Portobello mushrooms are genetically identical to the smaller white button and cremini mushrooms. All three are different stages of maturity of the species Agaricus bisporus. The Portobello name is reserved for the fully mature stage when the cap has expanded to a large size, exposing the dark gills. Cultivating this fungus outdoors involves recreating its cool, moist, and nutrient-rich natural habitat. This guide focuses on establishing and maintaining an outdoor growing bed to yield a reliable seasonal crop.
Essential Materials and Site Selection
Successful outdoor cultivation requires securing the correct materials and choosing an optimal location. The foundation of the growing bed is the substrate, a specialized, well-composted mixture of manure and straw. This nitrogen-rich compost provides the complex organic matter the mushroom mycelium needs for robust growth. The inoculation material, known as grain spawn, consists of a carrier grain colonized by the Portobello mycelium, which “seeds” the compost.
The final component is the casing layer, a non-nutritive top layer applied later to initiate fruiting. This layer is typically a mixture of peat moss and loam or garden soil, often buffered with lime to adjust the pH. The site must provide full, consistent shade, as direct sunlight quickly dries the bed and raises the temperature. It should also be sheltered from strong winds to maintain the high humidity necessary for the mushroom life cycle.
Step-by-Step Bed Preparation and Inoculation
Bed preparation involves using an outdoor bed or container at least eight inches deep. The container is filled with five to six inches of the prepared mushroom compost substrate. The compost must be moist, maintaining a consistency similar to a wrung-out sponge, but never soggy. Once the substrate is in place, the Portobello grain spawn is evenly distributed across the surface.
The spawn is then gently mixed into the top four to six inches of the compost, a process called inoculation, which allows the mycelial network to spread efficiently. This initial colonization phase requires consistent warmth, ideally between 70–75°F (21–24°C), and takes approximately two to three weeks. During this time, the bed should be covered with a damp layer of burlap or straw mulch to retain moisture and allow air exchange. Colonization is complete once a fine, white network of mycelium is visibly running throughout the compost.
Once the mycelium has fully colonized the substrate, the casing layer is applied to signal the start of fruiting. A one-inch layer of the moistened peat moss and loam mixture is spread uniformly over the colonized compost. This layer acts as a moisture reservoir and an environmental trigger that stimulates the mycelium to form mushrooms. The bed is then covered lightly with newspaper or a plastic sheet to maintain high humidity as the mycelium grows into the casing layer.
Managing Growth and Environmental Conditions
After the casing layer is applied, environmental conditions must be adjusted to trigger mushroom cap formation. The temperature must be lowered to a range of 55–65°F (13–18°C), as this temperature drop signals the mycelium to switch from vegetative growth to reproductive growth. This cooling period is followed by the appearance of tiny white growths known as pinheads, the first visual signs of developing mushrooms. Maintaining high relative humidity, around 85%, is achieved by lightly misting the surface of the casing layer multiple times a day.
Watering should be done gently with a fine spray to keep the casing damp without disturbing the pinheads or waterlogging the compost. Fresh air exchange is important once pinning begins, as developing mushrooms produce carbon dioxide. Insufficient airflow leads to elevated CO2 levels, causing mushrooms to grow with long, thin stems and small caps, a condition known as “legginess.” For outdoor beds, allowing natural circulation is often sufficient, though a fine mesh covering helps deter pests and animals.
Harvesting Your Outdoor Crop
Harvesting requires careful timing. The mushrooms are ready for picking when the cap has fully expanded and flattened out, typically reaching four to six inches in diameter. At this point, the veil—the membrane connecting the cap edge to the stem—will have fully detached, exposing the dark, mature gills. Harvesting mature mushrooms prevents them from dropping spores, which can inhibit the development of subsequent crops.
To harvest, gently grip the mushroom at the base of the stem and twist it slightly to separate it cleanly from the casing layer. This twisting action minimizes disturbance to the underlying mycelial network, which is essential for encouraging subsequent flushes of mushrooms. Alternatively, a sharp, clean knife can be used to cut the stem just above the casing layer. Freshly harvested Portobellos should be gently wiped with a damp cloth to remove any casing material, as washing them directly causes them to absorb water and become slimy. The mushrooms can be stored in a paper bag in the refrigerator for up to seven days.