Worm Casting in Potted Plants? Why, How, and When (and 5 Superpowers)

Worm casting is no more any less than worm feces. Despite sounds a bit gross, this is a gold mine for outdoor gardeners with higher harvest and healthier plants. However, does the same apply to your potted herbs and plant indoor? Here is all the research to find this out.

Hence, can you use worm casting in potted plants? Worm casting can be safely used in a modest amount in indoor potted plants and herb because of its:

  1. ability to attract and slowly release nutrients
  2. higher nutrient content (an organic fertilizer)
  3. hormone-like effect
  4. ability to trap moisture and high porosity
  5. high amount of bacteria that can improve the soil quality

Knowing what you can do does not mean you understand why and, more importantly, how you should use it.

One of my favorite worm casting for herbs is the wiggle worm here on Amazon if you have just a few plants. For a few pots, you might need a few more liters, so this one, always on Amazon, is another good option.

What is Worm Casting? Its 5 Superpowers (with Science)

Worm casting is the remnant of not digested plant material produced (yes feces) by earthworms. Worm casting looks like hummus. It is dark brown and slightly sticky when just produced. The one you buy at retailers (yes, worm pop is sold) is left sitting for a while to increase its bacterial actions (more on this later).

The video below shows you what it looks like when it is just produced.l.

As discussed in this study, there are different types of worm casting. Nonetheless, worm poop is more or less undigested food (as only 10% of nutrients are extracted) rich with microbes produced in the worm intestine and that cover the feces in a slimy gel.

What makes them unique is a substance called humic acid. Despite the “acid” word on it, this substance is totally safe and actually a gold mine for your soil. Not by chance, worm casting is also known as black gold.

Let’s see the 5 proven worm casting superpowers:

A “smart glue” for plant minerals

As detailed in this study from Abekuota University and the University of California, worm casting attracts important plant nutrients (like a glue) and release them only when the plants ask for them (a smart glue). This is a “slow-release” process (if you wonder, “slow” is good for plant nutrients) is due to a special coating that surrounds the worm poop.

Does it matter for potted plants? Yes. Indeed, over time, without this special glue, the plant nutrients that might be found in a good quality potting mix (or better the minerals needed to create the plant nutrient) tend to be washed away (when watering). Your plant will be then hungry, a common problem in potted herbs where their nutrient intake depends on us rather than on nature as it would happen if it was in a suitable environment outdoors.

A Natural Fertilizer

As stated by many studies (just to cite a few: the University of California, the University of Abakuota, and the University of Maryland), worm casting is rich in all those minerals that your herbs and plants need to survive. Hence, it does not only capture them in the soil, but it contains them. This is what fertilizer does. In particular, worm casting is rich in iron, sulfur, calcium, nitrogen, phosphorus, and potassium (measured as an NPK of 5-5-3, source, more on the NPK meaning here).

Just for your knowledge, the worm cast is around 10 times richer in minerals than compost, whose average NPK is 0.5-0.5-0.5 (source). Worm casting is highly recommended for greenery (houseplants and herbs) due to its heavier nitrogen content (over the other micronutrients).

Does it matter for potted plants? Yes! Indeed, every plant requires minerals that, again, due to the lack of a natural outdoor ecosystem (bugs, any other organic material), must be supplied by you. If not? Your plant will be stunted, as discussed in the article below.

Natural Growth Hormones

As discussed in this study from the University of Vigo, worm casting is rich in particular substances (called humates) and root hormones that boost growth. This is also consistent with what found through experiments by many scientists and what expert gardeners know from years of experience.

Just one of the several numbers, a study from the University of Umudike found a 180% increase in the harvest (yes, almost three times the original amount). The Italian Council Research also found an increase in the root system (a very positive point) that results in larger and more flourishing plants. This is called a “hormone-like” effect

Biological effect of earthworm casts is linked to microbial metabolites that influence plant metabolism, growth, and development

Italian Council Research

Does it matter for potted plants? Yes! All of the above findings apply to your potted herbs as well. If you care about growing prosperous plants (someone said more basil leaves!), having such hormones on your side is a great plus (and 100% natural).

Improve Soil Porosity

The fluffiness of the worm composting, when added to the soil, it increases its aeration as discussed in this study from the Mississippi State University. This is quite important for the development of healthy plants and herbs. Indeed, in case you didn’t know, plants also breathe quite a lot through roots. No air implies death for suffocation (like in case of overwatering due to root rot). So, the porosity of the soil is quite important.

