Every autumn millions of liberty caps spring up across Europe and parts of North America.

I joined the ranks looking for them in 2020.

I started by researching where and when liberty caps grow. I spent weeks scouring websites, forums and social media.

The only things everyone seemed to agree on were that they grow:

• on grassland

• after rainfall (mushrooms are 90% water).

Beyond that I found it difficult to separate woo-woo from wisdom.

Perhaps you've come across advice like this:

I wanted to make the most of the limited days I had to forage. But this initial research left me feeling confused and unsatisfied.

Surely there was a more effective strategy than picking fields at random?

What do we really know about liberty caps?

The liberty cap's scientific name is Psilocybe semilanceata. The mushroom itself is the reproductive body of an underground network of threadlike mycelium.

Like any organism, Psilocybe semilanceata evolved to survive and reproduce in particular environmental conditions.

I wanted to know which claims about these conditions had any real basis. So I tried tracing them all back to credible sources.

Most of these trails led nowhere. But some led to a handful of books by mycologists Gastón Guzmán, Jochen Gartz and Paul Stamets.

These confirmed that liberty caps prefer rich, acidic grassland:

• Rich soil is high in organic matter. All Psilocybe species are saprotrophs which get nutrients by breaking down organic matter. So this made sense – no food, no fungi.

• Acidic soil has a pH below 7. Guzmán analysed many Psilocybe species and found they all preferred acidic substrates. He didn't analyse Psilocybe semilanceata but it's unlikely to be an exception.

This provided a basic strategy to avoid wasting time looking for liberty caps in the wrong places. But there were still gaps:

• What levels of richness and acidity are optimal?

• How do I actually find rich and acidic grassland?

• Are there other important land characteristics?

• When should I look (i.e. how do weather conditions trigger reproduction)?

Going deeper: data-driven foraging

As a data scientist, I wondered whether data could help answer these questions.

I was delighted to find a dataset with thousands of crowdsourced liberty cap records – each with a date and coordinates.

This allowed me to link each record to information about the land and weather in that place at that time. I then used statistical models to learn about where and when liberty caps are most likely to grow.

My analysis confirmed that liberty caps do indeed prefer rich and acidic grassland (up to a point). But it revealed a third soil characteristic that's even more important. It also showed a clear link between weather and liberty cap sightings.

I now had a strategy to find more liberty caps in less time by searching:

• in areas with suitable grassland

• on days with suitable recent weather conditions

I built Magic Mushroom Map to share what I found.

It’s my ongoing effort at leveraging data to maximise your chance of being in the right place at the right time. I use it myself:

A step-by-step guide to finding liberty caps

1. Where to look

Liberty caps are most likely to grow on grassland with soil that is:

• rich – with organic carbon content between 5-15%

• acidic – with a pH between 4.0 and 6.0

• not heavily compacted – my analysis shows this is even more important than richness or acidity. My hypothesis is that grass roots struggle to penetrate compacted soil which limits nutrient sources.

You can use Magic Mushroom Map to identify suitable grassland.

I'm confident it’s the best possible starting point if you don't already know of any spots. I can't guarantee liberty caps grow everywhere with suitable grassland. But it’s much more efficient to search the 25% of grassland with suitable soil than it is to search all grassland.

I haven’t said anything about animals yet. This is because liberty caps don't have a direct relationship with animals like dung-loving mushrooms. Animals can affect soil characteristics, but we can account for that directly.

2. When to look

The liberty cap season runs from around August to November in the northern hemisphere. Within that broad window, the weather affects how likely they are to grow in a particular area.

Liberty caps are most likely to grow when weather conditions over at least 10 days reflect:

• averages of 11.5°C (suitable range of 8.5-14°C)

• lows of 9°C (suitable range of 6.5-11.5°C)

• highs of 14°C (suitable range of 10-17°C)

• the more rainfall, the better

You can use Magic Mushroom Map to identify when weather conditions are suitable.

I’m confident it’s the best possible guide to timing your outings. But nature is complex and – much like a weather forecast – it cannot be perfectly accurate.

All the best for the week ahead,

Nick