Blackberries occur naturally in woodland environments across the northern hemisphere and are widely cultivated for their sweet, dark-purple fruit. The rambling, thorny canes of the blackberry spread via underground runners, making them a prolific and somewhat invasive species to grow. Most gardeners take advantage of the aggressive nature of blackberries to create new plants by relying on vegetative propagation instead of seed germination, however, it is possible to grow blackberry plants from seed with a little patience and a few materials commonly found around the house.
Harvest the seeds from fresh blackberries. To do so, place the blackberries in a wire mesh colander and press them with the flat of your hand until pulverized. Run water over the berries as you continue to work the seeds from the flesh of the fruit. The seeds do not have to be perfectly clean, but they should not have large pieces of fruit matter attached to them. Give the seeds a final rinse and allow them to drain.
Lay a single layer of blackberry seeds along one end of a paper towel. Fold the paper towel in half over the seeds and slip it inside a sealable plastic baggy. Seal the baggy and place it in the bottom drawer of the refrigerator for 30 days. The temperature must be kept around 35 degrees Fahrenheit during this time.
Mix equal parts potting soil and compost. Add 1/4 cup vermiculite for every 2 cups of soil mixture and mix it thoroughly.
Fill the peat pots with the potting soil mixture and lightly press it to eliminate any air pockets. Water the soil until very moist but not sopping wet.
Create a 1/4-inch deep depression diagonally across the top of the soil. Sprinkle three seeds within the depression, then pinch the soil closed. Spritz the soil several times with a spray bottle.
Cover the top of the peat pot loosely with clear plastic wrap. Place the peat pots near a sunny window or outdoors in a warm area protected from wind.
Water the seeds with a spray bottle every other day for 15 to 20 days or until germination. Once the seedlings have reached 4 inches in height, they can be planted outdoors.