rocks at the bottom of the planter reduced the volume of the unsaturated zone.  

The perched water table of saturated soil without aeration becomes above the rock’s layer, providing less space for root growth.

In short, adding rocks at the pot’s bottom gives a result opposite to the drainage we want to achieve. 

Pots You Can Rocks on:

Large-sized pots are usually suggested to use because they have a lower perched water table. 

A wide and shallow container will hold water in closer contact with the plant’s root than the one that holds the same volume but is taller. 

The reason is that gravity pulls the water down through the container and out of the drainage holes. The height of the container affects the relative amount of water versus air. 

See post on covering potted plants with rocks.

How to Improve Soil Drainage

Using Soil Amendments

Adding soil amendments

Although rocks at the bottom of planters may seem somewhat of a soil amendment it does not give all the benefits that actually come with using recommended soil amendments like:

  • Perlite
  • Vermiculite
  • Dead leaves
  • Old Mulch
  • Peat Moss

If you go the route of removing the wet soggy soil you may want to apply soil amendment to the next potting mix to ensure the soil has good drainage properties so you won’t end up in the same situation again.

But what are soil amendments? These are additives that increase the drainage and aeration properties of the soil. These additives are usually perlite and vermiculite.

The proper ratio of perlite and vermiculite along with the intended soil can create the perfect potting soil in which you can repot the plant.

You can have a look at a more in-depth article on perlite and vermiculite which we have elaborated on their properties and how they help improve soil structure.

Bottom Watering Method

Bottom watering allows plants to absorb water from the bottom up through capillary movement throughout the soil when placed in a watering tray. 

The plant is then removed from the tray after 15 to 20 minutes and the excess water is allowed to drain freely from the soil.

The soil remains moist for 3 to 4 days before it can be bottom watered again.

Bottom watering works by the soil’s ability to absorb water through upwards capillary movement.

A point to note is that not all soils will have the same rate of absorption. 

The rate of absorption is dependent on the soil’s structure and the type and quantity of amendments in the soil.

Bottom watering plants infographic

Drainage holes at the Side of the Pot

Placing holes on the sides of a plant pot can improve both water drainage and aeration in plants. 

The number of holes should be between 4 to 8, at 1″ diameter closer to the bottom of the pot with a saucer. With smaller holes, there is less loss of soil and ensures the soil remains well-drained.

Using drainage holes at the side of a pot can bypass the saturation zone that exists above the rocks. 

Using “side holes†will allow for water to drain out while still having the saturated water level at the bottom with the rocks.

An advantage to this would be, the saturated zone will provide moisture for the plant long after watering as the water evaporates slowly and moves up into the soil.

It is like having a water reservoir at the bottom of a planter.

The time between watering the plant can be extended, meaning that it will take less attention from you.

Soil Type and Moisture Content

The soil type can play a role in having stones at the bottom when it comes to water retention.

The lighter the soil (sandy soils) the less water it will retain. The heavier the soil (clay soils) the more water it will retain in the saturation table with the stones.

The length of time for soil to dry varies by soil type. Some of which we have listed in the table below.

Soil TypeMoisture Equivalent % moisture after 24 Hours% moisture after 48 Hours
Light Soil12.518.520.2
Medium Soil22.725.027.0
Heavy Soil36.839.547.2

The Takeaway:

The practice of using rocks at the bottom of plant pots is one of the most controversial and often debated topics for gardeners. 

Although rocks are an inexpensive way to weigh planters down, they can increase the amount of soil moisture required.

Putting a layer of rocks in the planter makes the saturated zone closer to the soil surface approaching the roots. 

Many roots rooting diseases can affect plants growth because of the retention of a soggy environment around plant roots for a more extended period.

Placing coffee filters, a piece of screen or shade cloth can aid you in solving soil washing out problems.

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