Mushroom growing has come a long way since the early days. We used to use horse manure to produce them. Nowadays, sterilized compost is a common medium for commercial mushrooms, such as portobellos, buttons, and creminis. 

In fact, these three types of mushrooms all come from the same species, Agaricus bisporus. Cremini are the small, brown versions of portobello mushrooms that growers harvest before their caps open up. White button mushrooms grow from a special strain of A. bisporus.

Because portobello mushrooms are more mature than buttons and creminis, they require special attention and care to thrive. The process isn’t simple. It requires sterility, careful attention to detail, and consistent care. 

If you’re still up for the challenge, keep reading! We’ll dive into the hidden world of mycelium and spores. By the end of this article, you’ll be able to start growing portobello mushrooms at home.

What Is It?

A portobello mushroom is the fruiting body, or reproductive structure, of the A. bisporus species of fungus. It’s large, tender, and meaty, which is why it’s a favorite mushroom for burgers and plant-based alternative meals.  

Native Area

Several strains are found across North America.

Growers cultivate portobello mushrooms all over the world. They grow well indoors because they need a controlled environment, like a grow tent, to thrive. Though they’re crops nowadays, they still grow wild across North America, Europe, and Asia.

In North America, many strains of portobello grow outdoors. Some are North American strains, while others originated from Europe and escaped cultivation on the continent. They thrive in grasslands and meadows with plenty of rotting organic matter in the soil. 

Characteristics

Mushrooms don’t grow in the same way as other plants do.

Portobellos are brown with white stems. The upper part of the cap is light brown, and the underside reveals dark brown gills where the spores grow. When young, portobellos are cremini mushrooms, or button mushrooms if they’re the white strain.

Mushrooms do not grow like plants. They’re technically not plants at all, as they’re decomposers of the natural world. They use their root-like mycelial threads to break down organic matter such as manure, compost, and wood. 

Because mushrooms grow in the same manner as molds and mildews, they require a sterile environment for indoor cultivation. Careful planning, care, and maintenance ensure that they thrive without competition from pesky pests and diseases. 

Planting

You’ll need a substrate, or growing medium, to plant mushrooms. You’ll also need spawn, a substance that has the fungus growing inside of it. Mix the spawn with the substrate to get the mushrooms going. 

Make Compost Substrate

Mushrooms need rotting substances to thrive.

Substrate is the stuff your mushrooms grow in. Different mushrooms prefer different substrates. The easiest types to grow are those that thrive in wood chips, straw, and sawdust.

Portobello mushrooms are a bit different. They grow best in partially composted organic matter, as they need rotting substances to thrive. Commercial growers make compost in bulk, sterilize it, and use it to fill their mushroom beds for cultivation. 

At home, make manure compost yourself with grass stalks, straw, manure, and gypsum. When the compost decomposes halfway, halt the process and sterilize the medium. The oven is an easy place to sterilize, though it’ll make your house smelly! 

Simply lay the substrate on trays and bake it on low for three or more hours. Then, let the compost continue breaking down to reduce ammonia levels. 

The process is quite lengthy, and it may be easier to source mushroom manure compost from a reputable seller. Commercial producers often sterilize the compost before selling it, which saves you a step. 

Source Spawn

Find mushrooms from reputable sources online.

Making spawn is similar to planting seeds. Seeds grow new plants, while spawn produces new mushrooms. Mushrooms typically reproduce with spores that sprout new strains. Because new strains grow unpredictably, it’s best to use spawn culture from clones instead of spores. 

You can find mycelial spawn culture online from reputable sources. It typically consists of cooked, sterilized, and cooled grains that sellers inoculate with portobello cultures. Growers use cultures of the mycelia on agar-agar gel to inoculate the spawn, though you may use chunks of portobello stems to do the trick. 

After inoculation, the spawn will begin to show white, thread-like growths that are the mushrooms’ roots, called mycelia. Once the mycelia spread throughout the grain spawn, you may use it to inoculate the compost manure substrate. 

Incoluate Substrate

Make sure your environment is sterile.

Mycelia continue to grow so long as they have organic matter to sprout into. By mixing the spawn with the substrate, you give the fungus more food to consume. It’ll spread rapidly throughout the growing medium.

Sterility is a must throughout all steps of the process. Use clean, sterile containers to complete the planting process. You may use plastic totes, black trash bags, or any other closed vessels. Crumble the spawn into small pieces, then mix the pieces with the substrate in the containers.

Let the mix sit in containers for two or more weeks to allow the mycelia networks to grow. Keep them in a cool, dark location indoors, and mist the medium frequently to keep it moist, but not soggy. 

You’ll notice white filaments on the upper surface of the substrate in due time. Once you see these filaments, you may move on to the casing, growing, and harvesting stages. 

How to Grow

The inoculation stages are the most difficult parts of the process!. They require careful precision, sterility, and consistent attention. After the planting stages, growing portobello mushrooms for harvesting is a straightforward process. 

Casing

Start casing after the white mycelium filaments are visible.

Casing is a late-stage step that’s necessary for portobello harvests. Start casing after the white mycelium filaments are visible on the surface of the substrate. 

Casing isn’t as crucial for other mushrooms, like wine caps and shitakes. It involves placing a thin layer of organic matter on top of the substrate. The covering action encourages the fruiting caps to form.

Growers are unsure why casing encourages the caps to grow, though they know it helps. The layer insulates the mycelia, allowing them to stay moist underneath. The casing also provides support for heavy portobello caps as they swell and enlarge. 

For casing, use responsibly sourced sphagnum peat moss. This is what commercial growers use. As peat moss is often unsustainably sourced, consider using alternatives like pasteurized soil, finished compost, or coco coir. 

