Portobello Mushrooms: What Are Their Growth Stages?

Portobello Mushrooms: What Are Their Growth Stages?

Portobello mushrooms are not a separate species with their own life cycle. They are the fully mature, open cap stage of Agaricus bisporus, the exact same fungus sold young and white as a button mushroom and young and brown as a cremini. The growth stages run in one continuous sequence: mycelium colonizes the substrate, the mycelium rises into a casing layer, pins form, those pins swell into closed button caps, the caps brown and enlarge to the cremini stage, and finally the veil tears and the cap opens flat and wide into a portobello, after which the exposed gills darken and release spores.

I have sold Agaricus cultures and substrate out of Out-Grow since 2009, and the single biggest source of confusion I hear on this species is the belief that button, cremini, and portobello are three different mushrooms. They are one mushroom at three points on the same timeline. Once that clicks, the growth stages make sense and the harvest decision becomes yours to make, because you choose which stage to pick at.

Below I walk through why these are all one species, the full growth stage sequence in order, how to tell the button, cremini, and portobello stages apart, how long each stage takes, the conditions that move a crop from one stage to the next, and how to decide which stage to harvest at. If you are planning your first Agaricus grow, the stage identification section is the part worth bookmarking.

Are Portobello and Button Mushrooms the Same Species?

A woman at a farmers market table I was working a couple of summers ago picked up a jar of brown Agaricus culture, turned it over, and asked whether it was the cremini one or the portobello one, like they were two different jars on my table. That mix-up comes up more than almost anything else I sell. The answer is that portobello, cremini, baby bella, and white button are all the same species, Agaricus bisporus, and the difference between them is maturity and strain, not genetics from a different mushroom. A white button is an immature, closed, white strain cap. A cremini or baby bella is an immature closed cap of a brown strain. A portobello is that brown cap grown all the way on until the veil breaks and the cap opens broad and flat. The older names like Psalliota bispora that you sometimes see are just obsolete labels for the same organism.

This matters for anyone growing them, because it means you are not hunting for special portobello spawn. A single brown strain liquid culture of Agaricus bisporus gives you the full range. Whether you end up with buttons, cremini, or portobellos comes down to how long you let the caps develop before you harvest, not which culture you bought.

Here is the whole arc in three numbers before we walk through it stage by stage.

The Growth Stages of a Portobello Mushroom in Order

The first tray of Agaricus I ever ran start to finish sat on a shelf in my basement, and I remember checking it every morning convinced I was missing something because the visible mushroom stages took so long to show up compared to the invisible ones underneath. A portobello moves through six recognizable stages, from invisible mycelium to a wide open cap dropping spores. The first two stages happen down in the substrate and the casing where you cannot see fruit yet, and the last four are the visible mushroom growing up and opening. Here is the sequence in plain terms.

First, mycelium colonization, also called the spawn run. The grain spawn you mixed into composted substrate sends white mycelium through the compost until the whole bed is knit together. Second, casing colonization. A moist casing layer goes on top, and the mycelium climbs up into it, which is the trigger stage for fruiting. Third, pinning, when tiny knots of mycelium organize into primordia, the pinhead sized baby mushrooms. Fourth, the button stage, a small closed round cap. Fifth, the cremini stage, a larger brown cap with the veil still intact or just starting to tear. Sixth, the portobello stage, where the veil ruptures, the cap flattens out wide, and the gills are fully exposed and darkening as they release spores.

The chart below shows how much the cap grows across the visible stages. The jump from cremini to portobello is the dramatic one, which is exactly why the open cap looks like a different mushroom even though it is the same one grown on.

Here is the same sequence as a flow you can screenshot and keep next to your grow, from the hidden mycelium stages through to the open portobello dropping spores.

Button, Cremini, and Portobello: How the Stages Differ

A batch I ran last fall taught me to stop eyeballing cap width alone. I nearly pulled a tray of what I assumed were young portobellos, then flipped one over and saw the veil still sealed tight over pale gills. It was just a large cremini, not a portobello, and the veil was the only thing that told the difference. The fastest way to read the stage of an Agaricus bisporus mushroom is to look at three things: the cap size, whether the partial veil under the cap is still sealed or has torn, and the color and exposure of the gills. As the mushroom matures, the cap widens, the veil breaks, and the gills shift from pale pink to brown to nearly black. The table lays out the tells for each stage.

Notice that a cremini and a young portobello are the same brown strain at slightly different ages. The line between them is the veil. While the veil under the cap is still closed, you have a cremini. Once it tears and the cap opens flat with the gills showing, you have a portobello. That one visual cue is the whole distinction, and it is the same cue that fooled me that afternoon in the basement.

The profile below rates the same three harvest stages across the qualities growers care about, so you can see the tradeoffs at a glance before you decide when to pick.

The tradeoff is clear once you see it laid out. Growing the cap all the way to portobello gives you the deepest flavor and the meatiest texture, which is the whole appeal, but the open gilled stage does not keep as long once picked. Buttons and cremini store longer but taste milder. None of them is the correct answer, they are just different points on the same curve.

How Long Does Each Portobello Growth Stage Take?

I keep a wall calendar taped to the shelf above my Agaricus trays, and marking the day the first pin shows up is the habit that keeps me from guessing later. From a visible pin, an Agaricus bisporus mushroom reaches button size in about 3 to 4 days, cremini size in about 5 to 7 days, and a fully open portobello in about 10 to 21 days, depending on your strain and how tightly you hold the temperature and humidity. Cooler, steadier conditions slow every stage down but tend to give firmer, better formed caps, while a warmer room speeds things up and can leave you with thinner, faster caps.

