Are Worms Good Or Bad For Bonsai?


Earthworms are usually considered good for plants and you may be wondering if they are good for bonsai trees or not.

Earthworms are bad for bonsai trees because they can take away nutrients and water from trees, make bonsai soil compacted and may cause root damage.  They should be removed from pots if you find them.

What are worms?

(Not all worms in the soil are earthworms, and some worms do feed on plants, which is obviously bad for plants.  But for the purpose of this post, I will use the words worms and earthworms interchangeably.)

Earthworms are tube-like worms that live in the soil and feed on dead organic matter.  They are slimy, wriggly, creepy and disgusting which scares me off when I take care of plants in my garden.

Earthworms are generally classified into two groups: (1) large earthworms which are visible, and (2) small earthworms which are usually invisible to the naked eye.

Large earthworms are further divided into two families: the Lumbricidae, which are native to Europe, and the Megascolecidae (a.k.a. jumping worm), which are native to Japan, Southeast Asia, Australia, and New Zealand.

Although there are no native earthworms in the northern part of the US and Canada, both Lumbricidae and Megascolecidae earthworms are now widely found in North America due to human activities such as transporting plants and intentionally migrating them.

Small invisible earthworms are found in various parts of the world, but they are not commonly found in North America.

Are earthworms good or bad for plants?

Are earthworms in general good or bad for plants?

The presence of worms in the soil is good for plants.  Earthworms feed, excrete and move that make two good effects on the properties of soil: improve nutrient availability and improve soil structure.

Improve soil nutrient availability

When feeding, earthworms break down organic matter which is then broken down by microorganisms present in the earthworm’s gut.  The organic matter present in the casts excreted is further decomposed by microorganisms, increasing the available nutrients in the soil.

According to this scientific research, earthworm presence in the soil leads to a 25% increase in crop yield and a 23% increase in aboveground biomass.  Scientists believe that earthworms stimulated plant growth by releasing nitrogen locked in soil organic matter when feeding on and excreting it.

Improve soil structure

When worms move, they transport the organic matter to the lower layers through the tunnels they make.  The channeling and burrowing by earthworms loosen and aerate the soil and improve soil drainage as well.  It is said that soils with earthworms drain up to 10 times faster than soils without earthworms.

Are jumping worms good for plants?

Jumping worms (Megascolecidae) are native to Japan, Southeast Asia, Australia, and New Zealand.

In their native regions, jumping worms are considered beneficial to plants.  Most of the earthworms found in Japan are in the jumping worm family (Megascolecidae) and researchers find this type of earthworm present in the soil increased nutrients available in the soil and promoted crop growth.

We, as gardeners in Japan, normally think worms are good for plants and that they make the soil better.

On the other hand, people in North America think jumping worms are invasive and disruptive which may cause devastating effects on plants and the environment.  One of the reasons is that they swarm in large groups in the top few inches of soil and deplete nutrients that are otherwise available for plants.  When the population becomes high enough, they become destructive to plant roots as well.

The problem seems to be their overwhelming number, which is not a common problem in their native regions.  The reason for this is unclear now but there is a theory.  Although many animals (birds, snakes, and frogs) prey on worms, North American predators may not be familiar with jumping worms yet and not as effective as predators in the native regions at suppressing their populations.

Anyway, jumping worms can be harmful to plants and ecosystems outside of their native regions like in the US.

Are earthworms good or bad for bonsai trees?

Are earthworms good or bad for bonsai trees?

Now that you know earthworms are usually good for plants, what about the plants in containers like bonsai trees?  Are worms still good for bonsai?

Earthworms can do NO good to bonsai trees and should be removed from pots if you find them.  Here’s why.

  1. Overcrowding problem
  2. Soil compaction
  3. Root damage

Bonsai pots are very small compared to the size of the tree and contain a minimum amount of soil on which they can thrive.  As such, a single earthworm in your bonsai pot may be a serious competition to your bonsai tree for resources such as water and nutrients.

Also, recommended bonsai soil mix contains little or no organic matter for worms to survive.  If you are thinking about introducing worms, bonsai soil is not an ideal environment for them.

Recommended bonsai soil mix

Coniferous Deciduous Flower/fruit
Lava rock
60% 70% 60%
River sand 40% 20% 20%
10% 20%
Soil compaction

Soil compaction is the process by which soil particles are pressed together, resulting in a denser and more compact soil structure.

Earthworms produce vermicast (feces) as they digest organic matter in the soil.  Vermicast is a very rich source of nutrients and is in general beneficial for plants.  But for bonsai soil in a small pot, vermicast produced by worms can cause soil compaction because vermicast is denser and heavier than the soil mix, and can push the soil particles to be pressed together tightly.

The soil compaction may lead to reduced water drainage and airflow in the soil, which are detrimental to bonsai trees.

Root damage

Earthworms can cause root damage to plants as they burrow through the soil.  When earthworms move through the soil, they create tunnels that sometimes intersect with plant roots.  As roots cannot grow in hollow tunnels, they have to try to grow in what little soil is left in the pot.

Worms may dislodge or uproot bonsai trees as well as they burrow through the soil, especially when roots are pruned and the root system is underdeveloped.

How do earthworms get into bonsai pots?

There are a few possible ways that worms get into your bonsai pots.

Through drainage holes

One possibility is that earthworms may have entered the pots through the drainage holes at the bottom.  If you have bonsai pots sitting directly on the ground for some time, worms can crawl into the pots.

It is true that putting pots directly on the ground during hot summer days may save you from watering problems to some extent but letting worms into the pots do as much harm as water shortage.  And, as you might not notice their presence until your bonsai trees show some symptoms of issues, it is better to place the pots where there is no danger of worms sneaking into the pots, like on the shelf.

Present in the soil

Another possibility is that the (eggs of) worms are present in the soil when you planted or purchased the tree.  Use sterile compost if you are making your own soil mix.  It is quite uncommon that the soil you purchase contains worms, especially if it is high quality.  Using the compost you make at home is not a good idea not only because it may contain worms but also because of other insects that can harm bonsai trees.

When you are buying a bonsai tree, make sure to buy one from a reputable shop.  Look for a seller who has a reputation for selling high-quality products.  You can search online reviews or ask for recommendations from bonsai enthusiasts you know.

Or you can just address the seller of your concerns about the presence of worms in the pot.  They may be able to provide you with information about how they prevent any pests from lurking in the bonsai pots.

How to get rid of earthworms in bonsai pots?

If you want to get rid of earthworms in your bonsai pot, there are a few things you can try.

Remove worms by hand

One simple method is to remove the worms by hand when repotting.  You cannot do this at any time of the year but this is the simplest as well as the most certain way.  Simply lift the tree out of the pot and gently remove the soil around the roots.  If worms are present, they will fall off.

You can release the worm back in the yard or far away from home in the wild.

Use pesticide

Using pesticides is another option.  Carbamate insecticides are known to be very toxic to earthworms (reference here).  Some of the product names that include this compound are Sevin, Propoxur and Carbofuran.

Read the instructions written on the label carefully before using it.  Some products may be harmful to humans and pets if used incorrectly.  Some of them may have specific precautions or restrictions for use around certain types of plants or may need to be applied at certain times of the year to minimize the risk of harm to plants.

Using chemicals to kill earthworms can have unintended, unwanted consequences such as harming your bonsai tree or harming yourself.  For this reason, it is usually not a good idea to use chemicals to get rid of earthworms especially when worms will not kill your trees directly.