"Shocking" Shiitake Logs: What does is mean to force-fruit mushroom logs?

If you have been waiting for your mushrooms to show up that you planted many months ago, there are some "forcing" strategies you can use to encourage fruiting while ensuring your logs or mushroom beds stay healthy for future crops. 

man loading logs into a tank
covering logs with fruiting blanket
logs growing shiitake mushrooms

What is Force-fruiting?

Forcing an outdoor mushroom crop (this can be both shiitake logs or wine cap beds) usually refers to tricking the mushroom mycelium into thinking it needs to fruit; something in its interest because mushrooms themselves are the vehicle for reproduction and maintaining genetic viability for the species. One of the easiest ways to trick the mycelium into fruiting before it's natural fruiting time (usually in the late summer or early fall) is to make it think that time is NOW though it is naturally many weeks/months away. This is the primary way we are able to schedule weekly fruiting for a weekly market, and is commonly used by shiitake cultivators. Forced crops are concentrated so it makes harvest easier and allows the crop to be covered with protective fabrics while developing. Wine cap beds can also be forced (though with a little less reliability). Oyster logs as well as logs of other cultivated species do NOT usually force well.

10 Guidelines for Safely Shocking Shiitake Logs

Because forcing, especially for shiitake logs, can be a dramatic step (for the log) to take, it is important to observe the following guidelines.

  1. Forcing (for shiitake logs) requires immersing logs into cold water, usually for 12-18 hours. Logs are immersed in cold water that is about 20 F colder than the average air temp.  Trash cans, creeks, cold and clean water farm ponds, stock tanks can be used, just make sure logs can be secured weighted down underneath the water (see photo 1). Logs will float, so if you use a creekside catch pool, tie the logs up and secure them! If you are soaking logs in a short container and have half of the log sticking out of the water, you can soak for 8-12 hours and flip the log so the other side gets soaked. Logs can safely soak for 24 hours so if parts of a log are soaked longer than other sections, that is okay.

  2. Make sure the logs are inoculated with a strain that will respond to forcing. All Field and Forest Products Wide Range and Warm Weather shiitake strains will respond to forcing.

  3. Only force logs that are fully colonized... Usually if they have given you a few mushrooms sometime earlier in the season they are ready for forcing.

  4. Logs inoculated last year or older are usually ready after June 15 for their first soaking. Logs need to acclimate for several weeks  to warm temperatures so the success of the first forcing attempts will depend on how warm your spring and early summer has been. Usually in the northern USA, mid July and on is a good time to try).

  5. Let the logs rest to rejuvenate between forcing events, 8 weeks or more is ideal.

  6. Older logs (4 years or more) are best soaked only once per season.

  7. After pulling logs out of your soaking vessel, stack and cover with a humidity catching fabric (such as a fruiting blanket) to encourage nice development of the crop.

  8. You can use the same soak water twice long as the water stays cold. Soak your healthiest (often youngest) logs in the first soaking of an individual soaking vessel to avoid any transmission of disease that often is inherent on older logs.

  9. Last soakings of the season usually occur around the first frost date. Logs simply stop responding to forcing as temperatures get close to the soak water temperatures.

  10. Be gentle with your soaked logs, especially those with thin bark. Soaking loosens the bark and makes it more susceptible to physical damage.

Of course, you may choose to just let nature provide the water and temperature cues that your logs and beds evolved around for best fruiting (usually spring and fall), but allowing yourself some mushrooms "out-of-season" is such a treat.