Is a Third Flush Worth the Effort?
Every fall I get the same photo in my inbox: a grow bag that has already handed over two solid flushes, a scatter of thin little pins on the surface, and one question underneath. Do I keep going, or do I toss it? Here is my honest answer after more than twenty years of running blocks. For most gourmet mushrooms on a bag or block, a third flush is only marginally worth the effort, and for a lot of beginners it is not worth it at all.
By the time you reach a third flush, the bag has usually given up most of what it had in it. The first flush is roughly half the total harvest, the second is a quarter to a third, and the third is often only 10 to 20 percent, with smaller and more blemished mushrooms and a higher contamination risk. If your block is still firm, clean, and heavy, a third flush can be a pleasant bonus. If it is light, tired, or spotted, your time pays off better on a fresh grow.
Out-Grow Quick Answer
- 🍄 A third flush usually returns only about 10 to 20 percent of a bag's total yield, so it is a bonus, not the main event.
- 📉 Each flush drops off hard. First flush is roughly half the harvest, second a quarter to a third, third what is left.
- 🔎 Judge the block, not the calendar. Firm, clean, and heavy means try. Light, sour, or moldy means stop.
- 🌲 Wood-loving species like oyster and shiitake on solid blocks handle a third flush better than most.
- ♻️ Do not reuse spent substrate as spawn. In trials it gave one tiny flush and hit heavy contamination.
What This Means for Beginners
When you are new, chasing a third flush feels like the responsible thing to do. Waste not, want not. But the numbers tell a different story. The extra days of misting, watching, and worrying often buy you a small handful of second-rate mushrooms, while the same days spent inoculating a fresh block would set up your next full first flush. I would rather see a beginner nail two strong flushes and move on than babysit a fading bag for a week to squeeze out a few discolored caps.
Typical Yield Per Flush, and Where the Third Flush Lands
The first time I actually weighed every flush off a rack of oyster bags instead of eyeballing it, the pattern jumped out at me. The first flush was the giant. The second was respectable. The third barely moved the scale. That is not bad luck, it is how the substrate empties out. A mushroom block front-loads its harvest, and each round after the first pulls from a thinner and thinner pantry.
Across practical oyster data, the first flush runs roughly 40 to 60 percent of the total fresh weight, the second about 25 to 35 percent, and the third only about 10 to 20 percent, with anything after that under 10 percent. A published oyster trial at the University of Zambia put real grams to it: on one growing system the first flush averaged about 418 grams, the second about 334 grams, and the third only about 90 grams per unit, out of roughly 910 grams total. That third flush was about a tenth of everything the block produced.
People sometimes ask me about biological efficiency when they are weighing whether to keep going. It is worth knowing the term, because it is the honest way to score a grow.
The ratio of fresh mushroom weight harvested to the dry weight of the substrate, written as a percentage. Oysters commonly run about 75 to 150 percent, shiitake about 50 to 100 percent, and lion's mane about 50 to 90 percent across a full grow. A third flush usually adds only about 10 to 20 percent to that cumulative total, not a fresh round of the same size.
Here is the same story laid out flush by flush, because the drop-off is not only about weight. Size and quality slide at the same time.
| Flush | Share of total yield | Typical quality | Contamination risk |
|---|---|---|---|
| First | About 40 to 60 percent | Largest caps, best form | Lowest |
| Second | About 25 to 35 percent | Good, slightly smaller | Rising |
| Third | About 10 to 20 percent | Smaller, more defects | High |
| Fourth and beyond | Under 10 percent | Marginal, often not worth picking | Highest |
Why Third Flush Mushrooms Come In Small and Discolored
I remember a batch of oyster bags a couple of winters ago where the third flush pinned beautifully, then half the caps came up stunted and browning at the edges before they ever sized up. Nothing had gone wrong in the room. That is just what a tired block does. By the third flush the fungus is scraping the bottom of the barrel, working on the tougher, less accessible food left in the substrate, and it shows in the fruit.
The mechanism is straightforward once you picture the block as a food supply. The easy carbohydrates, the nitrogen, and the minerals get spent in the first two flushes. The substrate structure breaks down and holds water differently. The mycelium itself is older and more stressed, so it pushes fewer and weaker pins and it is slower to shrug off the bacteria and molds that are always waiting. On oyster in particular, published work has even found the third flush fruit shifts in composition, running higher in fat and ash and lower in the yield and the carbohydrate and protein content of the earlier flushes. Smaller, less vigorous, and more prone to blemish is the norm, not a sign you did something wrong.
None of this makes a third flush inedible. A clean, firm block that throws a modest third flush of decent-looking mushrooms is fine to eat. The rule I hold to is simple. Discoloration from age and a thin harvest is a quality question. Sliminess, a foul or sour smell, or fuzzy colored mold is a safety question, and anything in that second group goes in the trash, not the pan.
When a Third Flush Is Worth the Effort, and When to Skip It
A customer emailed me last spring, torn about a rack of shiitake blocks that had given two clean flushes and still looked solid. My answer was go for it, and it paid off. That is the situation where a third flush earns its keep: a robust wood-loving species on an intact block, low contamination pressure in the room, and the space and patience to run a slower round. Flip any of those and the math flips with it.
Push for a Third Flush
Block is firm, heavy, and clean. Species is a wood-lover like oyster or shiitake on a solid block. Low mold and fly pressure. You have the bench space and are not in a hurry.
Stop at Two Flushes
Block feels light or looks broken down. Any green mold, sour smell, or fly activity. You are short on space. You could inoculate a fresh block with the same effort.
