Why Are My Homegrown Mushrooms Coming Out Small and Thin?

Small, wiry, or undersized mushrooms are a reliable sign that something in your fruiting environment is not quite right. This is almost never a problem with the genetics of your spawn or an irreversible issue with the block. It is an environmental signal that can be corrected for your next flush. Identifying the specific cause is usually straightforward once you know what to look for.

Humidity Is the Most Common Cause

Mushrooms are 85 to 90 percent water by fresh weight. When ambient humidity is too low during fruiting, mushrooms cannot absorb enough atmospheric moisture and grow small and thin despite normal pin formation. Low-humidity grows produce lots of small caps that open and sporulate quickly rather than developing into the dense, thick-capped clusters you expect. Invest in a hygrometer if you have not already, place it in your fruiting chamber, and confirm humidity stays consistently at 85 to 95 percent throughout the fruiting cycle. Increasing misting frequency often resolves small mushroom issues immediately in the next flush.

CO2 Causing Elongation Without Bulk

High CO2 during fruiting causes mushroom stems to elongate rapidly while caps remain small. The mushrooms are responding to the CO2 signal by growing taller to reach fresher air, just as they do inside decaying logs in nature. This produces tall, thin, small-capped mushrooms with disproportionately long stems. The fix is more frequent fresh air exchange, either manual fanning or a fan on a short timer cycling every one to two hours. After improving airflow, you should see notably larger, thicker caps from the next set of pins.

Temperature Too High During Fruiting

Temperatures above the optimal fruiting range cause faster development but smaller final size. Mushrooms that develop in warm conditions go from pin to fully open cap in a day or two rather than the three to five days ideal conditions allow. This rush produces smaller caps because the cells expand quickly but do not accumulate as much mass. Moving blocks to a cooler location during fruiting often dramatically improves cap size and density without requiring any other changes.

Overcrowding and Late-Flush Resource Depletion

If too many pins set simultaneously on a block, they compete for the nutrients and moisture available, producing many small mushrooms rather than fewer large ones. This is most visible on the third flush of a heavily used block. Thinning clusters by removing some developing pins early gives the remaining ones more resources, though many growers find this counter-intuitive. Second and third flushes naturally produce smaller mushrooms than the first as substrate nutrients deplete, which is normal and expected. Soaking the block between flushes helps restore moisture but cannot fully replace depleted nutrients.