Can You Use Coffee Grounds As A Bonsai Fertiliser?

Coffee grounds are commonly said to be a great fertiliser and are used by gardeners all over the world, but when it comes to using them on your bonsai, they might not be so great.

Bonsai require slightly different care than your average garden plants and coffee grounds just don’t fit in to well with how we grow bonsai.

Why Cant You Use Coffee Grounds As A Bonsai Fertiliser?

If you want to start putting coffee ground on your bonsai, you need to understand that Coffee is not a fertiliser (well, not yet).

When we talk about coffee grounds being a fertiliser, this isn’t quite accurate. It’s quite a common misunderstanding.

The grounds themselves are not a fertiliser. They do contain a lot of nitrogen, which is great, but this is not readily available. The coffee grounds need to be processed so that this nitrogen can be released and used by your bonsai tree.  

When you brew the coffee the liquid will have very low levels of nitrogen. It is all locked within the beans themselves. Further watering of the grounds will not release this nitrogen.

If you want to use them as fertiliser, you need to let them fully break down and be processed by microorganisms.

When the coffee grounds are decomposed by the microorganisms the nitrogen will be slowly released and will go into the soil.

This is the same process that any organic matter will go through when it breaks down. Coffee just has good levels of nitrogen, so out of all the organic matters you could turn into fertiliser, coffee grounds would be a good choice.  

What Happens If You Use Coffee Grounds To Fertilise Your Bonsai?

If you try and use coffee grounds ground as a bonsai fertiliser you are going to run unto some issues which I will discuss below. People either usually just try and put them on the top of the soil surface, or actually into the soil mix. Both of these methods will not bring you the results you where hoping for.

Adding Coffee Grounds To The Soil Surface Of Your Bonsai

Placing coffee grounds directly onto your bonsai soil surface can cause some problems:

Timing/Dose Amounts

The first issue is that the coffee grounds are going to have to break down. This can take a while. So when you add them, it’s not really going to have any effect on the tree for some time.

As you have no real idea how long it will take for the nutrients to actually be released by the coffee grounds you have no idea how much “fertiliser” the tree is actaully getting.

When you use actual fertiliser you know from reading the packet how much nutrition the tree will actaully be getting from each dose. Although this may not be 100% accurate, it will still give you a decent guideline of how long your bonsai tree will be getting fed for and what actual strength it is getting.

Mould Can Develop On Coffee Grounds

To get the coffee grounds to release their nutrients, they have to break down, which means that they are going to get mouldy.

This is totally safe and fairly common with other organic fertilisers, especially the fertiliser’s cakes that are very popular in bonsai, but it can still shock and concern people. The mould is needed as this is what breaks down the coffee ground and gets them to release the the nitrogen that is in the coffee grounds.

However, If you have any indoor bonsai trees, this really wouldn’t be ideal. This mould can attract fungus gnats, which although harmless to the bonsai, you wouldn’t really want in your house.

Sometimes it can also smell, but this isn’t always the case.

Coffee Grounds Can Dry Out

The bigger issue with putting ground coffee grounds directly on to your bonsai soil surface is that if they get dry they can become very hydrophobic, which means they basically become water repellent.

If you have spread a layer of coffee grounds on your soil surface, and it gets too dry and becomes hydrophobic, this is a huge problem. You will now basically have a layer which is blocking water from getting into your pot.

If you have formed your coffee grounds into little cakes, then you bonsai tree should still get water, but the cakes might still dry out no longer absorb water. This means they will not start to decompose, as microorganisms need moisture to be effective, so basically they become useless.

The premade fertiliser cakes you can buy will not do this; they will absorb water and break apart really easily.

Mixing Coffee Grounds Into you Bonsai Soil

If you are mixing ground coffee into your bosani soil mix, you will face the same problems as placing them on the surface, as well as more issues.

If you are watering often, coffee grounds can end up staying moister for longer than the other components in your soil, so it could affect how long your soil stays wet for, which in turn could affect your bonsai.

Although it’s not the biggest issue, as you can just adjust your watering to counteract this, it is something you would need to take into consideration and be aware of.

When you add this problem along with the other issues I mentioned above, I would suggest that mixing coffee grounds into your bonsai soil mix is not the best way to do things.

Do Coffee Grounds Make Bonsai Soil Too Acidic?

Coffee is believed to be acidic and while it does usually lean on the acidic side of the PH scale when brewed, it is very weak.  

You will often hear people say it should be used to increase the acidity of soil or you will be warned that it will make your soil too acidic for your trees.

However, when it comes to coffee grounds, they are actually usually quite neutral in PH.

When you brew the coffee, all of the acidity is extracted into the drink and the grounds are left pretty neutral. So if you are going to use the coffee grounds in any form of gardening or bonsai, you shouldn’t worry. There is no real risk of it affecting your soils PH.

Using Coffee Ground In Other Ways In Your Garden

If you do have coffee grounds I would suggest not using them on your bonsai and instead using them in your garden. You shouldn’t run into as many problems when using them outside and you might be able to reinvest your results back into your bonsai.

Add Coffee Ground To Your Compost

The best use for your coffee grounds is going to be in the compost pile. As its breaks down it will enrich the soil and the final compost will be excellent.

You could then use this compost as a part of your bosnai soil mix.  

Give Coffee Ground To Worms

If you have a worm bin, this would be even better use for the coffee grounds. The worms will be able to process them into some seriously rich fertiliser. Worm castings and “worm juice” make some of the best fertiliser for bonsai.

The results from this will be far greater than just applying the coffee directly to your bonsai trees.     

Add Coffee Ground To Mulch

You can also mix the coffee grounds in with your mulch. I would probably add them so they make up about 20% of the mix at most. If you do any more than that it might start to clog up the mulch and and it will not be very effective.

If you have bonsai trees you are developing in the ground, this mulch would be perfect to use around them.

Scatter Coffee Ground Around Your Garden

You can also lightly scatter the coffee grounds on to the soil surface in your garden, or even gently mix the grounds in with the top layer of soil. Again I wouldn’t use too much; just a light dusting should be enough.

Although this method isn’t great on a bonsai pot, it works really well on the open ground. The extra size and depth of the ground stops it from causing the same issues you will get in your bonsai pot.

Conclusion

So it should now be clear that coffee grounds shouldn’t be used on bonsai. They basically have no effect on your trees until they start to break down.

(You can read about other fertilisers that you can use)

Until then you will just be watering your trees and letting weak coffee pass through your pot. Although this is harmless, it’s just a waste of time and your tree will not get the nutrients you want it to have.

You are better using real fertiliser and then putting your coffee grounds to work in the garden and allowing the benefits you get form this to help enrich your bonsai at a later date.