Mycelium-hemp brick walls for 4x8 raised beds, grown from local straw and hurds for frost-resistant herb enclosures

Begin by sourcing your primary materials locally. Contact nearby farms for straw bales, ideally wheat or oat straw, and locate regional hemp processors for bags of hemp hurds. For the mycelium, select a vigorous, cold-hardy strain like Pearl Oyster (Pleurotus ostreatus) or King Stropharia (Stropharia rugosoannulata) grain spawn, as these aggressively colonize cellulose-rich substrates.

Prepare the substrate by first chopping the straw into 1- to 4-inch pieces to increase surface area. Combine the chopped straw and hemp hurds in a 60/40 ratio by dry weight. Submerge this mixture in a large, clean container, such as a 55-gallon drum or an insulated cooler, and pasteurize it by pouring in boiling water until the substrate is fully saturated. Close the lid and let it steep for 12-24 hours, allowing it to cool completely to room temperature. Drain the substrate thoroughly; it should be moist but not dripping when you squeeze a handful.

Next, construct simple rectangular molds for your bricks from untreated plywood. A practical size is 16 inches long by 8 inches wide by 8 inches deep. This size facilitates easy handling and stacking. For a single-course 8-inch high wall around a 4x8 foot bed, you will need approximately 20-22 bricks.

In a clean tote or on a sanitized tarp, break up your grain spawn and mix it thoroughly into the cooled, drained substrate. Aim for an inoculation rate of 5-10% spawn to substrate by weight. Pack this inoculated mixture firmly into your wooden molds, ensuring there are no large air pockets. Cover the packed molds with plastic sheeting or place them inside large bags to maintain high humidity.

Incubate the bricks in a dark, undisturbed location with a stable temperature between 65-75°F, such as a basement or garage. Over the next two to four weeks, you will see white, web-like mycelium growing through and binding the straw and hurds. The bricks are fully colonized when they are solid white and feel firm to the touch.

Once fully colonized, remove the bricks from the molds and move them to a well-ventilated area with low humidity to cure. This drying phase is critical for stopping the mycelium's growth and creating a durable, inert, and water-resistant building material. Arrange the bricks on wire racks to allow for airflow on all sides. The curing process can take one to three weeks, depending on air circulation and humidity. The final bricks will be lightweight yet surprisingly strong.

To assemble the 4x8 raised bed walls, prepare a level foundation of gravel or sand. Stack the cured mycelium-hemp bricks directly on this base. For added stability between courses, you can mix a small amount of leftover mycelium spawn with pasteurized substrate to create a living "mortar," or use a simple lime-based mortar.

For maximum frost resistance and longevity, apply a protective coating to the exterior surfaces of the finished walls.