Plastic seed trays and cell packs can be reused for multiple growing seasons, but each reuse cycle carries the risk of introducing fungal spores, bacterial residue, and mineral deposits left over from the previous crop. A sanitation step before each use significantly reduces the likelihood of early seedling losses from contaminated tray surfaces.
This article describes three practical sanitation methods for home and small-scale growers: bleach solution, hydrogen peroxide solution, and hot-water soak. Each has different safety considerations and effectiveness profiles.
Preliminary Cleaning
Chemical sanitizers work most effectively on clean surfaces. Before any chemical treatment, trays and cells should be physically cleaned to remove soil, root fragments, and mineral scale:
- Scrub trays with a stiff brush under running water to dislodge attached soil and organic debris.
- For trays with mineral scale (white or grey deposits from hard water), soak in a solution of equal parts white vinegar and water for 15–20 minutes, then scrub and rinse.
- Rinse thoroughly with clean water before applying any chemical sanitizer.
This preliminary step is not optional — organic matter present on tray surfaces neutralises the active components of bleach and hydrogen peroxide, substantially reducing their effectiveness.
Method 1: Bleach Solution
Sodium hypochlorite bleach at low concentration is the standard sanitation method in commercial greenhouse operations and is broadly recommended by extension services for home grower tray sanitation.
Bleach Sanitation Protocol
| Step | Detail |
|---|---|
| Dilution | 10ml of household bleach (5–6% sodium hypochlorite) per 1 litre of water |
| Soak time | 30 minutes fully submerged, or spray and allow to sit for 10 minutes |
| Rinse | Rinse thoroughly with clean water × 2. Allow to air dry before filling with substrate. |
| Concentration note | Do not increase concentration — higher bleach levels can leave residues that injure seedlings. |
Safety: Use bleach solutions in a ventilated area. Wear gloves to protect skin. Do not mix bleach with vinegar, hydrogen peroxide, or any other cleaning product. Discard used bleach solution after each session — it degrades rapidly and should not be stored for reuse.
Method 2: Hydrogen Peroxide Solution
Household hydrogen peroxide (3%) is an alternative that some growers prefer for its faster degradation to water and oxygen, which means no residue risk if rinsing is incomplete. It is effective against many fungal spores and bacteria, though generally considered somewhat less potent than bleach for reducing pathogen load on porous surfaces.
Hydrogen Peroxide Protocol
| Step | Detail |
|---|---|
| Solution | 3% hydrogen peroxide (standard pharmacy grade), used undiluted for trays |
| Application | Spray onto clean tray surfaces until visibly wet, or soak for 10–15 minutes |
| Rinse | Rinse with clean water, or allow to fully air dry (H₂O₂ breaks down to water) |
| Storage | Hydrogen peroxide degrades when exposed to light; store in original opaque container |
Hydrogen peroxide at 3% is available at most Canadian pharmacies (typically sold as a wound antiseptic). Horticultural-grade hydrogen peroxide at higher concentrations (27–35%) is also sold by greenhouse suppliers, but requires significant dilution and carries hazards that make it impractical for home use without appropriate safety equipment.
Method 3: Hot Water Soak
For growers who prefer not to use chemical sanitizers, a hot water soak at sufficient temperature will reduce surface pathogen levels on plastic trays. This method is less effective against fungal spores than chemical methods but has no residue risk and requires no special materials.
Hot Water Soak Protocol
| Step | Detail |
|---|---|
| Temperature | Water at 82°C (180°F) minimum — use a kettle thermometer to verify |
| Soak time | 20 minutes. Keep submerged using a weight if trays float. |
| Container | Use a non-reactive container (stainless, enamel, or food-grade plastic rated for hot water) |
| Limitation | Does not reliably eliminate heat-resistant fungal spores and some bacterial endospores |
Tray Material Considerations
Most seed trays sold in Canada are made from polystyrene or polypropylene. Both tolerate dilute bleach and hydrogen peroxide without degradation. However, older trays with surface cracks or pitting are more difficult to sanitize effectively, as pathogens can persist in microabrasions that cleaning solutions do not fully penetrate. If trays show significant surface degradation, replacement is more practical than intensive sanitation.
Biodegradable cell packs (peat or coir-based) are single-use by design and should not be reused — they harbour pathogens in the degraded fibre and the sanitation methods above are not applicable to them. Recycle or compost them after use.
Drying and Storage After Sanitation
Sanitized trays should be allowed to dry completely in a clean location before filling with substrate. Stacking wet trays creates a humid microenvironment that can support rapid fungal growth, partially negating the sanitation step. Storage in a sealed bag or covered bin protects trays from recontamination between preparation and use. This is particularly relevant for growers in British Columbia and Ontario who may prepare trays weeks before indoor sowing season begins in February or March.