What To Do About Mould On Seedlings (When Growing Bonsai )

Growing bonsai form seed is a very long process, but very rewarding.  It is also quite difficult and you can run into many problems along way.

One of the first and most common problems with seedlings is seeing white mould growing in the pots. This can understandably be quite alarming and worrying. No one wants their bonsai to die, especially before it even had a real chance to grow.

Luckily this is normally harmless and also very common. You probably haven’t done anything wrong and you can usually sort the problem out fairly easily.    

Will Mould Kill My Bonsai Seeds?

Where and when you see the mould can make a difference:

Mould When Your Seeds Are In The Fridge

A common way to start seeds is to soak them, wrap them in a paper towel, bag them up and place them in the fridge for a couple of weeks. If you unwrap your seeds to find mould, this for me, is worrying.

I can’t find a definitive scientific answer on this, so take this information as my opinion, not fact. I feel that if mould is growing on your actual seeds, while in the fridge, is a bad thing.

Some say it’s normal and can actually help the seeds germinate as the mould and seeds have some sort of symbiotic relationship. Others say that it’s a sign the seed is not viable and the mould can also enter the seed and damage it or prevent it from growing. So there is a lot of conflicting evidence on this.

If I were faced with mouldy seeds in the fridge, I would separate them from the rest of the non-mouldy seeds. You could then either throw them away (which I have done) or try and wipe the mould off with a damp cloth then plant the seeds separately.

There is still a chance they will grow since there seems to be no real solid evidence to show otherwise (maybe one day I will try this experiment myself), but I think the chances are low.          

Mould Around Seedlings

If your seeds have germinated and you have potted them and they are growing …but you are now seeing mould on the soil surface, this is less of a worry.

Even if your seeds have germinated or not, mould on the soil surface is not going to affect them.

What Is Mould?

Mould spores are everywhere, you can’t escape them. However, they really only grow like crazy when they are given the right conditions.  Annoyingly for us, those conditions are the same ones that seeds also love to grow in.

Moulds are just a bunch of microbes that cause biodegradation, which means they just break down dead matter. Soil is made up of organic matter, which is the polite way of saying dead matter.

So moulds are just further breaking down the organic stuff in the soil (like pine bark, or peat moss).

In nature the mould breaks down the dead matter and the result of this process is that nutrients end up being released from the matter back into the surrounding soil. This is how compost is formed. This is how trees survive in the wild. This is how they get nutrients from the soil, which is actually made up of their broken down leaves.

This is the circle of life stuff. The trees need the mould and the mould needs the trees.      

So mould is actually really important and it’s a sign that your soil is actually really healthy.  

(You can read more about – How Do You Know If Your Bonsai Seedlings Are Healthy?)

What Causes Mould To Grow On Bonsai Seeds?

As I said, mould spores are everywhere, especially in soil. To understand what causes the mould to grow, we first need to understand that there are lots of different kinds of mould. They can grow in slightly different conditions and some are more harmful than others.

The two main type of mould we will come across when growing seeds are white mould and Damping Off Disease (which doesn’t sound very mouldy, but it is what causes it).   

What Is White Mould?

This is the mould that grows on the surface of the soil, and as the name suggests, is white and it’s also fluffy.

This is what you have probably noticed on your seeds and why you’re reading this. This mould is unsightly, but it harmless to the plants.  

It loves to grow in damp, warm conditions. It is also enjoys stagnate air and darker conditions.

These are the condition that seeds also like to grow in, so this is why it is so common.

What Is Damping Off?

This is a mould, you won’t see. You will only realise it is there when it starts to affect the plants. This mould is very bad for plants and usually kills them.

You will notice it with very young seedlings. The bottom of the stem will start to grow brown and also start to shrivel up. they will then just keel over and die. At this point you can’t really save the seedling. You can only try and stop it affecting the rest of the seedlings.

As the name suggest it likes damp conditions. It also likes the stagnant air and darker conditions too. However, it actually prefers growing in slightly lower temperatures than white mould.  

One of the main causes of damping off is the cool temperatures mixed with dampness. Most seed suppliers will suggest you germinate your seeds at room temperature, which is normally warm enough to avoid it. However, if your house is colder than normal, or it’s during winter, you really run the risk of having damping off.

This is also why you can buy seed heating mats and why people say they have more success when they use these mats. It’s basically reducing the chance of damping off by warming things up.  

How Do I Get Rid Of The Mould On My Bonsai Seeds?

The different moulds are treated differently:

Removing White Mould

You can simply scrape white mould off with a knife and throw it away.

