Therm-O-Rock Medium Horticultural Vermiculite | 4cu ft - Medium (36/PL) - OMRI Listed

Medium-Grade Vermiculite for Seed-Starting and Rooting Mixes, 4cu ft

Seed-starting and rooting mixes fail more often from uneven moisture than from anything else — a mix that dries out in patches loses germinating seeds, and one that stays waterlogged rots young roots before they establish. Therm-O-Rock Medium Vermiculite's finer particle size spreads water evenly through a propagation mix, holding enough moisture at the seed or cutting without the coarser, heavier texture that bulk container grades use.

This 4cu ft bag suits growers blending their own germination, plug, or rooting mixes at production volume, and it ships 36 bags per pallet so greenhouses and nurseries can order enough for a full propagation run.

Key Features & Benefits

• Medium-fine particle size — spreads moisture evenly through a mix rather than concentrating it in pockets.
• Lightweight structure — keeps propagation mixes from compacting around delicate new roots.
• OMRI Listed grade — certified for use in organic seed-starting and propagation programs.
• 4cu ft bag, 36 per pallet — packaged for bulk ordering by greenhouses and nurseries running propagation at volume.

Who Should Use Medium Vermiculite

This grade suits growers and nurseries building their own germination, plug, or rooting mixes who need even, gentle moisture retention around seeds and young roots. The 4cu ft bag and per-pallet packaging fit production-scale propagation more than a single small tray.

About Therm-O-Rock

Therm-O-Rock East manufactures horticultural perlite and vermiculite in graded particle sizes, from coarser grades for container mixes to finer grades sized for propagation.

Frequently Asked Questions

How is Medium grade different from Super Coarse vermiculite?

Medium grade has a finer, more uniform particle size that suits seed-starting and rooting mixes where even moisture matters more than maximum aeration. Super Coarse grades use larger particles better suited to bulk container mixes that need heavier drainage.

Can this vermiculite be used alone for seed starting, or does it need to be blended?

It's typically blended into a propagation mix alongside peat, coco, or a similar base medium rather than used as a standalone growing medium. The vermiculite's role is to even out moisture and lighten the mix, not to replace the base material entirely.

Is this vermiculite OMRI Listed?

Yes, this 4cu ft bag is OMRI Listed and approved for certified organic production. The listing covers this specific graded product and package size.

How is this vermiculite packaged for ordering?

It ships in 4cu ft bags, 36 bags to a pallet, matching the bulk format nurseries and greenhouses use when mixing their own propagation media. Individual bags can still be ordered outside of a full pallet.

Why does particle size matter for rooting mixes specifically?

A finer particle holds and releases moisture more evenly around a small root system than a coarse particle would, which reduces the risk of a cutting drying out in one spot while sitting in standing water in another. That even moisture is one reason medium-grade vermiculite is a common propagation-mix ingredient.