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Straw is the classic beginner substrate for oyster mushrooms. It’s cheap, easy to find and quick to colonise. But straw straight from the bale carries plenty of other microbes, so it needs treating before you add spawn. Treating it is not about making it sterile. It’s about knocking back the competition enough to give your mushroom spawn a head start.
There are two main ways to do it at home: soaking in hot water, or soaking in a high-pH lime solution. This tutorial walks through both, step by step, and looks at what on-farm and lab trials found when the methods were compared. If you’re new to the difference between pasteurising and sterilising, start with our guide to sterilisation vs pasteurisation.
The methods at a glance
Cornell University’s Small Farms Program describes the main options used by small growers in the north-eastern United States. Here is how they compare:
| Method | How it works | Time | Main drawback |
|---|---|---|---|
| Hot water | Hold straw at 140 to 160°F (60 to 71°C) for at least two hours | A few hours, including heating | Needs fuel, a heat source and temperature checks |
| Hydrated lime | Soak straw in water raised to about pH 12 to 13 | 16 to 18 hours | Caustic chemical and high-pH wastewater to deal with |
| Cold fermentation | Soak straw in plain water until it ferments | 5 to 10 days | Smelly, and Cornell describes contamination rates as high |
Cornell’s guide doesn’t recommend cold fermentation for commercial production because of the contamination risk. This tutorial focuses on hot water and lime.
Before you start: prepare the straw
Whatever method you choose, start with clean, dry straw that hasn’t gone mouldy in storage. Then chop it.
- Cut it short. A University of Florida IFAS Extension guide recommends cutting straw into 4 to 5 inch (10 to 13 cm) pieces. Cornell says yields “can substantially increase” when straw is shredded before treatment.
- Pick a shredding method. Cornell lists electric or gas shredders, a weed trimmer in a drum, or a lawn mower run over straw on a tarp. You can also buy pre-shredded bales.
- Use a bag or basket. Packing straw into a mesh bag, pillowcase or wire basket makes it much easier to lift out and drain.
Method 1: Hot water pasteurisation
Hot water is the method used in the University of Florida and NAMA home-growing guides. Recipes differ a little, but they share the same idea: hold the straw at a high temperature, below boiling, for long enough to kill many competing organisms.
| Source | Temperature | Time |
|---|---|---|
| Cornell Small Farms Program | 140 to 160°F (60 to 71°C) | At least two hours |
| University of Florida IFAS | 165°F (about 74°C) | 1 to 2 hours |
| North American Mycological Association (NAMA) | 160 to 170°F (71 to 77°C) | 1 hour |
Step by step
- Pre-soak (optional). The University of Florida guide soaks chopped straw overnight in water before pasteurising.
- Heat the water. For small batches, NAMA describes a large pot on a stove with straw in a pillowcase or mesh bag. For a whole bale, growers use a 55-gallon metal drum over a propane burner or similar heat source.
- Submerge and hold. Push the bag under the water and keep it in your chosen temperature range for the full time. Cornell suggests a fuel-saving variation: heat the water and straw to 180°F (82°C), turn off the heat, cover with a lid, and let it cool slowly through the target range.
- Check the temperature. Use a thermometer rather than guessing. Cornell recommends an infrared thermometer for quick checks. Stir or check in more than one spot, because a big drum doesn’t heat evenly.
- Drain. Lift the straw out and let it drain. NAMA’s test is to squeeze a handful: only a few drops of water should come out.
- Cool. Spread the straw on a clean surface, handling it with a clean, dedicated tool. Cornell says it is ready to inoculate once it’s at or below 100°F (38°C).
- Inoculate promptly. The University of Florida guide advises inoculating as soon as the straw has drained and cooled, to reduce the risk of unwanted fungi moving in. It suggests a spawn rate of 5 to 10%.
Cornell notes that some growers report clean straw after only about an hour at temperature, but it frames this as something to experiment with rather than a rule. If you’re starting out, stick to the full time.
Safety: you’re working with a large volume of very hot water. Use a stable stand for the drum, keep children and pets away, and wear heat-resistant gloves when lifting wet straw.
Method 2: Hydrated lime (high pH)
Lime treatment uses chemistry instead of heat. Cornell explains that raising the straw’s pH to 12 or 13 kills most competitors, and the straw can then be inoculated.
Choosing the right lime
- Use hydrated lime (calcium hydroxide). Cornell says to use hydrated lime with a magnesium content below 10%, and calls this “critical to success”. Check the guaranteed analysis on the bag.
- Don’t swap in any garden lime. Products sold as lime vary, so check that the label says hydrated lime or calcium hydroxide and shows the magnesium content.
Step by step
- Fill your container with water. A food-grade bucket works for small batches, and a 55-gallon barrel for larger ones.
- Add the lime. Cornell suggests starting at 0.35 to 0.5 lb of lime per gallon of water (roughly 40 to 60 g per litre), then testing with pH strips because products vary. The aim is a pH of 12 to 13.
- Submerge the straw. Weigh down the bag so all the straw stays under the water.
- Soak. Cornell recommends 16 to 18 hours, and adds “no longer!”
- Drain. In the on-farm trial described below, straw was drained for 20 minutes before bagging.
- Inoculate. There’s no cooling step, which is one of lime’s big advantages.
