How to Make Mushroom Fruiting Blocks

Making DIY mushroom blocks involves mixing sterilized substrate with spawn to create a 6-pound block that's shaped in a box for uniform growth. Once colonized, you cut an X-shaped opening for mushrooms to fruit directly from the block, eliminating the messy pinning issues common with loose grow bags.

The core problems this method solves: Traditional grow bags create frustrating side pinning where mushrooms grow randomly across any substrate surface exposed to light, making harvesting difficult and messy. Additionally, fully inflated grow bags take up enormous amounts of storage space in grow tents, limiting how many you can manage at once.

Loose Bags	Fruiting Blocks
❌ Mushrooms pin everywhere on exposed substrate surfaces	✅ Controlled fruiting from single X-cut opening
❌ Difficult, messy harvesting	✅ Clean, easy harvesting from one location
❌ Takes up massive space when fully inflated	✅ Compact, stackable storage in grow tent
❌ Uneven moisture distribution	✅ Uniform moisture across fruiting surface
❌ Unpredictable second/third flushes	✅ Reliable multiple flushes with soaking method
❌ Professional appearance impossible	✅ Clean, commercial-quality presentation

 

Whether you're tired of dealing with mushrooms pinning all over your grow bags or want to maximize space in your grow tent, learning to make professional mushroom growing blocks will revolutionize your mushroom cultivation. This comprehensive guide will walk you through every step of creating professional-quality fruiting blocks that produce consistent, high-yield flushes.

Why Make Fruiting Blocks Instead of Using Loose Bags?

Traditional mushroom growing often involves inoculating substrate in bags that remain loose and shapeless. While this method works, it creates several problems that fruiting blocks solve:

Problems with loose bags:

• Mushrooms pin randomly across the entire surface area
• Difficult to manage multiple bags in limited space
• Uneven moisture distribution leads to inconsistent yields
• Harvesting becomes messy and time-consuming
• Second and third flushes are unpredictable

Benefits of fruiting blocks:

• Controlled fruiting from a single surface
• Better space utilization in grow tents
• More predictable flushes and higher yields
• Easier harvesting and maintenance
• Professional appearance and consistent results

Essential Materials and Equipment

Before starting, gather all necessary materials. Having everything prepared beforehand ensures a smooth, contamination-free process.

Primary Materials:

• Sterilized substrate (we recommend 50% fine hardwood sawdust + 50% soybean hulls)
• Quality rye berry spawn (or liquid culture)
• 8x8x6 inch cardboard boxes
• Packing tape or duct tape
• Sharp, sterile knife or blade
• Polypropylene mushroom grow bags with filter patches

Additional Supplies:

• Digital scale for precise measurements
• Clean work surface
• Isopropyl alcohol (70% concentration)
• Disposable gloves
• Face mask
• Large mixing container
• Spray bottle for misting

Optional but Recommended:

• Still air box or laminar flow hood
• Timer for tracking colonization periods
• Labels for dating and identifying blocks

Substrate Preparation and Weight Ratios

The foundation of successful fruiting blocks lies in getting the substrate-to-spawn ratio exactly right. This recipe has been tested extensively and produces consistent results across different mushroom varieties.

Target Weight Formula:

• Total finished weight: 6 pounds (2.7 kg)
• Sterilized substrate: 5.5 pounds (2.5 kg)
• Spawn addition: 0.5 pounds (0.2 kg)

This 11:1 ratio provides sufficient spawn for rapid colonization while maintaining the bulk necessary for substantial mushroom production. Too much spawn wastes valuable inoculum, while too little results in slow colonization and increased contamination risk.

Important Note: This recipe has been specifically tested and optimized using rye berry spawn and a wood-based substrate mixture of 50% fine hardwood sawdust and 50% soybean hulls. If you're using different substrates with higher volume (such as straw pellets, wood chips, or other bulky materials), the weight ratios and 8x8x6 box dimensions may not work properly due to volume differences. For best results, we recommend sticking to the tested substrate combination or adjusting box sizes accordingly for bulkier materials.

Mushroom Variety	Recommended Substrate	Supplement (% by weight)
Oyster mushrooms	Hardwood sawdust, straw pellets	None or 5-10% wheat bran
Lion's Mane	Hardwood sawdust	10-15% wheat bran
Shiitake	Oak or beech sawdust	10-20% wheat bran
Wine Cap	Hardwood chips/sawdust	High lignin content preferred

 

Ensure your substrate is properly sterilized before use. If making your own, steam sterilize at 15 PSI for 90-120 minutes depending on bag size. Allow to cool completely before inoculation.

