**Appendix B**

Details about sieving non-spherical objects and the sieving procedure conducted are described here. Due to the high aspect ratio between the length and width and/or thickness of the chipped material, the use of sieves is not as straightforward as with more spherical objects. The amount of material passing through the sieve depends on the amount of material placed on the sieve, the movement produced for sieving, and the sieving time. If too much material is placed on the sieve, particles are more likely to be arranged in a position perpendicular to the sieve holes (vertically). The horizontal sieving technique was chosen to limit the vertical repositioning of the particles after being positioned on a sieve. The speed of the circular sieve movement can also cause the particles to reposition vertically. Therefore, these three variables were kept constant throughout the sieving of the chipped straw.

The sieving process for the 5.7 mm sieve consisted of placing 140 g of chipped material on the 5.7 mm sieve and making 10 horizontal circles with the sieve for 8 s in a clockwise direction, then in an anti-clockwise direction. The sieving process for the 1.5 mm sieve consisted of placing 70 g of chipped material on the 1.5 mm sieve and making 5 horizontal circles with the sieve for 4 s in a clockwise direction, then in an anti-clockwise direction.

## **Appendix C**

The details of the procedure performed to quantify the particle sizes of both mixture are presented here. First, particles were positioned on a light table while making sure that they did not touch each other. If particles were touching each other, the algorithm would recognize them as one, and resulting dimensions would be false. A picture of these particles was taken from a top view with a Sony alpha 7R II camera (Sony, New York City, NY, USA) mounted on a tripod. The following settings were used: f/4, 1/100 s, ISO-100 at a focal length of 35 mm. The 35 mm focal length was used to reduce lens distortion. The picture was then opened in Adobe Lightroom (version 6.10.1, AdobeTM, San Jose, CA, USA) to crop, increase contrast, and correct the lens distortion. The adjusted pictures were then imported into a Python script (version 3.7, Python Software Foundation, Wilmington, DE, USA). The script turned the image into a binary image (e.g., only composed of black or white pixels), labeled each individual item, and calculated the number of objects and the area, perimeter, eccentricity, major axis length, and minor axis length of each item. A ruler integrated in the light table was used to scale the image by converting the number of pixels from the picture to mm.

### **Appendix D**

The pasteurization process can be divided into the following steps: first, the substrate mixture (S or L) is placed in a large steamer with a rotating arm in its center. Then, the required amount of water is added to the mixture based on the necessary percentages from the inoculation recipe (detailed in Section 2.3). The steamer is then closed and both mixing and steaming are started. The steamer is kept on for one hour at a temperature of 64–65 ◦C to allow proper pasteurization. A thermal blanket is placed around the mixer to retain the heat. The mixture is then allowed to cool for one hour until it reaches a temperature of 27 ◦C. To speed up the cooling, the mixer is kept on and the fan of the steamer blows filtered outdoor air through the mixture. To enhance the pasteurization process, water is introduced before pasteurizing the substrate. However, the pasteurization process also brings moisture in. To quantify the quantity of water added to the mixture during the pasteurization process, the team ran the steamer without any substrate. The steamer produced approximatively 5.90 L of water, which represented 13.40% of the water in our inoculation recipe.

