Why the second flush is often smaller, and how to get a third

After the first harvest from a grow kit the same email almost always follows: the second round is much smaller, am I doing something wrong? Usually not. A grow block is a pantry, and every flush eats part of it. The second flush being smaller than the first is normal. That it is half the size for some people and nearly the same size for others, that is where it gets interesting.

Why is the second flush smaller?

In the weeks before the first flush, the mycelium in a block builds up a huge reserve: sugars and proteins from the substrate, stored in the fungal tissue. The first flush is paid for out of that full pantry and is therefore the largest. After that the mycelium has to gather new reserves from substrate that is already partly digested. That goes slower and yields less. Every following flush comes from a poorer cupboard. For an oyster mushroom block of just over three kilos, a ratio of roughly 50, 30 and 20 percent over three flushes is typical.

What makes the second flush smaller than it needs to be?

Three things, all avoidable. First, drying out: many people mist faithfully until the first harvest and then leave the block alone. It is precisely during the rest period that it must stay moist, otherwise a dry crust forms that the next flush cannot break through. Second, harvesting too late: an oyster mushroom left on the block past its prime releases spores and costs the block energy the second flush should have had. Harvest while the cap edges still curl slightly downward. Third, old stubs: bits of stem left behind after harvesting rot and attract mould. Cut them off flush with the substrate.

How do you force a third flush?

With a cold shower. After the second harvest a block often looks exhausted, but there is usually one more flush in it. The method:

1. Remove all stubs and loose bits from the harvest site.
2. Submerge the whole block in cold water for twelve to 24 hours. A bucket, tub or bath, with a plate or stone on top to keep it under.
3. Let it drain for an hour and return it to a cool, bright spot with fresh air.
4. Mist daily and wait. The third flush often comes within two to three weeks.

The soak does two things at once: the block regains its lost moisture, and the temperature shock mimics an autumn rain, the signal the fungus fruits on in nature. It is by far the most effective trick we know, and it works on logs too, as described in our log piece.

And a fourth?

Sometimes. Repeat the cold shower after the third harvest, and accept that the yield gets small: a handful, not a meal. At some point the block weighs only half what it did on arrival and feels bone dry and springy. Then the pantry is empty. Crumble it into the garden over a layer of wood chips, and who knows, autumn may bring a surprise.

Does this apply to other species too?

Yes, each with its own rhythm. Lion's mane often gives two fine flushes and little after that. Maitake puts almost everything into one big flush and usually one small one afterwards. Shiitake on a block can reach four or five small flushes with cold showers. The pink oyster is the fastest of all and also the fastest to finish: three quick flushes in a month and then it is done.

In summary: the first flush is a gift, the second you earn by misting and harvesting on time, the third you force with a cold shower. Take all three and you get out of a block what is in it.