Light

Grow mushrooms away from direct sunlight.

Portobellos need little light to thrive, and too much sunlight dries them too quickly. Grow them in a sheltered, dark location indoors away from direct sunlight. 

You may grow portobellos outdoors, though they need sterility and protection from the sun to produce well. Cover their containers with lids, and place them in full or dappled shade away from direct sunlight. 

Water

Portobello mushrooms need consistent moisture to thrive.

Water is crucial for proper mushroom development! Fungi thrive with regular moisture, and portobellos are no exception. Water is necessary throughout all stages of the cultivation process.

Manure compost must be 50% moist, like a wrung-out sponge, to break down properly. The sterilized grain spawn needs to be moist for the mycelium to spread. After mixing the spawn and substrate, water is necessary to keep the mix wet. 

The casing layer also requires consistent moisture. Have a hose nearby, and add a nozzle to it to soften the water’s stream. Don’t let the medium dry out. Instead, water consistently to maintain a moist substrate.

Fertilizing

Mushrooms generally don’t need fertilizer like regular plants do.

Mushrooms do not need fertilizer, though they benefit from having nitrogen in their substrate. When making the compost for the substrate, add manure from chickens or cows to boost the nitrogen levels.

Allow the manure to decompose before using the substrate, as excessive ammonia levels prevent mushrooms from forming. You’ll be able to smell the ammonia. If it smells like manure, the mix needs more time to decompose. 

Do not fertilize portobello mushrooms like you would plants. It will cause unnecessary growths from molds, pests, and bacteria. Instead, ensure the substrate is full of nutrients by making compost with manure. If you’re vegan, use alfalfa or soybean meal instead of manure. 

Maintenance

Maintain the right environmental conditions to promote growth.

Fungi prefer specific environmental conditions while they grow, and each step of the process relies on different parameters. We haven’t even touched on temperature and airflow.

Generally, portobello mushrooms need different temperatures depending on the step of the process. Keep the room between 75-77°F (24-25°C) while allowing the spawn to colonize the substrate. Then, during the casing and fruiting process, lower the temperature to between 60-66°F (16-19°C). 

Airflow isn’t necessary during the spawning process, but it’s crucial when the caps form. Excessive CO₂ builds up while the heads swell, and a fan provides them with fresh air. Aim the flow so it blows nearby, but not directly on, the mushrooms. 

Harvesting and Storage

Your portobello mushrooms will be ready to harvest within a few weeks.

After the casing process, it typically takes between two and three weeks for the heads to reach a harvestable size. Watch your crop closely, and pick the caps when they reach between four and six inches wide. 

Continuous harvesting ensures that more caps form. Pick the portobellos with your hands, and dust off any dirt or debris. After harvesting, maintain consistent moisture and airflow to encourage more flushes to form. 

Store fresh-picked mushrooms in a brown paper bag in the fridge for a week, or in a container in the freezer for three months. Alternatively, you may dry the caps and store them for a year. Dried mushrooms go great in soups, stews, and casseroles. 

Propagation

A single bed can produce two to three harvests, or flushes, of portobello caps. After the third flush, the quality declines and fewer caps form. Use propagation if you’d like another harvest. Indoors, you may grow and propagate a fungal supply year-round.

Spores

This propagation method can be challenging.

Spores are one challenging way to propagate portobellos. They’re microscopic and unpredictable. Some spores may sprout new mushrooms that look and behave differently from your desired cultivar.

To propagate with spores, you’ll need to collect the tiny particles into a container. Set the portobello cap on top of the jar, and let its spores dump into the container. Then, seal the container until you’d like to make spawn. 

Mushroom Chunks

Propagate in a closed and sterile environment.

This is the best method for easy propagation. You’ll need some supplies. Source a chunk of portobello stem, agar-agar gel, and petri dishes with lids. Prepare the agar gel, then let it set inside the petri dishes. 

Taking the portobello stem, place a piece of it onto each petri dish. The fungus will colonize the agar-agar gel, after which you’ll see white growth forming on the surface of the gel. 

Remove the old portobello stem and close the containers until you’re ready to make spawn. Store the closed petri dishes in the fridge at 41°F (5°C).

It’s best to do this process in a closed, sterile environment where contaminants aren’t abundant. Anything can waft onto the gel and grow in an uncontrolled environment. Sterilize all your tools, put on gloves, and wear clean clothes anytime you handle the dishes.

Common Problems

Portobello mushrooms often encounter issues with fly larvae, competing fungi, and bacteria. This is why sterility is important. It prevents unruly invaders from messing with your crop. If pests do appear, take stock of the growing area to see how they’re invading the space. 

Pests

Outdoors, watch out for slugs and snails.

Flies lay eggs near mushroom stalks. The eggs hatch into wormy larvae that tunnel into the stems and move upwards as the mushrooms grow. They’re common in outdoor grows, as it’s difficult to prevent them from entering the growing site. 

Indoors, flies shouldn’t pose a problem. Use a closet, pantry, or dark room for growing where flies cannot enter. 

Outdoors, also watch for slugs and snails. They love munching on tender plants and fungi, and they’ll decimate the crop before you can harvest it. Use traps or copper bands to control their populations. 

Diseases

Maintaining a clean environment prevents common diseases.

Diseases are often present when growers reuse the spent mushroom substrate, or mushroom compost. Commercial growers tend to pasteurize the finished product and sell it to gardening supply companies. 

Instead of reusing old substrate for mushroom growing, consider using it in your garden. Mushroom compost is superb as an organic soil amendment, and it helps boost crops and wildflowers. 

Avoid diseases by using fresh manure compost as your substrate. Also, keep a clean and sterile grow room. 

Frequently Asked Questions