Is Bat Guano Safe to Use in the Garden?

We gardeners are always looking for natural sources of pest control and fertility to help our gardens thrive. Bat guano has gained much popularity as an organic fertilizer thanks to its potent nitrogen and phosphorus concentration. But is it safe?

This natural resource has many safety and ethical concerns to consider before adding it to your garden beds. We must consider the endangerment of bats, how guano could potentially harm human health, and why you might want to avoid store-bought guano fertilizer and opt for personal bat boxes instead.

Let’s dig into everything you need to know about using bat guano in your garden without harming yourself, your plants, or the ecosystem as a whole.

What is Bat Guano?

Bat guano is a nutrient-rich manure harvested from insectivorous and fruit-eating bats. This excrement is an organic fertilizer because it is particularly high in readily available nitrogen and phosphorus, helping plants grow stronger and greener to produce higher yields.

Unlike chicken or cow manure, guano is low-odor, fast-acting, and safe to apply directly to plants without as much risk of fertilizer burn.

The Long Answer: There are Nuances to Safe and Ethical Use in The Garden

The safety of products used in our homes and gardens is often far more nuanced than a label like “safe” or “dangerous.” Some pesticides were considered safe according to government regulation in the 1960s but later became outlawed due to new science that they cause cancer or birth defects.

Some plants are considered safe to handle but may cause rashes or problems for individuals with certain health conditions. Certain garden inputs are safe for humans but may harm bees and butterflies. 

Safety occurs on a spectrum and requires zooming out to see the big picture of how our decisions affect us, our gardens, and the world around us. To understand how bat guano fits in this picture, let’s examine its safety from three angles:

It’s (Mostly) Safe for Humans

While many people associate bats with vampires and rabies, these nocturnal mammals don’t pose much risk to human safety. In fact, they can help prevent many human diseases by devouring thousands of mosquitoes per night, including those that harbor the West Nile virus.

When it comes to handling bat guano in the garden, the safety risks are about the same as spreading chicken manure or handling dog poop. Of course, all forms of animal excrement contain potentially harmful pathogens, but as long as you wear gloves and thoroughly wash your hands, they are unlikely to cause major health problems.

In my humble opinion, as a novice bat researcher and an experienced organic farmer/gardener, I avoid buying dried guano altogether. I don’t want to risk my lungs or contribute to declining bat populations.

Instead, I recommend collecting the droppings that fall naturally below eco-friendly bat boxes you can build in your garden. You get free manure fertilizer and natural pest control without the safety or ethical concerns associated with store-bought guano fertilizer!

It’s Safe for Plants

This manure-based fertilizer is especially popular thanks to its safety for garden crops. Due to the unique nutrient composition of bat guano, it is mild on plants and unlikely to cause fertilizer burn.

It is similar to worm castings because it can be applied directly to the garden. Guano is gentle on your crops, unlike chicken or cow manure, even if you don’t compost it first. It is rich in microorganisms that make the nutrients readily available to plants. 

However, a little goes a long way! The low application rate of guano is another benefit to the plant. To increase plant growth, you only need about 1 part guano to 20 parts soil. No need to dump it on!

Questionable Safety for Bats 

When considering the safety of this fertilizer for bats themselves, things get a bit more complex. When the droppings are commercially harvested for fertilizer, collectors enter many caves to collect them. In the process, they may spread a deadly fungal disease called White Nose Syndrome between bat colonies. Fertilizer collection is the primary reason for contamination of caves, leading to major risks for wild bat populations. 

Although this pathogen is transmitted from bat to bat, the U.S. Geological Survey highlights that humans inadvertently contribute to faster spread because the disease-causing spores attach to their clothing gear. Note that these fungal spores differ from those that cause human respiratory illness. However, it’s pretty interesting (and sad) that guano fertilizer harvesting poses health risks to humans and bats! 

The story’s moral is that if you care about saving bats, don’t buy store-bought guano. You don’t know how it is being harvested, and it likely isn’t safe for endangered bats.

