Seed Storage Containers – seven materials and what they hold

A dry seed sitting in an open paper bag draws in the moisture of the room air over weeks, and after a year it holds the same water content, the share of its weight that is water, as one never dried. The same seed in a sealed metal can stays at the level it was packed at for decades. Between those two ends sit five further materials, each with its own strength and its own weak point.

Gas and water vapour travel through plastic walls without any hole being needed. The molecules dissolve in the material on one side, move through the wall thickness, and emerge on the other. How much arrives depends on the material, the thickness and the temperature; cold slows the process considerably.

Sealed metal cans

Tin plate and aluminium pass neither gas nor water vapour. Machine-sealed cans are the reference of genebanks, the institutions that keep seed for decades, and count as the most reliable choice, because no lid and no gasket stands in the way. The drawback is practical: a sealed can cannot be closed again. Opening one means needing a new vessel.

Glass jars

The glass wall is as impermeable as metal. The weak point sits at the lid: a rubber ring ages, a sealing insert turns brittle. Whether a jar still holds after five or fifteen years is decided there.

Aluminium laminate pouches

A laminate of polyethylene, aluminium foil and kraft paper or polyester combines the impermeability of metal with the handiness of a pouch. Heat-sealed, this combination proved completely impermeable to water vapour across two years, even with a steep humidity difference between inside and outside. Lightproof, flat, cheap. The pouch is spent once opened and needs a fresh seal.

Barrier bags of nylon and polyethylene

These are the bags vacuum sealers use. The wall consists of two layers: polyamide on the outside, better known as nylon, which holds gases back, and polyethylene on the inside, which can be welded shut with heat. The combination holds a vacuum far longer than plain polyethylene, because the nylon layer supplies the gas barrier that polyethylene alone lacks.

Against moisture the barrier is weaker than metal or aluminium laminate, but at fridge or freezer temperature permeability drops sharply. A sound choice for some years under refrigeration; for decades an aluminium laminate or metal container is safer.

Polyethylene film

Plain polyethylene restricts moisture exchange but does not block it. Thicker film resists better than thin, high-density better than low-density: a film of low-density polyethylene, LDPE for short, 25 µm thick passes between 16 and 23 g of water vapour per square metre per day at 38 °C and 90 % humidity, one of high-density polyethylene, HDPE, of the same thickness between 4.7 and 7.8 g.

For gases polyethylene is more permeable still than for water vapour. A vacuum in a plain polyethylene bag is therefore lost within a short time. Usable for one to five years under refrigeration, not for long spans.

Rigid plastic containers

Tubes and pots of polypropylene or high-density polyethylene protect mechanically and close via their lid. The wall is thicker than a film and correspondingly less permeable. The seal depends on the design of the lid and cannot be judged in general. The advantage over pouches: the container can be opened and closed again without being destroyed, and it shields the seed from pressure. A desiccant, a substance such as silica gel that takes up water from the air, does not belong inside as an addition: even a tenth of the seed weight pulls a batch at 6 % water content below 5 % at first, below the 5 % to 7 % target range. Anyone wanting to monitor moisture puts an indicator, such as a small bag of silica gel that changes colour when damp, into a larger outer container holding the individual ones.

Paper, cloth and jute

Freely permeable to gas and moisture. Useful during drying, because that exchange is exactly what is wanted. Unsuitable for storing dried seed, because the seed takes on the moisture of the room air.

What every material demands

Three mistakes cost more than the choice of material. Sealing damp seed shuts the moisture in: in an 18-month study on wheat, maize, cotton and quinoa, maize and quinoa that went into airtight bags at 14 % water content no longer germinated at all by the end, and wheat only at 18 %, while the same bags at 8 % gave the highest share of germinating seed in all four species. Opening a cold vessel in warm air brings condensation onto the seed; let it reach room temperature first, then open it. And frequent opening lets damp room air in every time; several small vessels protect a batch better than one large one.