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Cathy's Lab: Running Oysters - Sawdust vs Grain Spawn and Other Insights.

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For simplicity, only Straw is referred to in this blog. What is meant by “Straw” is any cellulosic agricultural waste product that can be used to grow oyster mushrooms; for example corncobs, cottonseed hulls, sugarcane bagasse, nut and seed shells, legume hulls, lawn clippings, cereal straws, and the list goes on and on.

Growing oyster mushrooms can often be a low margin business. Booms and bust are to be expected. To make money it is necessary to keep your cost inputs low. The beauty of running straw is that the material and equipment costs are inexpensive, and most importantly you can pasteurize the substrate. Pasteurization is much faster and cheaper than the time, equipment and energy required for sterilization.

You can get more spawn for your money in sawdust over grain spawn. Grain spawn is much more expensive than sawdust because it has to be sterilized and the cost of grain is higher than sawdust. Grain is great for lab situations. The grain is sterilized for hours before it is cooled and spawned from perfectly clean petri plates. The grain spawn is then transferred to sterilized wood bags. All processes are performed in a laminar flow hood in a squeaky clean lab by meticulous well washed professionals. Any lab person will tell you the most important part of growing mushrooms is to keep it clean.

This brings us to the second problem with grain - It is delicious, not only to oyster mushroom mycelium but to almost every microbe out there (Bacteria, Fungi and Mold), and animals (rodents, birds and insects). For this reason it is not the best material to use even in a semi-clean environment. As the grains are degraded by the fungus it respires the signature smells of rotting grain that attract fruit flies and other dirty creatures like no ones business. Any grower will tell you that the best way to control insects is to not get them in the first place.

Straw/wood on the other hand is not delicious, except to fungi. Not much else is good at chowing down on its complex cellulose molecules. Primary sugars are not easily accessible in straw so primary invaders like bacteria may show up to the party, but they don’t stay and get drunk and throw up all over your couch. All straw should be fresh or used quickly, kept clean and away from moisture. Mold and fungal invaders will start to grow on wet straw almost immediately.

The only small benefit to grain spawn is that your inoculated material might run a little faster. Instead of using grain spawn you can add the same nutrients that allow the mycelium to run faster. Grain spawn primarily provides nitrogen in the form of proteins that are slowly broken down by the mycelium. Don’t use plant fertilizer cuz it will burn the mycelium. There are specific slow release supplements for mushrooms that are used by commercial growers but they are pretty expensive. Cheap and free things like coffee grounds, hair, feathers, crab meal, corn gluten meal or just about any meal made from the waste of soy bean plants (or other legumes) work well for oysters. A little bit goes a long way. Experiment with 1 to 5% weight to your total straw run. For most oysters, coffee grounds will need to be drained for a few days or pressed to get rid of excess moisture. They need to be kept clean so you don’t have to pasteurize, which will suck up the moisture once again. Hair and feathers are ideal in that they are almost all protein and can be pasteurized along with the straw. The meals usually aren’t free but are largely clean so they can be added after pasteurization. Corn meal is also a pre-emergent weed killer if you are growing mushrooms in beds outdoors, and it has that additional benefit.

In side by side tests in our green house coffee grounds did not increase days to fruiting or mushroom fruit yields.


Studies have shown that supplements of 1-2% Calcium carbonate (lime) and/or 1-5% Calcium sulfate (Gypsum) can increase oyster fruiting (Chen, Lin et al. ; Tabata and Shinohara 1995; Raja, Mallesha et al. 2013), but do not have a discernible effect on the mycelium run. Lime raises pH, while gypsum does not change pH but it has the added benefit of increasing airflow by facilitating gas exchange. Lime works well with straw (I’m talking actual straw here, not necessarily the other types of agriculture waste) because the pH of straw is low. Mushrooms like slightly acidic pH and other invaders like bacteria don’t, so it is best to keep the pH below 7 but above 5. Green molds like the pH low so if you have a mold problem it is good idea to raise the pH. In addition, mycelium growth lowers pH, when the pH decreases to 4 the mycelium will stop growing. Optimal pH for fruiting is around 5-6 though mushrooms will fruit up to a pH of 7.5. Both gypsum and lime should be mixed well with the straw before or after pasteurization. If you add it before it will be dissolved better but you will be losing some in your pasteurization water. On the other hand if you add after you are more likely to have uneven mixing and un-dissolved bits that can cause pockets of slow or absent growth.

The most important thing is that you spawn at the correct ratios, which is 1 pound of spawn (sawdust or grain) for every 5 to 20 pounds of straw. We recommend for new growers that they start with a 1:5 ratio of sawdust spawn: straw. Get a good fast run and start to get a feel for how well a strain does in your farm conditions. Move up to experiments with supplements and lower ratio of spawn:straw runs. To get a real handle on the best conditions it is always good to do side by side tests with no supplement controls. Take good notes, it is surprisingly hard to remember how much of an ingredient was added and how it correlated with results. Often when you go back and look at results you can figure out external conditions or seasons that lead to contamination or poor yield.

Straw should be weighed and spawned somewhere between 1:5 and 1:20 W:W ratio.


Here in Tennessee we have highly variable weather. Summers are so hot and humid you feel like new life forms are evolving right on your skin. Our mold and pollen counts are often some of the highest in the nation. Here we have found that nitrogen supplements do increase the run, thus decreasing the days to harvest. However, nitrogen supplements also increase the chance for contamination and invading mold tends to set in on supplemented bags soon after the first flush. So here at Everything Mushrooms we just do a straight straw run from sawdust spawn even though we have ready access to grain from our lab.

Chen, W., Z. Lin, et al. Culture medium useful for cultivating Pleurotus nebrodensis, comprises corncob, lotus seed shell, peanut shell, grape seed, corn powder, lees and calcium carbonate, LIANYUNGANG SENBAO EDIBLE FUNGUS CO LTD (LIAN-Non-standard): 6.

Raja, S., B. C. Mallesha, et al. (2013). "Effect of micronutrients on growth and yield of mushrooms." Mysore Journal of Agricultural Sciences 47(1): 66-69.

Tabata, T. and H. Shinohara (1995). "ABSORPTION OF CALCIUM FROM CALCIUM SALTS ADDED CULTURE MEDIA BY HIRATAKE (PLEUROTUS-OSTREATUS (FR) QUEL) AND NAMEKO (PHOLIOTA-NAMEKO J-ITO)." Journal of the Japanese Society for Food Science and Technology-Nippon Shokuhin Kagaku Kogaku Kaishi 42(9): 682-686.