Handling lime safely
Hydrated lime is caustic. The US National Institute for Occupational Safety and Health (NIOSH) lists it as a hazard to the eyes, skin and respiratory system, with possible eye and skin burns. Its guidance is to prevent skin and eye contact, and to irrigate the eyes immediately if lime gets in them.
- Wear eye protection and waterproof gloves when mixing and handling lime and wet straw.
- Add the powder to water gently to avoid raising dust, and do it outdoors or in a well-ventilated space.
- Keep the soaking container covered and away from children and pets.
- Deal with the wastewater. Cornell describes the used water as high-pH and toxic, and advises neutralising it with an acidic product such as hydroponic “pH down” before disposal. Check your local rules for disposing of it.
What happened when the methods were compared
An on-farm trial in Massachusetts
Fungi Ally, a mushroom farm in Massachusetts, compared four straw treatments in a SARE-funded farmer trial, with technical advice from the University of Massachusetts Amherst. Each treatment was tested on six oyster strains, using 10 lb bags.
| Treatment | How it was done | Average biological efficiency |
|---|---|---|
| Lime | 1 gallon of hydrated lime in a 55-gallon barrel, about pH 12 to 13, 16-hour soak | 41% |
| Hot water | Water heated to 180°F (82°C), straw submerged for 2 hours | 29% |
| Wood ash | 4 gallons of wood ash, about pH 11, 16-hour soak | 20% |
| Cold fermentation | Soaked in water for 5 to 7 days | 18% |
Biological efficiency is the weight of fresh mushrooms divided by the dry weight of the substrate. The farmers also reported that:
- Fermentation and wood ash produced many more fully contaminated bags than lime and hot water.
- The lime cost about $2, compared with about $10 of propane for hot water.
- Lime took about 10 minutes to prepare. Hot water took about 30 minutes to set up plus roughly six hours of on-and-off monitoring, including about four hours to heat the barrel.
- The fruiting room was not temperature-controlled and often reached 75 to 95°F (24 to 35°C) in summer, which the farmers say hurt yields.
The farmers concluded that lime was the most practical method for commercial growers, while any of the four methods could work for home growers.
A university trial in Germany
A 2024 study by Grimm, Sonntag and Rahmann, published in the Journal of Microbiology, Biotechnology and Food Sciences, found a different result. It used maize straw and mixed crop straws rather than wheat straw, and compared several treatments for oyster mushrooms.
| Treatment | Conditions | Biological efficiency (first harvest) | Days to first harvest |
|---|---|---|---|
| Hot water | 63 to 70°C for 1.5 hours | 75.7% | 26.8 |
| Autoclave | Full sterilisation | 66.8% | 23.3 |
| Lime | About 5 g per litre, pH 9.5, 8 hours | 52% | 28.2 |
In this trial, hot water did better than lime. The researchers also found that pest problems were low across all the pasteurised treatments.
Why the results differ
The two trials differ in straw type, strains, conditions and lime dose. The German lime treatment used a much lower pH (9.5) than the pH 12 to 13 that Cornell and the Massachusetts trial aimed for. We can’t say from these studies alone which factor explains the difference, so treat both methods as workable, and test them yourself.
Which method should you choose?
- Choose hot water if you’re treating a small batch in a kitchen pot, don’t want to handle chemicals, or already have a burner and drum.
- Choose lime if you’re treating larger volumes, want to save fuel and time, and can handle and dispose of a caustic solution safely.
- Avoid cold fermentation as a first method, given the contamination reports. It’s a fallback when you have no heat and no lime.
Whichever you pick, keep notes on the batch, method, temperature or pH, and outcome, so you can compare results over time.
Common problems and fixes
- Green mould soon after spawning: review your treatment time and temperature or pH, and your handling after treatment. Our pressure cooker sterilisation guide covers sterile handling habits that also help with straw.
- Soggy, slimy straw: drain longer and use NAMA’s squeeze test before bagging.
- Slow colonisation: check the spawn rate and spawn quality. If you make your own, see how to make grain spawn.
- No growth after hot water treatment: if the straw was still hot when you added spawn, that may be the cause. Wait until it’s at or below 100°F (38°C).
For more on choosing between straw and other materials, read our guide to mushroom substrates. To see how straw fits into a full grow, see our complete guide to oyster mushrooms.
Sources and further reading
- Cornell Small Farms Program: Methods of Commercial Mushroom Cultivation in the Northeastern United States, part 4 (Four Methods of Commercial Cultivation)
- Crosby, W. and Kessler, D. (Fungi Ally). Alternative growing practices for oyster mushroom cultivation in the Northeast. SARE project FNE15-825, final report
- Grimm, D., Sonntag, E. and Rahmann, G. (2024). Evaluation of different pasteurization and sterilization methods for oyster mushroom substrates. Journal of Microbiology, Biotechnology and Food Sciences 13(5): e10428
- Hameed, K., Cotter, V. T., Liao, H.-L. and Hsu, C.-M. D.I.Y. FunGuide: Grow Your Own Oyster Mushrooms at Home (SS662). University of Florida IFAS Extension
- North American Mycological Association: Preparation of Mushroom Growing Substrates
- NIOSH Pocket Guide to Chemical Hazards: Calcium hydroxide
Featured image: ink caps on straw by Brittany Darwell via iNaturalist (CC0 1.0).