### **Appendix E**

The inoculation procedure of both batches and summary of the quantity of materials grown are described here. Mixture L of the first batch was inoculated as follows: first, 9303.3 g of sterilized dry substrate was placed inside a sterilized drum mixer. With the drum mixer on (i.e., rotating), 28 L of distilled water was slowly poured inside the mixer. The mixer was left running to mix the substrate and water for 10 min. Then, 4134.8 g of *Pleurotus ostreatus* grain spawn was added. The whole mixture was mixed for an additional 30 min. The molds were sterilized with 70% isopropyl alcohol. After 30 min of mixing, the mixture was placed and compacted into the sterilized molds. The mixer was kept running while the team filled the molds to prevent the mixture from sitting still for too long. An acrylic disk was used for pushing by hand as hard as possible to compact the mixture multiple times during the filling process. For the 17 cm tall PVC tubes, the mixture was compacted twice: when the mixture filled half of the tube height and when it filled the entire height. For the 34 cm tall PVC tubes, the mixture was compacted four times (i.e., at each quarter of the height). Once the molds were filled, specimens for compression testing were covered with a layer of non-woven housewrap (Everbilt, Home Depot, USA) to maintain constant humidity, reduce the exposure to contaminants, and limit mycelium growth on the outer surfaces. In mycelium materials, the surfaces exposed to ambient air display increased mycelial growth. This phenomenon impacts mechanical behavior as shown in a study about jacketing the materials [44]. The bias emerging from increased mycelium growth on external surface was reduced in our study by compacting mixtures in the molds and growing all specimens in PVC tubes with top and bottom surfaces covered with housewrap. Specimens for bending testing were placed inside a ridge-like tent. The ridge-like tent was made of non-woven housewrap and clear plastic, to ensure mycelium growth was visible throughout the growth period. All covered molds were finally placed inside a sterilized large tent in order to lower the risks of contamination. An opaque tarp covered the large tents containing the specimens during the growth process to block sunlight, which is a stimulus for the formation of fruiting bodies. Filtered air flowed into this tent throughout the mycelial growth period. The same process was performed for mixture S with the following quantities: 11,841.3 g of sterilized dry substrate, 35.5 L of distilled water, and 5262.8 g of *Pleurotus ostreatus* grain spawn. The leftover inoculated mixtures, S and L, filled four additional rectangular molds each. The filled molds were then sealed with a layer of non-woven housewrap.

For the second batch, water was added before the pasteurization process and the mycelium spawn after it. Once the mixture had cooled to 27 ◦C, the mycelium spawn was added. After 5 min of mixing, the inoculated mixture was placed inside opaque trash bags to be transported to the growth facility (redhouse studio's warehouse in Cleveland, OH, USA). The temperature of the inoculated mixture should remain constant to avoid early growth and contaminants. Therefore, bags were only half filled with an average of 13.4 kg of inoculated mixture. These bags were then spread on a grid to reduce the heat produced by this biomass. Due to the time needed to fill all the molds, mixture S was placed in the molds on the day following inoculation, whereas mixture L was placed two days after inoculation. After being sterilized with 70% ethanol, the molds were filled with the inoculated mixture and pressed to achieve the desired heights akin to the previous batch. Finally, the filled molds were weighed and placed in their respective growth environments. The growth environments used for the first batch were reused for the second batch. In the first batch, the closed PVC tubes were set on a planar surface to maintain a flat surface on the specimens' bottom sides. However, this configuration did not let the specimens fully breathe during the growth period resulting in high humidity of the lower parts of the specimens and the presence of higher rates of contamination. Therefore, for the second

batch, the filled PVC molds were placed on a metal grid to let them breathe. The leftover inoculated mixtures, S and L, filled eight additional rectangular molds each. Similar to the first batch, a layer of non-woven housewrap was used to cover these filled molds.

A total of 120 specimens were grown to evaluate the effects of the following variables on the compressive properties of mycelium-based composites: fungal species (*Pleurotus ostreatus* and *Coprinus comatus*), substrate particle sizes (with or without micro-particles), and post-growth treatments (dried, baked, compacted then dried, and compacted then baked). A total of 40 specimens were grown for the first batch with *Pleurotus ostreatus* to achieve a sample size of five. Due to increased availability of materials, space, time, and levels of contaminations observed in the first batch, the sample size was increased to 10 (80 specimens total) for the second batch grown with *Coprinus comatus*. For bending testing, the effects of the same variables were evaluated with a total of 148 specimens. The sample sizes used were 6 for the first batch (48 specimens) and 12 or 15 for the second batch (100 specimens), depending on the number of molds and material available.
