Growing Mushrooms on Coffee Grounds: Myth or Miracle?

Introduction

Coffee is one of the world’s most consumed beverages — but what happens to all those used coffee grounds? Instead of ending up in the trash, many mushroom cultivators are turning this rich waste into substrate for growing gourmet mushrooms. But is this really a miracle of sustainability, or more of a myth? In this article, we'll explore how and why mushrooms like oyster, shiitake, and Lion’s Mane can (or cannot) grow out of spent coffee grounds — backed by science, best practices, and real-world experience.

Why Use Coffee Grounds as Substrate?

• Nutrient-rich: Used coffee grounds contain nitrogen and minerals that fungi need.

• Pre-pasteurized: Brewing actually pasteurizes the grounds, reducing many competing microbes.

• Waste recycling: Using coffee grounds helps divert organic waste from landfill and turns it into local food production.

• Low barrier to entry: Especially in urban settings, it's relatively easy to collect spent grounds from cafés.

Which Mushrooms Grow Best on Coffee Grounds?

Oyster Mushrooms (Pleurotus spp.)

• Oyster mushrooms are the most commonly grown species on spent coffee grounds because they colonize quickly and tolerate this nitrogen-rich medium.

• According to a scientific study, caffeine in coffee grounds can inhibit mycelial growth and delay fruiting, especially if there’s a high share of grounds in the substrate.

• However, the same research found that Pleurotus ostreatus can partly degrade caffeine into other compounds, showing a biological capacity to detoxify the substrate.

Lion’s Mane (Hericium erinaceus)

• Lion’s Mane can grow on coffee grounds, but it’s more sensitive: many growers recommend blending the grounds with sawdust or straw to improve aeration.

• The fine texture of grounds alone can compact easily, which restricts airflow — a problem for more delicate species.

Shiitake (Lentinula edodes)

• Shiitake is less common on pure coffee grounds, but some growers use supplemented mixes (coffee grounds + woody substrate) to give shiitake the lignin-rich environment they need.

• Because shiitake has different growth needs, coffee grounds are rarely the only substrate; they’re usually part of a hybrid mix.

Challenges & Risks

1. Density and Compaction

   • Coffee grounds are very fine and compact easily, which reduces oxygen flow and slows colonization.

   • Solution: mix with bulkier materials (straw, sawdust) to improve structure.

2. Contamination Risk

   • Despite being pasteurized during brewing, the grounds can pick up contaminants quickly if left too long.

   • It's best to use grounds within 24 hours of collection, or store them properly (e.g., freeze if needed).

3. Chemical Inhibitors

   • Caffeine can delay mycelium growth or reduce overall yield in some strains.

   • Not all strains degrade caffeine equally; success depends on strain choice.

4. Yield Variability

   • While studies show oyster mushrooms can fruit on high proportions of coffee grounds, too much coffee (100%) may reduce biological efficiency; some experiments suggest mixing with other substrates yields better results.

Best Practices for Growing on Coffee Grounds

• Collection: Gather fresh coffee grounds from cafés or your own brewing. Use them quickly — ideally the same day.

• Mixing Ratio: Use a mix — for example, 70–80% coffee grounds + 20–30% straw or sawdust to balance structure and nutrition.

• Spawn Rate: Use a relatively high spawn rate (10–15% by weight) to help the mushroom mycelium outcompete contaminants.

• Incubation Conditions: Maintain around 20–24 °C in a dark, warm spot. GroCycle suggests colonization can take 2–3 weeks.

• Fruiting: Once colonized, move to a humid, well-ventilated area, cut holes in the bag, mist regularly, and wait for flushes.

• Reuse & Disposal: After the final flush, spent substrate can be composted or used as a soil amendment (if contamination is low).

Is It a Myth or a Miracle?

It’s more of a miracle — with caveats.

• The environmental benefit is real: turning coffee waste into mushroom food is a beautiful example of upcycling.

• Scientifically, mushrooms like Pleurotus can grow on coffee grounds and even break down caffeine, but growth may be slower and yields may vary.

• The technique isn’t foolproof: contamination, compaction, and substrate formulation are real risks.

If done correctly — with the right species, spawn rate, and substrate blend — growing on coffee grounds can be both sustainable and productive.

If you are interested in trying out mushrooms that haven't been grown on coffee grounds it's right here.

If you are interested in learning more here is a thorough study published on PubMed.