Disadvantages of Worm Castings in the Garden

As much as we love worm castings, there are some downsides to using them in the garden. We’ll explore the pitfalls of vermicomposting and what you can do to overcome these drawbacks in this guide to the disadvantages of worm castings.

Disadvantages Worm Castings


  • worm castings is not a complete fertilizer solution
  • it can take months for your worms to produce enough worm castings
  • worm bins can attract unwanted pests

It is not in our nature to sugarcoat things on the Garden Bench Top.

We always like to present a fair and impartial view of all gardening-related things. And unfortunately, it’s not all peaches and cream with worm castings.

credit: tumblr

So if you’re ready to have your rose-tinted glasses shattered, let’s get into the disadvantages of worm castings.

Worm Casting Disadvantages for Gardening

Worm castings take a long time to cultivate

Worm Castings Production Times

By far, the biggest gripe many vermicomposters have when using worm castings in their gardening endeavors is the time it takes to produce the black gold.

worms can take up to 6 months to produce enough worm poop to sustain your garden

If you have a small worm farm, don’t expect to harvest castings until at least 4-6 months into the process. Sometimes you may find a decent amount of worm castings as early as 2-3 months after you first set your farm. However, you will likely have to do a bit of sifting or filtering of uneaten bits of food before you have pure 100% worm castings.

Garden Bench Top Tip

If you are impatient and want results quicker, we recommend cherry-picking the types of worms you use for your worm farm. Red wriggler worms (Eisenia fetida) have the most voracious appetites and can eat their weight in food (or organic matter) daily.

Red wriggler worms are, by far, one of the most popular types of worms to be used for vermicomposting. However, other varieties also do a good job, like Indian Blues, Alabama Jumper, and various nightcrawlers. We have a rundown of the pros and cons of each worm in our Worms for Vermicomposting guide.

Won’t Replace Fertilizer

Worm castings won't replace fertilizer

As much as we wish worm castings were the one solution for all our fertilizing needs in the garden. Sadly, this is not the case.

Don’t get us wrong.

Worm castings are an excellent general fertilizer for most plants, as they contain the essential building blocks of healthy plant growth, nitrogen, phosphorus, and potassium (aka NPK).

However, the concentration levels of NPK in worm castings are generally not high enough for some plants. The ratio levels of NPK also depend on their diet (what you feed them in the worm farm).

It is also vital to note worm castings will not provide the essential nutrients and minerals for plants that require a specific fertilizer. For example, you will need to include soil amendments and synthetic fertilizers for blueberries, which benefit from fertilizers high in magnesium chloride(MgCl2) and potash (KCI).

Garden Bench Top Tip

Supplement your worm castings with an all-purpose fertilizer to ensure your plants receive sufficient minerals to grow healthy and strong.

The worm castings will provide a good base with the added benefit of microfaunae like beneficial bacteria and microbes—an essential element in any healthy garden soil system.

Plants Require Soil Diversity

Have you ever wondered why when you open a pre-made bag of soil, it looks like it has haphazardly been thrown together?

And even though there is sometimes a questionable nail, piece of plastic, or the odd piece of wood in your soil mix, there is a method to the madness. Plants need rich soil health diversity that consists of a variety of matter.

Earthworm castings are the remains of vegetables, fruits, and other organic matter consumed by worms. Making it 100% organic matter.

While worm castings are fantastic for germinating and rearing seedlings, worm castings alone are not enough for plants to survive on and will lead to sub-optimal growth.

Garden Bench Top Tip

Let’s be honest; if you were to try to grow a garden on worm castings alone, it would take years to generate enough worm castings.

Suffice it to say we do not recommend attempting this.

The sensible approach is to create soil blends incorporating worm castings into your soil.

Worm castings are a great way to revitalize old soil. So your plants receive the benefit of a varied soil composition. As well as a boost from the nutrient-rich worm castings.

Check out our recommended recipe for reinvigorating old soil with worm castings HERE.

Worm Farms Can Attract Pests

Worm farms can attract pests

Unfortunately, this is one disadvantage of composting with worms we are all too familiar with.

Like compost bins, vermicomposting farms can be pest magnets—particularly rats, mice, and raccoons. Any animal with a similar diet to humans will find the aromas from your farm irresistible.

In our case, we had a determined rat chew its way into our very first DIY worm farm. Not even heavy-duty plastic could prevent rats from breaching our farm.

Rats eating into our worm farm

Here is a picture where it managed to create an opening, access the food scraps, and dig its way through the organic material. Who knows? It may have even had some of our worms as appetizers 😢.

Close up of rat chew marks on our worm farm

At least the tomato seedlings are able to get some light!

Garden Bench Top Tip

Pests are attracted to your worm farms because they can smell the kitchen scraps.

We find burying the food scraps, rather than scattering them over the surface of the worm farm, helps to hide the smell.

You can also use a worm bin blanket to cover the contents of your worm farm and minimize the risk of attracting unwanted visitors.

Worm Farm Upkeep and Maintenance

It is fruitless to hide the fact that worms are animals and therefore require maintenance and care (albeit much less than more common household pets like dogs and cats).

The environment that a worm lives should be regularly monitored. They require a neutral pH environment that is moist but not saturated. In addition, the worm farm should be maintained at a temperature between 55-80° Fahrenheit (12-27° Celcius).

For more information on how to start and maintain a worm farm, check out our beginner’s guide HERE.

It IS NOT a set-and-forget type project, and you need to be clear that you are responsible for the creatures in your farm.

Garden Bench Top Tip

Once you accept the responsibility for these black gold producing wonder creatures, looking after them is not inconvenient.

Especially if you are feeding them regularly, we perform a few simple checks on the farm at each feeding session.

Here’s what we look for:

  • Moisture levels – what does the surface of the worm farm look like? If the food is moldy, you may be feeding them too much. It also indicates the farm could be too moist.
    If the material looks dry, give your farm a spritz of distilled water OR feed food with high water content, like watermelon peel.

Why Bother with Worm Castings?

Hopefully, we haven’t turned you off the opportunity to start a worm farm or use worm castings in your gardening exploits.

They are still an eco-friendly and budget-conscious method of giving your plants a rich environment to thrive.

With each disadvantage, we’ve provided a solution to help you overcome the problem or negate the deficiencies that worm poo pose.

We wholeheartedly encourage you to pursue your worm castings dreams. We certainly intend to.

If you’re ready, you may be interested in reading some of our other vermicomposting guides: