How to Seal Agar Plates for Long-Term Use
- Harold Evans
- Aug 22, 2025
- 2 min read
Updated: Jul 22
Agar plates are one of the most important tools in mushroom cultivation, but only if they're sealed and stored properly. Parafilm is the standard choice. It stretches to conform to the seam, holds moisture in while still allowing gas exchange, and won't contaminate your culture.
That gas exchange matters. Mycelium needs oxygen, so a truly airtight seal works against you on an actively growing plate. Parafilm strikes the balance.
Why Sealing Matters
Protects the vulnerable seam where the lid meets the base
Keeps moisture in so the agar doesn't dry out
Extends shelf life when storing plates for future projects
What You'll Need
Parafilm (preferred)
Grafting tape (acceptable alternative)
Plastic wrap (workable for cold storage of dormant cultures, but it seals tighter than parafilm and isn't ideal for plates still growing)
Scissors
Step-by-Step: Sealing an Agar Plate
Cut your strips
Cut parafilm into strips about ½" to 1" wide, enough to comfortably cover the seam where lid and base meet.
Grip the plate
Hold the plate between your index and middle finger, one finger resting on the flat top of the lid and the other on the bottom. This gives you a stable grip while keeping the plate flat so the culture inside doesn't slide or shift. Your thumb rests against the side of the seam, ready to press the parafilm into place.
Anchor the parafilm
Place one end of the strip along the seam. Use your thumb to press and hold it firmly against the edge of the plate.
Stretch and wrap
With your free hand, gently stretch the parafilm while guiding it around the plate. Rotate the plate smoothly with the hand holding it, keeping it flat to avoid jostling the culture. Maintain steady tension so the strip seals evenly without gaps.
Overlap and finish
When you reach the starting point, overlap the strip slightly and press it down to complete the seal. It should feel snug and secure, but not so tight that it bends the lid or base.
Storage
Short-term: store sealed plates at room temperature while the culture grows.
Long-term: place sealed plates in a clean container in the fridge. Never the freezer — ice crystals rupture hyphae and kill the culture. Properly sealed plates stay viable for months, which is what lets you maintain a library of clean cultures.
Conclusion
Sealing plates is a simple skill that takes a little practice to do quickly. Once the grip becomes automatic, you can seal a stack of plates in a couple of minutes and know your cultures are protected.
Where This Fits in Your Grow
Before this: [How to Do an Agar to Agar Transfer] — the work you're sealing up
Next step: [How to Make Liquid Culture] — scaling a clean plate into volume
Also useful: [Still Air Box] — the environment this work happens in
A culture library starts with clean plates. Ours ship pre-poured and made to order, with a 0.27% contamination rate across more than 750 plates.
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