Step-by-Step Block Formation Process

Step 1: Prepare Your Workspace

Clean your work area thoroughly with isopropyl alcohol. Wear gloves and a mask throughout the process. If working without a laminar flow hood, work quickly and minimize air movement around your workspace.

Step 2: Prepare the Forming Box

Take your 8x8x6 inch cardboard box and securely tape the bottom seams with packing tape. Leave the top completely open. The cardboard box serves as a temporary mold to give your substrate the desired uniform block shape - you're not permanently housing the mushrooms in cardboard. Think of it like using a cake pan to shape a cake; the box creates consistent dimensions and proper density that would be difficult to achieve by hand-shaping. Ensure all seams are sealed to prevent the substrate from leaking through gaps.

Step 3: Mix Spawn and Substrate

In a clean mixing container, combine your 5.5 pounds of cooled, sterilized substrate with 0.5 pounds of spawn. Mix thoroughly but gently to distribute the spawn evenly throughout the substrate. Avoid overmixing, which can damage the spawn and reduce colonization rates.

Work systematically, folding the substrate over itself rather than stirring aggressively. The goal is even distribution without creating excessive dust or breaking apart the spawn chunks.

Step 4: Fill and Seal the Bag

Place your inoculated substrate mixture into the mushroom grow bag and seal it completely. Proper sealing prevents contamination while allowing gas exchange through the filter patch.

Step 5: Shape the Block in Box

Place the sealed bag inside the prepared cardboard box. Now comes the critical shaping process:

• Press the bag down firmly into the box
• Use your hands or a clean tamping tool to compress the substrate
• Work systematically across the surface to eliminate air pockets
• Tamp down on a sturdy table surface for best results
• Level the top surface by pressing down evenly across the bag

The substrate should conform closely to the box shape. Proper compression is essential for even colonization and prevents hollow spaces that can harbor contaminants.

Step 6: Position for Colonization

Once your substrate is properly shaped and compressed, position the top of the sealed bag so it extends upright outside the top of the box, creating a tent-like appearance. Ensure the filter patch remains unobstructed and exposed to air - don't let the bag fold over and cover the filter. The block should sit firmly in the box with the bag material standing upright above the rim, allowing proper gas exchange through the filter patch during colonization.

Colonization Phase: Patience and Monitoring

Place your formed blocks on a shelf in a clean, temperature-controlled environment. Ideal conditions for most mushroom varieties during colonization:

• Temperature: 70-75°F (21-24°C) for most species
• Humidity: 80-90% (but avoid direct moisture on the blocks)
• Air circulation: Minimal to prevent contamination
• Light: Not necessary during colonization phase

Week	What You'll See	Action Required
Week 1	Initial white mycelium patches appear	Monitor daily for contamination signs
Week 2	Mycelium spreads rapidly across substrate	Continue monitoring, avoid disturbing blocks
Week 3	Complete colonization - solid white throughout	Ready for fruiting preparation

 

Monitor daily for signs of contamination. Healthy mycelium appears bright white and fluffy. Green, black, or colored patches indicate mold contamination and the block should be discarded immediately. For safe disposal, seal contaminated blocks in plastic bags before placing in regular trash - never dump contaminated substrate outdoors as it can spread mold spores to your garden or local environment.

Colonization Troubleshooting:

• Slow colonization: Temperature may be too low, or spawn-to-substrate ratio insufficient
• Uneven colonization: Poor mixing or air pockets in the substrate
• Contamination: Usually indicates sterility issues during preparation

Preparing Blocks for Fruiting

Once colonization is complete (solid white mycelium throughout), it's time to prepare your blocks for mushroom production.

Step 7: Remove and Reshape

Carefully remove the colonized block from the cardboard box. The substrate should hold together as a solid unit. Gently press out any excess air from the bag, working from bottom to top.

Fold the bag material tightly around the block, leaving one surface exposed. This creates a neat, compact package with a clear fruiting surface.

Step 8: Position in Fruiting Setup

Place the wrapped block back into the cardboard box with the exposed surface facing upward. The folded bag material should be on the bottom and sides, with only the top surface of the substrate visible.

Step 9: Create Fruiting Opening

Using a sharp, sterile blade, cut an X-shaped opening in the exposed substrate surface. Make each cut about 2-3 inches long, creating flaps that can be gently pulled back. This opening becomes the primary fruiting site for your mushrooms.

The X-cut should penetrate about 1/4 to 1/2 inch into the substrate. Deeper cuts aren't necessary and may damage the mycelium network.