Those are just the visible fruiting stages. The full crop takes longer, because before any pin appears you have the spawn run and the casing colonization to get through. From spawning the compost to your first harvest usually runs about 6 to 8 weeks under good home conditions. The scale below shows the visible stage timing from the first pin.

One point worth knowing for the far end of the timeline. Developmental studies on Agaricus bisporus fruit bodies record a lifespan of up to about 36 days in protected conditions, with visible signs of aging appearing around day 18 of the fruit body's life. In practice you harvest a portobello well before that, as soon as the cap is open and the gills are clean, because eating quality drops off as the mushroom ages toward spore drop and senescence.

Growing Conditions for Each Portobello Stage

The first Agaricus room I ever set up ran on a single thermostat for the whole grow, spawn run through fruiting, because I figured one steady number would be simpler to manage. It was not. The caps that came up were thin and the pinning was patchy, and it took me a full crop to understand that this species wants a warm sealed phase and then a cool fresh phase, not one setting the whole way. Each stage is driven by a different set of conditions, and moving the crop from one stage to the next is mostly a matter of changing temperature, humidity, and fresh air exchange at the right moment. The mycelium runs warm and sealed, then the crop fruits cool and fresh. The table shows the target conditions stage by stage.

The trigger that starts the visible stages is the drop from the warm spawn run into the cool fruiting range in the mid teens Celsius, combined with more fresh air to bring the carbon dioxide down. That shift is the signal to stop running mycelium and start making mushrooms. Once pins are up, the temperature stays cool and steady while you ease the humidity down slightly so the expanding caps do not stay wet, which invites bacterial blotch on the developing portobello.

Growers coming over from oyster mushrooms often underestimate how much more this species asks of them at this stage of the process. Here is roughly how the effort stacks up against an easier crop, based on what I see in my own grow room.

What Stage Should You Harvest Portobello Mushrooms?

I let a tray go two days too long one September because I was traveling, and came back to caps that had gone soft at the edges with gills so dark and wet they had shed spore dust over half the shelf. That crop still cooked fine but the texture was gone. Harvest at whatever stage matches what you want to cook, because the same bed can give you buttons, cremini, or portobellos depending on when you pick. For a true portobello, wait until the cap has opened broad and flat and the gills are fully exposed, usually around 10 to 15 centimeters (4 to 6 inches) across. That open, dark gilled, meaty cap is the point of growing them. If you pick while the cap is still domed and the veil is sealed, you have a large cremini, which is perfectly good, just not a portobello.

The judgment call is the late end. Leave a portobello too long and the flesh softens, the gills blacken further and drop spores heavily, and the eating quality falls off, the way that September tray taught me. I harvest when the cap is flat and the gills are open and clean but before the flesh goes soft. To pick, twist gently at the base to lift the mushroom with minimal disturbance to the casing, then fill the small hole left behind with a little casing so the surface stays intact for the next flush. Agaricus typically gives 2 to 3 flushes about 7 to 10 days apart when you keep the casing moist and the climate steady.

Common Bad Advice About Portobello Growth Stages

A grower emailed me a link to a care sheet a few months back that gave one flat temperature for the whole grow, then wrote back two weeks later wondering why nothing was pinning. That care sheet is a good example of the kind of claims that circulate in beginner content, and each one costs people either a crop or a good harvest.

Bad advice: portobello, cremini, and button are different mushrooms. They are one species, Agaricus bisporus, at different maturities. Treating them as separate mushrooms sends beginners hunting for special portobello spawn that does not exist, when a single brown strain culture grows all three. The stage is set by when you harvest, not by which culture you bought.

Bad advice: grow the crop at one steady room temperature through all the stages. Many beginner guides claim a single ideal temperature around 65 to 75 degrees Fahrenheit for the whole grow. In reality this species runs its mycelium warm, around 23 to 25 degrees Celsius (73 to 77 degrees Fahrenheit), then needs a drop into the cool mid teens Celsius, about 15 to 18 degrees Celsius (59 to 64 degrees Fahrenheit), to pin and fruit. Hold it at one warm temperature the whole way and you often get poor pinning and weak caps, which is exactly what happened in my own first room and what happened to the grower who emailed me. The stages need different temperatures.

Bad advice: more fresh air exchange is always better once pins appear. A common rule you will see is to just give the mushrooms as much air as possible. Too much airflow dries out forming pins and the casing surface and drives aborts, while too little leaves carbon dioxide high and gives you long stems and small caps. The real target is balance, enough fresh air to move the crop through pinning and cap growth without drying the bed, which is why I mist surfaces to hold humidity rather than blasting the developing mushrooms with air.

Relevant Out-Grow Supplies

The four things that land on my own bench every time I run this crop are the same four I will point you to here. I am not going to pad this with gear you do not need.

• Portobello (Agaricus bisporus) liquid culture: The brown strain culture that gives you button, cremini, and portobello from one starting point. This is the genetic start of the whole stage sequence.
• Agar plates and culture supplies: For cloning tissue or cleaning up a culture before you expand it to spawn, the reliable route to your own portobello genetics.
• 50/50 horse manure and straw mix substrate: The composted base Agaricus needs to run mycelium and reach the fruiting stages, so you skip building a compost pile.
• Grain spawn bags: To expand your liquid culture into the grain spawn you mix into the compost before the colonization stage.

One honest note. Agaricus fruits from a moist casing layer, and I did not find a dedicated peat casing product in our catalog to link you to, so source a peat and limestone casing separately. The casing is what triggers pinning, so it is not a step to skip.