Basil plants are pollinated by small flying insects. The different varieties will readily cross pollinate so it is important to isolate the variety you wish to harvest seed from by at least 150 feet (45.5 m) apart. This will prevent another variety from polluting your strain. The seeds are contained in the spent flower head. Be aware that Basil will readily re-seed itself by spilling seeds on the ground when flower heads are left on plant to long. Use a fine colander for basil seed collecting, as the black seeds are very tiny. Cut off the brown and spent flower heads and let them dry for a few days in a warm, dry location. Crush the heads over the colander and pick out the old petals and any chaff. Basil seed collecting is that simple. You can also put the dried seed heads into a paper bag and shake it, then crush the bag with a rolling pin, tip the pulverized plant material into a shallow tray and blow out the chaff. You now have home-harvested basil seed which will be of the parent plant’s strain, provided they didn’t cross pollinate. If they are properly stored, basil seeds are viable for up to five years or more. Label and date your seeds and rotate them so the oldest are used up first. Seeds that are completely dry and kept in a dry, dark location should be viable for years after basil seed collecting. Place the dried seeds in a plastic bag or glass jar with a sealable lid. Put the bag or jar in the freezer for a couple of days to kill any insect pests that may still be in the plant material. Ensure there is no air in the container and store the seed in a cool, dark place. Seed viability will be affected if the seeds are exposed to more than minimal light.