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.
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 oC storage without loss of vigour. Only when sufficient frost hardiness is reached are seedlings packed and placed into cold storage.
Our main facility can store up to 65,000 box/cartons in a series of six individual, temperature controlled rooms. With over 200 orders/request keys in storage in a season, careful handling, computer recorded inventory /location information enables access to every carton. Rooms are maintained at -2.5 +/- 1.0 oC through constant air flow from an extensive ducting system.
The cartons are stacked in a proprietary rack and pallet system (not shown) with air space along several walls of each individual box. Spaces are maintained to provide maximum air flow each carton. This facilitates tight in box temperature control at 2.5 +/- 0.5 oC. 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 -2.5 oC compared to the rates associated with above-freezing storage temperatures.
Prompt freezing is essential to keeping your seedlings fresh. This minimizes carbohydrate loss and mould development during the storage period.
Our Cold storage facility is 10 minutes from our nursery. Within hours of the start of lift at the harvest line, stock has been packed, palletized, and arrives at cold storage. Typically it arrives at 4 to 10 oC. Before freezing, a significant amount of cooldown must occur. Heat removal is accomplished in rooms at -2.5 oC with constant air flow. The cartons are stacked in configurations 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 -0.5 oC in 3-6 days.
The foliage and stems freeze first, which traps free moisture and halts possibility of development and spread of cold storage diseases. Root mass and plugs contain more water, and therefore more thermal mass. They freeze in 7-14 days.
Over freeze is prevented by our relatively mild room temperatures and thorough control and monitoring. We maintain a safe buffer to prevent that damage. Even though the foliage has be conditioned and tested to handle temperatures to -18 oC, we need to care for roots that could be damaged at temperatures below -6 oC.
Our Cold Storage Manager monitors the cold storage facility full time during the storage period. Seedlings are inspected at regular intervals to ensure overall plant health is maintained during the storage period. Careful control of our storage environment is achieved by regularly monitoring room temperatures, carton temperatures and visual assessment of seedlings.
All rooms have multiple digital probes connected to computer controlled monitoring system. Room temperatures and box temperatures are monitored continuously and are checked in real time onsite or via internet connection. Temperatures are recorded every 7.5 minutes from all probes. This data is graphed to look for trends and data is backed up daily for a historical record.
In addition internal carton temperatures and room temperatures are checked manually. Checks are done using a digital thermometer: are conducted daily in the critical freeze down period, and weekly during the store period. These checks are systematically random throughout each storage room.
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.
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 oC 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.