This is generally most effective with slightly, as opposed to deeply, dormant species. The seed is mixed with damp sand (not wet) in a sealed polythene bag and is placed in a fridge at approximately 4°C for between 4 and 12 weeks, depending on the species. It is also useful for reducing erratic emergence in non-dormant species. For recommendations on prairie species, see the guidelines produced by Prairie Nursery (2002a). Fridge chilling is not as effective as chilling in situ for more deeply dormant species due to the constant temperature and lack of leaching. Dropping the fridge temperature to 1°C improves germination for some of the latter species. Overall, however, where possible chilling in situ is to be preferred as it is less complicated and avoids risks of the seed being exposed to anaerobic conditions in excessively wet sand or germinating prematurely in the fridge. Fridge chilled seed is more sensitive to soil moisture stress during germination.
Mechanical abrasion
This is used for seed with impermeable seed coats, as in the pea family. Samples of the seeds are placed between sheets of fine-medium sand paper and are rubbed together until there is evidence of seed coat abrasion. This is effective, however it is rather difficult to be sure when sufficient abrasion has occurred. An alternative is to place the seed into hot or boiling water for various time-periods. This is easier to standardise but finding specific recommendations for a given species is often difficult, however see Hartmann et al. (2001).