Does it matter for potted plants? Yes again. Indeed, especially for potted plants, where there is a lack of soil movement (due to worms or other insects/animals), good soil aeration should be provided by a well designed potting mix where worm casting can be a great addition.

Packed with (potentially) Helpful Bacteria

In this topic, I read quite a bit of debate. Despite the fact that worm casting is full of microbes as discussed in this study by the Environmental Microbiology Laboratory of Braunschweig, it is quite hard to determine their positive effects compared, for instance, to those found in compost.

Pro: A study from the Italian Council Research states that “microbial activity [in worm cast] can explain many of the effects, sometimes paradoxical, attributed to earthworm activity and casts.” Many other studies have similar claims.

Against: However, other experts, such as Robert Pavlis in his article, claims that very likely the effects of such microbe might be marginal if not proved entirely (yet). Bottom line: more research is needed, although all the studies always suggest, one way or the other, that (good) microbes can be of great help, also preventing root rot and another disease by simply out-competing those bacteria responsible for this problem.

How Much Worm Casting to Add? How? How Often?

Despite gardeners recommending slightly different amounts (yes, gardening is not an exact science), a good rule of thumb is to use around 10 to 30% of the total potting mix (example: if you have a bag of 10kg of potting mix, add 1-2kg of worm casting).

Ideally, you want to add the worm casting since the very beginning. This is because the worm cast is evenly spread within the growing medium (more on growing medium here) and so enhancing the quality of the potting mix evenly, especially at the root level.

How to apply worm casting in case you have an already potted herb or plant?

In this case, some gardeners recommend repotting. I would avoid that if I was in you for two reasons:

  1. Repotting will lead to transplant stress (check this article here for more)
  2. Worm casting is an organic material that after a few months will be totally broken down (so you might need to apply again)
  3. It can be messy, especially if you do for the first time

The easiest solution is to:

  1. Deposit half an inch of worm casting (I would say 5-6 generous tablespoons for a 10-inch diameter container)
  2. Move the soil gently with the worm compost on top with a spoon. Your objective here is to incorporate the worm casting in the soil for faster absorption. If you can create some holes (with a stick) 1-2 inches deep (without affecting the roots), this will allow the worm casting to sink into the soil.
  3. Repeat after a month (if in summer weather), especially if you have herbs

This technique allows, over time, the worm casting to reach the plant roots through watering. Indeed, each time you pour water on the soil (avoid wearing leaves), some worm casting will be moved down through the rest of the soil, close to the plant roots.

Be careful, in this case.

The only case I would not recommend worm casting is for seedlings. Some claimed that depending on how the worm cast has been produced (and bacteria present on it) it was found responsible for the death of the seedlings.

Another suggestion that I would personally avoid is to have worms in potted plants. Indeed, in such a confined environment it is very difficult to handle them and you might found them on your kitchen counter if you are not careful. For more check the article below.

Worm Casting Q&A

Here some of the most common questions you might have on worm casting and its use on potted plants.

Can You Use Only Worm Casting as Soil?

Although none has actually tried, a suitable growing medium, especially in the potted herb, requires a good combination of physical and chemical properties that can last at least 1-2 years. If you have houseplants, chances are that you are not (and you should not) change the soil often.

Hence, the problem with worm casting is that as every organic matter (compost, for instance), is its decay over time. So, expecting to use only worm casting, especially in the long term, is not a solution.

Do Worm Casting Burn Plants?

No, worm casting does not burn the plant due to its lower nitrogen concentration. Indeed, as any organic material, the concentration of minerals is way lower than their synthetic counterparts.

Indeed, the “burn” effect is due to a very high concentration of salts (coming from processes involving unused nitrogen in the soil) that, over time, keep water away from the plants and affect the pH triggering a large variety of (negative) effects.

Worm casting and other organic products do contain a relatively small amount of minerals (I call them weak fertilizer), so preventing such phenomena from happening.

Further Questions

Will worm grow from worm casting? No, worm cast has been separated by any worm eggs, so the chances for worms to develop in worm casting are minimal

Do Worm Casting Smell? Yes, the smell of worm casting is typical of wet soil. However, during the production of worm casting, the presence of excess water, or worm “food” might trigger a decomposition process that will then make the whole worm cast pile smell badly. However, this is a problem of inadequate procedure in producing the worm casting that normally should not have an unpleasant smell.

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A young Italian guy with a passion for growing edible herbs. After moving to the UK 6 years ago in a tiny flat, it was impossible to grow herbs outside. So I start my journey in growing indoor and so I decided to share my knowledge.

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