Managing Flushes for Maximum Yield

First Flush

After cutting your fruiting opening, place blocks in your fruiting chamber with proper humidity and air circulation. Most mushrooms will emerge within 7-14 days under optimal conditions.

Fruiting Environment:

• Humidity: 90-95% during pinning, 80-85% during development
• Temperature: Species-specific (generally 60-70°F for most varieties)
• Air circulation: 4-6 air exchanges per hour
• Light: Indirect natural light or LED grow lights

Mist the blocks 2-3 times daily, avoiding direct water on developing mushroom pins. Focus spray on the walls of your fruiting chamber rather than directly on the substrate.

Second Flush Preparation

After harvesting your first flush, prepare the block for additional mushroom production:

1. Remove the block from the cardboard box
2. Soak the entire block in clean water for 24 hours with the X-cut facing downward
3. Drain thoroughly and return to the box in fruiting position
4. Wait 2-3 weeks for the next flush to develop

The soaking process rehydrates the substrate and triggers the formation of new mushroom pins. Each successive flush typically produces smaller quantities but maintains good quality.

Third Flush and Beyond

Most blocks will produce 2-3 viable flushes before nutrient depletion reduces yields significantly. Repeat the soaking process between each flush, but expect diminishing returns after the third harvest.

Flush Number	% of Total Yield	Typical Weight from 6lb Block
First flush	60-70%	1.5-2.1 lbs
Second flush	25-30%	0.6-0.9 lbs
Third flush	10-15%	0.3-0.4 lbs
Total Expected	100%	2.5-3.5 lbs

Note: Actual yields vary by mushroom variety, environmental conditions, and substrate quality.

Troubleshooting Common Issues

Problem: Mushrooms Growing from Bag Sides

Cause: Inadequate sealing around the block or moisture accumulation
Solution: Ensure bag is tightly folded around the block; improve air circulation

Problem: Poor Pin Formation

Cause: Insufficient humidity, poor air exchange, or over-misting
Solution: Adjust environmental conditions; check humidity and air circulation

Problem: Contamination During Fruiting

Cause: Usually bacterial contamination from excess moisture
Solution: Reduce misting frequency; improve air circulation; safely dispose of affected blocks by sealing in plastic bags before regular trash disposal

Problem: Soft or Mushy Substrate

Cause: Over-watering during soaking or poor substrate composition
Solution: Reduce soaking time; ensure proper drainage before returning to fruiting

Advanced Tips for Professional Results

Substrate Supplements:
Add 10-15% wheat bran or similar nitrogen source to your base substrate for increased yields. Calculate supplements as part of your 5.5-pound substrate weight.

pH Optimization:
Most mushrooms prefer slightly acidic substrates (pH 6.0-6.5). Add small amounts of gypsum if your substrate tests alkaline.

Batch Processing:
Prepare multiple blocks simultaneously for consistent production schedules. Stagger preparation by 1-2 weeks for continuous harvests.

Quality Control:
Keep detailed records of substrate recipes, colonization times, and yields. This data helps optimize future batches.

Storage and Shelf Life

Properly prepared fruiting blocks can be stored for extended periods:

• During colonization: Blocks remain viable for several weeks at room temperature
• After colonization: Refrigerate unused blocks for up to 6 months
• Between flushes: Store soaking blocks in cool, humid conditions

Always inspect stored blocks for contamination before returning to production.

Conclusion

Creating mushroom fruiting blocks transforms chaotic bag growing into an organized, predictable system. By following these detailed instructions for how to fruit mushrooms in blocks, you'll produce professional-quality blocks that maximize yields while minimizing space and maintenance requirements.

The key to success with DIY mushroom blocks lies in precise measurements, proper sterile technique, and patience during colonization. Once you've mastered the basic process, experiment with different substrate combinations and supplements to optimize production for your specific mushroom varieties.

Start with this proven 6-pound formula, then scale up as your experience and space requirements grow. Within a few months, you'll be producing consistent, high-quality mushroom harvests that rival commercial operations.

Remember that mushroom cultivation is both science and art. Each batch teaches valuable lessons that improve your technique. Keep detailed records, maintain sterile conditions, and don't be discouraged by occasional failures – they're part of the learning process that leads to mastery.

Ready to Get Started?

If you'd rather skip the sterilization and prep work, we've got you covered! Check out our ready-to-inoculate sterilized substrates and premium grain spawn that take the guesswork out of mushroom cultivation. Visit our collection at Out-Grow.com to get everything you need for successful mushroom growing blocks, delivered right to your door.

Whether you're making your own substrates or using our pre-made options, the block formation techniques in this guide will help you achieve professional results every time.