I’ve seen a lot of people recommend a light dusting of cinnamon powder. I have a feeling I did this years ago, but I can’t fully remember. Apparently it has anti fungal properties and stops the mould growing. I would just scrape the mould off, but you can try cinnamon too. Just remember to apply a very light dusting if you do try it.

You could also repot your seedling. However, for me, this is way too drastic. The mould is harmless, where as you disturbing the roots has a high chance of causing damage. Once your seedling is large enough I would repot it, but not while its super tiny and young. It will just end badly.

Stopping Damping Off

Annoyingly there is no way to treat damping off.

Once a plant is affected by this, it can’t be reversed or cured. You can only try and prevent it from spreading to the rest of the seedlings, which isn’t easy. I would suggest you try and warm the seeds up, but if you haven’t got a seed mat, this can be difficult.

You would just have to move them to a warmer area and hope that is enough.

How To Prevent It From Happening Or Returning

As we are in control of the conditions the seeds are growing in, it means we can try and prevent the mould growing in the first place, or coming back again. We can do this by controlling a number of factors:

Water

Controlling the water is a fine balancing act. Mould will not develop if it is too dry. However, if the seeds are too dry, they will also not grow.

Ideally you can let the very top surface dry out, so the mould will be prevented. But you need to be very careful and only let a very thin layer dry out. Hopefully the seeds are buried below enough and still remain damp. This is quite hard to achieve, so I would be very careful trying this.  

Temperature

Temperature plays a huge role in mould growth and again also plays a role in seed growth. Room temperature is about right, if you start to notice damping off, you will want to raise the temperature by a few degrees. If you do not have a seed mat, you could try moving your seeds to a warmer location in your house.

If you are seeing white mould, your temperature may be too hot, but again this may be the ideal temperature for the seeds, so it may be best to just leave this.    

Airflow

Adding some airflow to your seeds is one of the most effective ways of reducing the risk of mould. Depending on how your seeds are set up you can do this a number of ways.

You can use a small fan to move air over your pots.

If your seeds are covered in a bag, or small plastic greenhouse/propagator, just open the bag or remove the lid daily. You don’t need to do this for very long. Replacing the stagnant air that is trapped inside will really reduce the chance of a mould outbreak.

Adding airflow can also help the water escape, both from the soil and from the air in the form of humidity, meaning your soil and air is not as damp, which as above, will help a lot.

Airflow, especially in an enclosed environment, like a bag or propagator can help lower the temperature.  These little environments can get quite warm compared to the outside temperature, so adding fresh air can cool this right down.  

UV Light

Mould likes darker conditions and can actually be killed by strong UV light.

The strongest UV light we all have access to is the sun. UV light doesn’t really get through glass, which is why you don’t get a sun burn inside and also why white mould is very common on the soil of house plants.

It is possible to take your seeds outside and give them a little sunlight. However, I would be careful with this. Seedlings are very delicate and too much sunlight can actually burn and kill them.

You also need to consider the temperature changes form bringing seeds in an out. Too much change can shock them. If you are going to do this, I would wait until your seedlings are a few months old and strong enough to cope with this. However, this is a very good way to get rid of the mould.    

Use Less Organic Soil

When you are starting your seeds, try and use a soil that has less organic matter. Bonsai soil is a great soil to use. Especially some of the finer particle soil, it’s perfect for seeds. Using soil like this does three things.

It firstly stops the soil staying super wet and should keep it the right level of dampness which should keep the mould levels down.

It will also dry out faster, which will also keep the mould levels lower.

Lastly, the mould spores are only found in the organic elements of the soil. The bonsai soil I use is only 1/3 organic matter. So this means it’s going to have a lot less mould spores than a soil that is 100% organic matter.

Sterilise The Seed Trays

Some people suggest you should sterilize your seed trays before you start. This does make sense. You would be cleaning any spores off the seed trays.

However, it makes no sense when you then remember you are filling the clean trays with soil. Soil that is full of mould spores.

This is the equivalent of being really dirty, taking a shower to get beautifully clean …only to dry yourself off and put your dirty clothes back on.

Sterilise The Soil

Again some people suggest you should sterilise your soil before using it. This is usually done by heating the soil up and it kills any microorganisms in the soil.

It will certainly get rid of the mould spores, but it will also get rid of any of the beneficial microorganisms.  

Soil is affectively alive, and sterilising it, will kill it. I don’t think you should be doing this. I think the beneficial elements far outweigh any negatives that might in the soil.

Conclusion

Hopefully now you have a clearer understanding of what mould is and how it works and how you can avoid it as well as how to deal with it, if it does appear.

Although this information is aimed at growing seedlings for bonsai, the information is still relevant to any other bonsai or house plants you may have.  

(You can read more about – Should You Grow A Bonsai From Seed Or Just Buy One That’s Already Developed?)