How to Cold Stratify Raspberry Seeds
Raspberries (Rubus spp.) make excellent garden fruits in U.S. Department of Agriculture plant hardiness zones 2 through 7. Often planted as young plants, they do not produce their fruit until the second year. If you collected your own raspberry seeds, they need a period of cold stratification to break dormancy. This naturally occurs outdoors if you plant the seeds in the garden in fall or winter. If you want to start raspberry seeds indoors, you must simulate the cold stratification process before planting.
Moisten a handful of sand, vermiculite, peat or another type of medium. Squeeze the medium with your hands and dump out any excess water. It should be moist, not saturated.
Take a portion of the moist medium and an volume of raspberry seeds and mix them together in a zip-top plastic bag. Seal the bag and place it in the refrigerator that is set to 37 to 40 degrees Fahrenheit.
Check on the seeds once a week to ensure the medium and the seeds are still moist. Add water, if necessary, and squeeze out any excess so it is not overly wet.
Remove seeds in after days, or if they begin to sprout. The raspberry seeds are now ready to plant.