Bat guano? Safer homemade fertilisers for growing cannabis

In late 2024, two Rochester residents (New York), aged 59 and 64, died after contracting a severe fungal lung infection. The common factor: both were using bat guano as fertiliser for their medical cannabis grows.

One of the growers had bought the guano online, seeking a natural nutrient-rich fertiliser. The other had taken advantage of a bat infestation in his attic to collect the droppings himself. Neither of them took the necessary precautions when handling this organic material.

Both men developed alarming symptoms, including a high fever, a persistent chronic cough, significant weight loss, and respiratory failure. Despite receiving medical treatment, they both succumbed to acute pneumonia and blood poisoning, which were both direct consequences of inhaling fungal spores present in the guano.

The cause of death was histoplasmosis, an infection caused by the fungus Histoplasma capsulatum, which thrives in warm and damp environments. This microorganism occurs naturally in the droppings of birds and bats and becomes a potential danger when its spores are inhaled during handling.

Why do growers use bat guano?

Bat guano has become increasingly popular among cannabis growers due to its exceptional nutrient content – it is rich in nitrogen (essential for vegetative growth), phosphorus (key to root development and flowering), and potassium (important for bud formation).

This combination of macronutrients has earned bat guano a reputation as a "superfood" for cannabis plants, being seen by many as an organic alternative to commercial synthetic fertilisers. Moreover, its availability online and in specialist shops has made it easily accessible to home growers.

However, as the Rochester case demonstrates, the risks associated with bat guano far outweigh its potential benefits as a fertiliser. Handling this material without the appropriate protective measures can have fatal consequences, which is a price no grower should have to pay.

Safer alternatives: effective and affordable homemade fertilisers

Fortunately, there are numerous ways to feed your cannabis plants without turning to hazardous bat guano. The following homemade methods are not only safer but also cost-effective and highly beneficial:

1. Composting: the foundation of sustainable growing

Composting transforms organic waste such as kitchen scraps (onion skins, salad leftovers, fruit peels) and crop residue (stalks, leaves, roots) into a nutrient-rich medium that improves soil health for cannabis plants. To prepare compost, mix the waste in a well-aerated container, add vermicomposting worms if possible, and leave to ferment for several months, turning occasionally. Use 20 - 30% of the soil volume to replenish nutrients and save money.

2. Actively aerated compost tea

Also known as worm tea, this liquid fertiliser is made from premium worm castings. You'll need 1 cup of castings per 5 litres of water, a strong air pump, and a diffuser stone, leaving it to ferment for 48 hours to ensure full oxygenation. Apply every two weeks throughout the growing cycle, either via root irrigation or as a foliar spray to prevent disease.

3. Fermented plant and fruit juice

This method uses nutrient-rich plants (nettle, comfrey, aloe vera) or fruits (bananas, melons, berries) fermented with brown sugar. Mix equal parts of chopped plant matter or fruit with sugar; cover with a cloth; and leave to ferment in a cool, dark place for 7 - 14 days. Strain and store in the fridge, and then dilute 4 tablespoons in 3.5 litres of water before use.

4. Coffee grounds

As simple as it sounds, used coffee grounds are rich in nitrogen, which makes them ideal during the vegetative stage. They're also an excellent source of organic matter as they contain other vital nutrients. As they break down, coffee grounds improve soil structure by creating aggregates that enhance aeration and water retention. Mix around 2 grams of coffee grounds (or even freshly ground coffee) per litre of soil.

5. Black sesame tea

This simple tea uses a common kitchen ingredient, which makes it highly economical and sustainable. Prepared by boiling black sesame seeds in water for 20 - 30 minutes, it provides calcium, magnesium, and phosphorus, which are essential nutrients for strengthening the stems and promoting healthy root development. After cooling and straining, dilute with water in a ratio of 1:5 and apply during the vegetative phase.

6. Crushed eggshells

Another excellent way to ensure organic fertilisation. Eggshells are rich in calcium and other minerals that greatly improve plant structure, health, and overall development. In fact, many gardeners and farmers routinely use them to stimulate plant growth, and they work just as well with cannabis.

7. Banana peels

A fantastic natural source of potassium, which is vital for strong flowers and sturdy stems. Boil the peels in water for 15 minutes, let cool, and use the liquid for watering; or bury fermented peels near the roots for a slow-release effect. This method is affordable as it uses kitchen waste, and aligns with organic practices, offering a safe alternative to the potassium in guano.

8. Wood ash

Untreated wood ash is high in potassium and calcium, so it can help to strengthen cannabis stems and improve flower quality. Sprinkle a thin layer over the soil and mix it in gently (use just 1 or 2 grams of ash per litre of substrate). Be careful not to overuse it, as it can make the soil too alkaline. Wood ash is ideal for the flowering stage, and you're making use of a by-product of home heating (provided you have a fireplace, of course).

Why opt for homemade fertilisers?

Homemade fertilisers make use of materials you would otherwise discard, offering significant savings when compared to commercial products. Unlike synthetic fertilisers, which can degrade soil quality over time, homemade fertilisers gradually improve the structure and fertility of the substrate, allowing it to be reused in future grows.

That said, you should still take basic precautions. For instance, you should wear gloves when handling any organic material; work in well-ventilated areas to avoid odour or gas build-up; and store your preparations correctly with clear labels. Always avoid over-fertilising, which can "burn" your plants: start with lower concentrations and increase gradually based on how your plants respond.

The tragic case of the Rochester growers reminds us that not all "natural" solutions are safe. Responsible growing means prioritising not only the health of our plants but also our own physical wellbeing. As responsible growers, we have a duty to properly inform ourselves about the products we use and the potential risks involved.