Your leftover seeds will maintain a high germination percentage if stored under proper conditions. How long your seed remain viable will depend on the temperature and moisture levels the seeds are exposed to. Seeds which are improperly stored will quickly deteriorate if unprotected against high humidity and dramatic temperature fluctuations day after day.
For best results, store any unused seed in a water resistant container. Common household items such as zip-lock storage bags, glass jars or plasic containers with snap-on lids work well. Place the unused portion of seed in the storage container that you have chosen. Before sealing the container add a packet of desiccant to the seed such as silica gel. The silica gel will remove any moisture that remains in the storage container after it is sealed. Seed stored under these conditions will remain viable for many years.
If you collect seed from your own wildflower area, the seed should be thoroughly air dried on newspaper. Seeds which are not completely dry prior to storage will contain excessive moisture which will cause mold to grow and damage the seed. After the seeds are completely dry, the seeds should be cleaned to remove as much chaff and leaf litter as possible. Follow the above procedure to store you home grown wildflower seeds for future use.