Overwinter cold storage must be done well if spring plant seedlings are to be in good physiological condition when planted and are to achieve optimal survival and growth performance.

Our cold storage service encompasses equipment, materials, techniques and procedures designed to prevent physical damage to the seedlings, maintain good seedling physiology, retain seedling vigour and prevent the development of disease.

Frost Hardiness Testing

Prior to packing for cold storage, samples of a wide variety of seedlots are tested using our biofreezer, growth chamber and fluorometer. This ensures that the seedlings are sufficiently dormant to withstand several months of -2 C storage without loss of vigour. Only when sufficient frost hardiness is reached are seedlings packed and placed into cold storage.

Pre-Storage Cooling

Heat removal is accomplished in rooms at -2 C with constant air flow. The cartons are stacked only 2-3 high with good separation to allow maximum air flow past the carton surfaces. This allows heat to be drawn out more efficiently, lowering the internal temperature to between 0 and -2 C in 5-7 days.

Prompt freezing is essential to minimize carbohydrate loss and mould development during the storage period.

Cold Storage

Our main facility can store up to 42,000 cartons in a series of six individual, temperature controlled rooms which are maintained at -2 C through constant air flow from an extensive ducting system.

The cartons are cross stacked 6 high in rows 2 cartons wide. A 30cm space is retained along each wall, and alternating 30cm and 15cm spaces are retained between each row to provide maximum air flow to at least one side of each carton and to enable access to every carton. The cartons remain in freeze storage until delivery or pre-delivery thawing is requested.

Freezing the water inside the cartons traps the moisture as ice and prevents it from being accessible for the development of fungal diseases. There is no risk of cell damage as much lower temperatures are required to freeze the intracellular contents of dormant seedlings.

Mild freezing reduces seedling respiration. Since early post-planting photosynthetic activity is generally insufficient to support new growth, reducing respiration during storage is essential for conserving stored carbohydrates and optimizing early seedling survival and growth. Respiration and carbohydrate loss rates are significantly reduced at -2 C compared to the rates associated with above-freezing storage temperatures.

Regular Monitoring

Our Cold Storage Supervisor is in the cold storage facility full time during the storage period, regularly monitoring room and carton temperatures and seedling condition.

Ambient room temperatures are checked daily using a digital thermometer to check probes located throughout each storage room. Internal carton temperatures are checked weekly by randomly selecting several cartons in each room in the same way.

Seedlings are inspected at regular intervals to monitor their overall condition and ensure the correct storage environment is being maintained.

Root Growth Capacity Testing

Prior to shipping, seedling samples are removed from cold storage and tested for their ability to develop new roots when placed in controlled conditions of light, temperature and humidity. The measured results and visual assessments of the amount and quality of new shoot growth provide an indication of their ability to grow vigorously once planted.

Prompt Thawing

Cartons are thawed in one of two separate thaw rooms which have a combined capacity of 4,500 cartons. The rooms are maintained at +5 C and provided with constant air flow.

The cartons are stacked 2 high on pallets with good separation between them to allow maximum air flow past the carton surfaces. This allows the internal cold to be drawn out more quickly and provides a prompt, non-damaging, relatively uniform thaw rate achieving complete carton thaws in 7-9 days.

Prompt thawing is essential to minimize seedling carbohydrate loss and storage mould development during the thaw period. Prompt thawing also permits seedlings to be thawed on relatively short notice, minimizing the potential for delivering frozen or partially frozen seedlings.