Which pebbles should I choose for my terrarium?
What pebbles to choose for your terrarium will largely depend on what you're after in terms of the look of your terrarium, and whether you'd like pebbles for drainage or for decor.
The first layer of drainage is usually the pebbles or the gravel. This captures any excess water that drains through the substrate levels.
We highly recommend going for clay pebbles (otherwise known as 'leca' or 'pon', or 'hydroponic clay pebbles'. This is a very lightweight and porous substrate and is excellent for drainage as they create lots of airy space for roots. It is a baked clay substrate that has a neutral pH level and no nutrients. A sustainable solution that can be re-used because it can be cleaned, it also has a lovely warm red-toned colouring that complements any terrarium beautifully. If you ever need to re-make a terrarium, try not to throw away the clay pebbles at the bottom so you can reuse it for your new terrarium.
We also use perlite as a drainage layer. These are much smaller, and are similarly lightweight allowing for lots of drainage. They look like little pieces of polystyrene but are in fact 'popped' volcanic glass that expands the surface of the perlite. This creates another light and porous drainage level substrate with good water absorption.
In addition to using it as the first drainage layer in your terrarium, perlite is often mixed into soils with sand to create an airy, draining planting medium for plants like succulents and cacti that do not do too well with compact, wet soil. They are white in colour so are perfect for modern, classic and clean aesthetics or monochrome looks.
We don't recommend using pebbles or gravel for the base layer of your terrarium because these pebbles tend to be quite heavy and are not porous so do not allow for the same level of drainage. This can be a problem for terrariums as it can lead to root rot if the water is collected in the base layer and allowed to pool there without being absorbed. However, if you really do insist on using these heavier pebbles or gravel as a base layer because you are in love with the way they look, make sure you use a little bit more to create more space at the bottom of your terrarium, and be very careful with watering, making sure to do a little bit at a time. The layers of sphagnum moss should help to buffer the effects of this as well, and you might even want to pour a hidden layer of perlite or hydroponic clay pebbles in between the sphagnum moss and the soil to add that extra level of protection.
Pebbles, gravel, stones, rocks and crystals make excellent decoration and provide a lot of interest and texture to your terrarium. Sometimes just one larger statement rock is enough, but other times you might like to use a couple of river rocks in a couple of central places, and smaller pebbles scattered in between plants to fill empty spaces or even to create little pathways through your terrarium to recreate scenes. Slate is great for replicating steps and pathways too and its dark colour can add some beautiful and understated touches.
We would recommend you avoid using these heavier pebbles to completely cover the surface of your terrarium, especially in a closed terrarium. This is because a closed terrarium is supposed to be a self-sustaining ecosystem where it waters itself through a process of condensation. If there are too many pebbles covering the surface of the soil, the flow can get interrupted. There is also more likelihood of your plants rotting because of an aggressive containment of moisture.