When it comes to discussions about mycelium type, you’ve probably heard the terms rhizomorphic and tomentose and their association with mushroom pinning, but then, there comes another term, “aerial mycelium.”

So what is it? Is it a third type of mycelium? Where does it fall between rhizomorphic and tomentose?

How does aerial mycelium delay mushroom pinning? Is aerial mycelium a good or a bad thing?

In this article, we will answer the most commonly asked questions about aerial mycelium including how it affects mushroom growth.

What is Aerial Mycelium: Meaning and Definition

Mycelium is a network of hyphae that grow from mushroom spores in a substrate with plenty of nutrients. In certain conditions, hyphae may grow upward or outward from the surface of the substrate.

This outward growth is called “aerial hyphae.” Coming from the term “aerial” which means “existing, happening, or operating in the air.” A network of outward hyphal growth is collectively termed “aerial mycelium.”

In appearance, it presents as loose cotton balls or cauliflower growth above the substrate. Unlike molds, aerial hyphae are bright white and do not overgrow overnight.

1. Vegetative Mycelium vs Aerial Mycelium

In contrast to aerial mycelium which grows above the substrate, there is what you call substrate mycelium.

Also called as penetrative mycelium, it is the portion that penetrates the substrate and absorbs nutrients.

Basically, this is the main mycelium that develops from the germinating spores. That is why it is also termed as “primary mycelium.”

Next, we have surface mycelium which is obviously the mass of hyphae growing right on the surface of the substrate.

Then there goes the aerial vegetative mycelium which is the non-reproductive mass of hyphae growing right above the substrate’s surface.

Last is the aerial reproductive mycelium which produces spores.

Meanwhile, some scholarly articles only describe two types of mycelium.

Vegetative mycelium as the mass of non-reproductive hyphae penetrating the substrate. Then, aerial mycelium as the reproductive outward hyphal growth.

Given that, they refer to vegetative mycelium as the “primary mycelium” and aerial mycelium as the “secondary mycelium.”

2. Aerial Hyphae Definition and How Does It Form

Firstly, aerial mycelium may grow as an extension of the substrate mycelium. It may start occurring when the substrate is only partially colonized or when it has reached full colonization.

Often, growers cannot tell whether they are seeing aerial mycelium or already the formation of primordium before the Fruiting Phase.

This confusion stems from the fact that both growths characterize an appearance of extended mycelium.

Primordium (plural primordia) also referred to as pinhead is the earliest identifiable stage of development of a fungus’ fruiting body.

3. What Does Mushroom Pinning Look Like?

Give it a little time and see if more branching hyphae stem in a few days. With no noticeable tiny pins, the mycelium in your growing room or medium might not be pinning at all.

In other words, the conditions in your mushroom growing kit are not optimal and you need to do some adjustments.

Usually, the overgrowth of aerial hyphae is caused by two correlated conditions: poor air exchange and excessive humidity.

Branching outside the substrate is like the mycelium’s cry for fresh air.

However, it is unclear if all mushroom-bearing fungi go through aerial hyphae phase. Or if it is an abnormal growth that prevents pinning.

In other scholarly articles, it was described that after the primary substrate mycelium has been established, aerial hyphae are formed. From that form, it may develop into reproductive structures such as fruiting bodies.

Probably, in healthy mushroom growth, aerial hyphal growth is supposed to be short-lived before it starts pinning.

So aerial mycelium has earned its bad reputation because its overgrowth and stagnation are associated with delayed pinning.

4. How Do Aerial Mycelium Affect Mycelium Pinning and Mushroom Growth?

For the mycelium to produce good pin sets, you need to tweak the conditions like adjusting humidity and controlling air temperature.

You may introduce fanning to encourage airflow and prevent CO₂ buildup. Also, you can just limit misting with water spray bottle, to bring down moisture and humidity.

According to mushroom expert “TVCasualty” in the Mycotopia forum:

“… aerial mycelium is a sign of high Relative Humidity (not necessarily too high, just somewhere above 90%). In the case of most home growers, this would also usually occur in conjunction with high carbon dioxide.

(This is because) most people don’t build complex automated FAE systems. They either end up with air that’s a little too dry (but very fresh), or with 90+% rH but is a little stagnant. It’s far better to err on the side of too dry but fresh air.”

There is no universal growing process for all mushrooms at home. So parameters for optimal mushroom development vary depending on the strain you’re growing.

But commercialized mushroom strains (such as shiitake mushrooms, oyster mushrooms, or white button mushrooms) can fruit in the following approximate conditions:

• Humidity
  • 80% to 95% Relative humidity
• Fresh Air Exchange (O2 vs. CO2 levels)
  • 4 to 6 fresh air exchanges per hour
• Temperature
  • 50°F to 70°F (10°C to 21°C)
• Lighting
  • 12-hour cycles of light/dark

Aerial mycelium overgrowth, as a result of poor air exchange, may delay your mushroom pins. A lot of growers report ending up having flush of mushrooms with stunted growth or stems that are too long.

While others have little mushroom yield or not going through the pinning phase at all.

5. Is Aerial Mycelium Rhizomorphic or Tomentose?

There is no strict rule in terms and definitions when it comes to mushroom growing. The meanings of some terms have not yet been established.

Thus, growers often rely on reading discussions and asking around online forums. Then, from there, discover new words that best describe their mushroom appearance…

until those words become “FORuMALLY” widely accepted terms, such as calling any string-like growth as rhizomorphic and a fluffy growth as tomentose. 😉

So if your aerial mycelium looks like string-like growth, call it rhizomorphic aerial mycelium. No one can judge you.

Key Takeaways:

• Aerial mycelium a.k.a. secondary mycelium grows from the surface of the substrate or the primary mycelium.
• Substrate mycelium, vegetative mycelium, or primary mycelium is the portion of the mycelium that penetrates the substrate.
• Though not necessarily bad, overgrowth and stagnation of aerial mycelium often correlates to delayed pinning and inferior mushroom growth.
• Lastly, improving Fresh Air Exchange seems to prevent further aerial mycelium growth and encourages pinning.