What’s the Problem?

The main problems come from two angles: environmental ethics and human health.

Environmental Ethics

While bat poop is tremendously beneficial in gardens and on farms, it poses some environmental and health safety challenges. Bats are facing widespread decimation worldwide, with more than half of the bat species in the U.S. listed as severely threatened or endangered. 

The causes for bat declines are multifaceted, including pressures like:

• Climate change
• Habitat destruction
• Pollution
• Lack of food (when pesticides and chemicals kill their insect food sources)
• Pesticide bioaccumulation (when bats eat a lot of insects sprayed with pesticides, they are exposed to increasingly high levels of toxins)
• Hunting and poaching
• White Nose Syndrome (a fungal disease that disrupts bat’s natural hibernation cycles)
• Commercial harvesting

Those last two bullet points are the most problematic for gardeners because our fertilizer could come from unethical sources. You may wonder how collecting an animal’s poop could harm them. The issue is not related to the poop collection itself but the fungus that may spread via the clothes, shoes, and tools of commercial guano harvesters.

White Nose Syndrome and Fertilizer Harvesting

White Nose Syndrome is a severe fungal disease that is killing bats worldwide. It is the most dire threat to bats because it spreads rapidly from bat to bat and severely disrupts their hibernation cycles, causing them to awaken in the middle of the winter, fly out of their caves, and starve or freeze to death. 

How does that relate to gardening? Commercial harvesting is a key cause of the spread of White Nose Syndrome. When humans enter caves to collect the poop to sell as fertilizer, they can inadvertently spread the fungus from cave to cave, causing more and more bats to get infected. Much like a super contagious human disease, when one member of a colony (family) of bats is exposed to the fungus, it rapidly spreads to the rest.

Unfortunately, sanitation measures and guidelines have still been unsuccessful at slowing this spread. If we want to use this fertilizer in the garden ethically, the best choice is to install our own bat boxes and collect the guano from beneath them, much like you would from your chicken coop.

Human Safety Issues

A serious respiratory disease called histoplasmosis is another key concern when handling and using guano in the garden. The microscopic spores of a fungus called Histoplasma capsulatum are sometimes found in dried bat guano and can harm humans if inhaled. This rare disease most commonly occurs when people breathe in these spores while demolishing or cleaning up attics and houses where bats have overwintered. 

Interestingly, fresh bat droppings do not pose the same risk. This dangerous fungus prefers dried, aged guano. In addition to the ethical issues mentioned above, this key safety concern can be mitigated by building bat boxes near your garden and collecting guano yourself rather than buying it.

Collecting from Bat Boxes

Bat boxes are an emerging trend that offers amazing benefits for gardeners and bats! Official state governments even have conservation projects to promote building bat boxes to aid in ecological health. Bats are some of the best pest-control agents you can have in your garden. Installing a bat box in your yard can help protect these threatened mammals while enjoying reduced insect pest pressure for your plants. 

U.S.-native brown bats are particularly voracious predators of mosquitoes, cucumber beetles, and moths. When bats eat night-fluttering moths, they can dramatically reduce infestations of pests like cabbage loopers, tomato hornworms, and army cutworms, all of which are larval stages of adult moths.

Better yet, you can use the droppings that accumulate under a bat box as fresh fertilizer in your garden! Much like roosting chickens, bats sleep on elevated stands inside bat houses. The bottom of a bat box is open so the poop won’t accumulate inside. 

While they sleep, their guano drops to the ground and can be collected by placing a bucket, saucer, or potted plant underneath the bat box. Suddenly, you have a free, abundant, and ethical supply to use in your garden! Installing bat boxes and using the guano from local bats is a far safer, eco-conscious option for this nutrient-rich fertilizer. 

Remember that bats sleep during the day, so it’s important not to disturb the box. This shouldn’t be an issue if your bat box is installed at the recommended 20 feet or higher above the ground. As we mentioned above, take care to handle the bat droppings just like you would handle any poop— with obvious sanitation measures!