Pumice 3/8 mm - 2 litre bag

What it is and where it comes from

Pumice is a light, highly porous volcanic rock, formed when lava solidifies while trapping gas. Here it comes sieved to a 3/8 pumice grain and packed in a 2 litre bag. It is an inert material: it provides no nutrition, it works on the structure of the substrate.

How it behaves in the pot

Water flows through and never stagnates. The pores within each grain hold a reserve of moisture that the plant draws on between waterings, while the rest drains away and leaves room for air. It is precisely this alternation between pores filled with water and pores filled with air that pushes the root system to branch into fine roots.

In terms of durability, pumice is among the most reliable materials available. The grain does not turn to paste, does not break down with frost and reaches the next repotting with the same shape it had the day it went into the pot: no compaction, no suffocating clay around the roots.

Specifications

• Material: pumice, natural inert material of volcanic origin
• Grain size: 3/8 mm
• Pack: 2 litres
• Use: draining component for bonsai substrates

Which grain and for which plants

3/8 mm is a medium-coarse grain. It works well in the body of the substrate for medium and large bonsai, while in the shallow pots used for shohin it is bulky and is better kept for the bottom layer only. As a drainage layer under the roots it is perfect: it never closes up and keeps the drainage holes clear.

It pairs well with any component that holds more moisture, from akadama to organic soil, and on conifers many growers use it with a heavy hand because they tolerate wet feet poorly. It can also be used on its own, a common choice during the early establishment phase of nursery plants or freshly collected material, when air matters more than nutrition.

How to use it at repotting

Sieve it before use. Even though the grain is graded, transport generates dust which, once wet, clogs the gaps between grains: two passes over a fine sieve are enough. If you prefer, you can also rinse it.

At repotting, spread a layer of this grain over the bottom of the pot, above the drainage mesh, then carry on with your chosen mix. Among the roots the material is settled with a chopstick, working from the bottom upwards to close the voids without crushing. Water generously the first time, until clean water runs from the holes.

Frequently asked questions

Does 3/8 mm pumice break down like akadama?

No. It is volcanic rock, not fired clay: the grain keeps its shape and porosity for the whole period between one repotting and the next, even after freeze and thaw cycles. That is the main reason it is chosen for substrates that must stay open for a long time.

Can I grow a bonsai in pure pumice?

Yes, it is common practice during the establishment phase, when air around the roots matters most. Being inert it does not feed the plant, so fertilising needs careful management and watering will be more frequent than with a substrate containing moisture-retentive components.

How many pots will 2 litres cover?

It depends on the volume of the pots and how much pumice goes into the mix. A 2 litre bag of pumice suits a few repottings: enough to fill one small pot or to prepare the drainage layer for several plants, ideal for trying the material out or topping up a mix.

What is included

• Volcanic pumice, 3/8 mm grain size
• 2 litre pack

If you prefer to measure out the components yourself, you can build your own mix on the made-to-measure soil page.