Can You Use Gravel As Bonsai Soil?

Bonsai soil can be made from all sorts of different components and I often get asked about gravel and what roll it can play in bonsai soil.

Gravel is probably not going to be your first choice when it comes to bonsai soil, but it can be useful in some mixes.

I will go through some reasons why you might want to use some gravel, and why you might not. 

Could You Use Gravel As The Only Component Of Your Bonsai Soil?

When we look at other components commonly found in bonsai soils (like akadama or molar clay) they can usually be used on their own, but can we do this with gravel? Could you really only use 100% gravel as your bonsai soil?

In theory this would be possible. A tree will grow in anything so long as it has enough water and light. However, the reality would be that using only gravel is probably not going to be ideal.

It is pretty heavy, so the roots would really need to really dig into it, but there is also enough little air pockets that they would be able to find their way through and be fine.

I think the bigger issue is the fact it doesn’t hold any water. You may get some collecting on the surface of each stone but this will quickly dry out. I think you would be watering so frequently that it would not be a viable option.

If we look at other components like lava rock, molar clay or akadama, they will hold a little bit of moisture …most of it will flush through, but they will just hold enough to keep the tree happy for a good few hours, or at least even a full 24 hours at certain times of the year which is much more manageable than having to water every 30 minutes or something crazy like that which is what I think would happen if you used 100% gravel as your soil.       

You Can Add Gravel Into Your Bonsai Soil Mix

If you want to use gravel with your bonsai, then the best use for it is going to be adding it into your soil mix to give it extra drainage.

Hopefully you are mixing your own bonsai soil and tweaking it fit your exact requirements. If you feel your soil is holding too much water then some gravel can be an excellent edition to help fix this.

The gravel is not going to hold any water itself and is going to help add more little air pockets within the soil, which can of course help water flush through as well as hold some air that the roots can use.

If we look at the general gardening world, grit and gravel will often be sold for this exact reason. If I look on my local hardware stores website I can find it sold under the “soil improvers” section. People add gravel to their soil to help with aeration and drainage and we can do exactly the same with our bonsai soil.

(You can read more about – Can You Reuse Bonsai Soil?)

You Could Use Gravel As A Drainage Layer

A classic use of gravel is to use it as drainage layer at the base of your pot.

The science seems to say this isn’t really needed and I would kind of agree. I’ve done this and not done it and I don’t really see any difference in my soil performance.

I have usually just found it to be useful if I am using a very light pot (as in plastic food container I have cut holes into) I find the gravel just adds a little bit of weight to the pot, but apart from that it doesn’t really seem to add any extra benefits to have a drainage layer, but feel free to experiment yourself.

You Could Use Gravel As A Top Layer

One use for gravel is using it a top layer for you soil.

Most people do this for aesthetic reasons as they just like the look of it and they may even pair it with moss to get some sort of contrast in the pot.

However, doing this isn’t really going to help with your drainage, in fact it is probably going to do the opposite and help keep the soil moist for longer.

The gravel is going to be blocking the direct sunlight from hitting the soil surface, so it should take slightly longer for the soil underneath to fully dry out.

(You can read more about – Why Do People Put Stones Around Bonsai Trees?)

Are There Any Issues With Using Gravel In Your Bonsai Soil?

While it can be totally acceptable to use gravel in your bonsai soil, it is not perfect and does have some issues, this is why a lot of people do not use it, which I will go through now.

The Particle Size May Not Be Right

Probably the biggest issue you will find with gravel is that the particle size can be too big in comparison with the rest of your soil mix.

Ideally you want all the particles in your mix to be about the same size so your soil can be even though out. If something is a lot bigger or smaller than the rest than it can sort of end up falling through the gaps as such and building up in one area, rather than being consistently mixed throughout.

Usually gravel will be slightly larger, but really it depends on what you buy. If you are buying something that is usually used in a driveway, then this will be massive. However, you can buy aquarium gravel or pea gravel and this is usually a decent size.

Personally I found it was still a little big, but it was an acceptable size to use with my larger trees in larger posts. When I was using my smaller particle size soil for my smaller trees …the pea gravel looked like a massive boulder sat in it, and it wasn’t a good fit at all.

Generally if you get some small gravel it will mix in well with the rest of your bonsai soil and shouldn’t be too much an issue for you but it is something to be aware of.

Gravel Is Heavy

Gravel can be quite heavy, which means your pots are going to be heavy if you use a lot of it in your soil mix.

This isn’t always a bad thing and if you are using very light plastic pots, this may actually be what you want.

I have certainly done this in the past to make sure things do not topple over so easily.

It can be a little confusing when you first start using gravel in your soil mix as you will be used to the lighter weight of your usual soil, then when you go to check if your pot needs water or not it can throw you off. You might pick the pot up and think “this is full of water” when it isn’t, it is just the weight of the gravel.

However, you will get used to the slightly heavier weight and learn to feel the difference when it is dry or full of water.

There Are Questions Around Gravels Ability To Hold Heat

I often hear people suggest that you should not use gravel in your pot. When asked why they give some sort of answer around the temperature.

Basically the theory is that the gravel will absorb heat, meaning your soil/pot may get hotter than you wish it to be.

I think there is some sense to this, just think of any concrete on a hot day …it can get very hot and burn your feet. However, does that actually happen to the little stones inside you pot? I have no idea. I guess it is possible.

I live in a very mild climate, so this doesn’t even cross my mind, but if you do live somewhere warm, this is something worth thinking about. Maybe it is just an urban legend, maybe it is legit. I don’t even know how you would go about testing for this. I would just suggest you are at least aware that this might happen before you use it, just in case.

Conclusion

As you can now see, gravel can be used in bonsai. It isn’t the most popular component, but it can certainly be used effectively in a number of different ways. 

I think the benefits outweigh the negatives and while it is probably not wise to use it on its own, it can be a great solution to add some extra drainage into your soil mix.

I have always had a little bit incorporated into my own soil mixes over the years to help with drainage and I have no complaints. It is also cheap and easy to find, so it really is a good solution that you should consider for your own soil. 

(You can read more about – A General Guide To Bonsai Soil)