Species matters more than most beginners expect. Wood-loving mushrooms on firm sawdust or supplemented blocks hold their structure and tend to give a usable third flush. Softer or faster-spending setups run out sooner. Here is roughly how I rank the common gourmet species for third-flush potential on a home block.
| Species | Third flush potential | Notes |
|---|---|---|
| Oyster | Fast and forgiving, often gives a small third flush | |
| Shiitake (block) | Dense blocks can flush several times if rested and soaked | |
| Lion's mane | Often gives two or three, third is usually small | |
| King oyster | Puts most of its energy into the first flush |
After more than twenty years of running blocks and shipping bags, my rule has not changed. Two strong flushes and a fresh start beats babysitting a fading block for a thin third. I only push for a third when the block is still firm, clean, and heavy in the hand. Everything else is opportunity cost.
How to Get a Third Flush From Oyster and Block Grow Bags
When I do decide a block has earned a third try, I do not just keep misting and hoping. I reset it the same way every time, and the first step is the one people skip. Before anything else, I strip off every aborted pin and bit of harvest debris from the last flush, because that rotting material is exactly what contamination feeds on. Then I rehydrate carefully, put it back in fruiting conditions, and give it a set window to prove itself.
- Block still feels firm and heavy, not light and crumbly
- No green, black, or fuzzy mold anywhere on the surface
- No sour, foul, or ammonia smell when you open the bag
- Wood-loving species on a solid block, not a spent soft substrate
- You have space and time to run a slower, lower-yield round
If the block passes that checklist, here is the sequence I follow to coax out a third flush.
When to Stop Harvesting Mushroom Blocks and Toss the Substrate
The hardest habit I try to break in newer growers is the belief that a block owes them more mushrooms. It does not. I have watched people mist a dead block for two weeks, convinced the next flush is coming, while contamination quietly takes over. Knowing when to stop is a cultivation skill on its own, and the block gives you clear signals when it is done.
Retire a block when it feels light and crumbly, when the color has gone dull or patchy, when any green or black mold appears, or when it starts to smell sour or off. Those are the block telling you the nutrients are gone and the contamination load is climbing. A common question is whether you can get the substrate working again by using it as spawn for a new grow. The answer from real testing is a hard no.
That does not mean the block is garbage. A retired block is excellent compost, and outdoors it will sometimes throw a surprise flush if you bury it in a shady bed and let nature handle the humidity. What it will not do is perform as a reliable indoor producer, so I treat the end of the third flush as the end of that block's working life and move its remains to the garden.
Common Bad Advice About Third Flushes
I hear the same handful of myths about later flushes over and over, usually from folks who read one confident guide and took it as gospel. The most damaging one is the idea that you should always push for as many flushes as possible to maximize yield. That advice ignores what actually happens to a tired block, and it costs people clean grows.
A common rule you will see is to always push for a third flush to maximize your yield.
The third flush is often only 10 to 20 percent of the total, at lower quality and higher risk. Two strong, clean flushes usually beat three uneven ones.
Just pour plenty of water on the block and you will keep getting flushes.
Over-soaking an aged block speeds up discoloration and rot. Rehydrate to field capacity, and if a block is spent, no amount of water revives it.
You can reuse spent substrate as spawn and get similar performance.
In testing, spent substrate as spawn gave one tiny flush and 100 percent contamination. Use it as compost, not as spawn.
The bigger lesson underneath all three myths is that the number of flushes is the wrong scoreboard. What matters is total quality yield and keeping your clean grows clean. Three poor flushes that end in a contaminated block are worse than two strong ones and a fresh start.
Relevant Out-Grow Supplies
You do not need to buy anything to decide whether a third flush is worth it. The honest read is on the block in your hands. That said, if the answer for you keeps coming back as start something fresh, these are the supplies that actually match that advice.
- Mushroom grow bags. The filtered bags I use for block grows. Good fit when you have decided a spent block is done and you want to run a clean new grow instead of chasing a fourth flush.
- All in one grow bag. A colonize-and-fruit setup for beginners. Honest fit note: the same two-strong-flushes pattern applies, so plan your yield around the first two rounds, not a big third.
- Wood Based All In One Mushroom Grow Bag. A wood-based block for the wood-loving species that hold up best to a third flush, like oyster and shiitake. Fits if you specifically want the best shot at a usable later flush.
- Mushroom supply. The broader supply collection for the humidity and fresh air exchange gear that makes a slower third flush actually pin instead of drying out.
The Honest Verdict on the Third Flush
A third flush is a bonus, not a goal. Most of a grow bag's harvest lands in the first two flushes, and the third typically brings in only 10 to 20 percent, smaller and more prone to blemish, with contamination climbing the whole time. Let the block decide. If it is firm, clean, heavy, and a wood-loving species, give it a rehydrate and one more window. If it is light, tired, or spotted, retire it to the compost and put those same days into a fresh grow. Two strong flushes and a clean start will out-produce a babysat third flush almost every time.
About the author
Mike Wiberg is the founder of Out-Grow, a mushroom cultivation supply company he has run since 2009. He developed the original all in one mushroom grow bag in 2014, a format now used across the hobby, and over more than 20 years of hands-on mycology he has grown hundreds of species and researched substrate formulations from the lab bench. He maintains one of the largest commercial culture libraries online, including a working collection of more than 300 species under a USDA permit. His cultivation work has been cited in peer reviewed research published in Frontiers in Cellular and Infection Microbiology, and he is one of only a handful of growers with a documented successful clone of Matsutake, one of the most difficult species in cultivation. He has helped growers troubleshoot cultivation problems for nearly two decades, and he writes from the lab and the grow room, not from a keyword list.