For many years it is debated among bonsai growers about the necessary drainage. There can be no simple answer to this question for many factors.

A drainage layer is necessary to remove surplus water from the plant roots. This process is dependent on several things and factors:

• quality and composition of drainage and soil;
• average size of granules or grains of drainage and soil;
• the size of the container or pot;
• thickness proportion of drainage and soil layers;
• humidity (amount of water when watering) ;
• external conditions (temperature, sun, wind) and location;
• method of watering (watering can, spraying, from a tray, dipping, frequency of watering).
• number, size and location of holes in the container.

Physics of the drainage process

Two forces are created during watering:

• gravitational, which promotes water to flow to the bottom;
• capillary forces, which hold moisture in and between the drainage granules, especially in the fine pores of the drainage material.

Gravitational forces try to draw excess water downward, helping to create conditions where roots have access to water but are not exposed to overflow.

Capillary forces provide to retain part of the moisture in the soil, creating a reserve for the plant and preventing complete drying out of the substrate.
The process is directly dependent on the porosity and spacing of the grains, as well as the water permeability of the drainage material.

Large pores efficiently allow water to pass through and help it to drain away, small pores retain water through capillary forces, thus providing the plant's need for water.

Small pores hold water in the upper layers of the pot, creating stagnation and reducing oxygen access to the roots. At the same time, excessive amounts of coarse particles will lead to rapid water run-off and insufficient moisture retention. Thus, optimal drainage is provided by a combination of macro- and micropores, which promotes both water drainage and retention.

During irrigation, water molecules are held on the surface of soil particles by adhesion (bonding to particles) and cohesion (bonding between water molecules). These phenomena create a “film” around the soil particles that allows the roots access to water.

After watering, the excess moisture is drawn away from the bottom of the pot by gravity, leaving only the moisture in the soil that is held by capillary forces. At this stage, the water at the bottom of the pot is retained mainly by hygroscopic pressure, so that the plants still have access to moisture, but the roots are not at risk of rotting.

Drainage layer at the bottom of the pot is a layer of porous material (e.g. expanded clay, lava, pumice, etc.) that helps water to drain downwards quickly by creating an air barrier between the soil and the bottom of the pot. This layer helps to accelerate the removal of excess moisture and increase air gaps, facilitating oxygen access to the roots.

Drainage holes are an essential part of the pot as they allow water to escape, preventing it from accumulating at the bottom. They also allow air to enter from below, keeping the roots ventilated and preventing the growth of mold and harmful bacteria.

Water balance. When the topsoil dries out, water from the drainage rises up into the base layer of soil by capillary forces, which helps to maintain humidity and utilize the full amount of water available in the pot.

Temperature and vaporization.The temperature and humidity of the air around the pot also affect the drainage process. In hot and dry conditions, water evaporates more quickly, requiring more frequent watering. In high humidity, water evaporates more slowly, causing water to accumulate in the pot.

Cases where drainage is necessary

A drainage layer is necessary if:

• regular watering is difficult or impossible;
• it is not possible to constantly control the humidity of the soil;
• the plant is growing in variable environmental conditions;
• the plant is being cared for by different people;
• no experience in plant care.

Refusal of the drainage layer

Scientific research in different countries over the past decades has led to the conclusion that the role of drainage can be significantly reduced if certain conditions are met:

• use of special substrates and soil mixtures;
• keeping plants in pots and containers made of porous material;
• precise control over the frequency and volume of watering;
• constant control of soil humidity;
• drainage holes for moisture drainage and root ventilation;
• more often repotting of the plant to check the condition of the roots;
• mulching and regular cultivation of the surface layer.

Which system to choose depends on your capabilities, experience and knowledge.