Members of the onion family may cross if there are different types flowering at the same time. So to keep heritage varieties pure only let one variety flower at any one time. Spherical or teardrop shaped flower heads will form on long stalks of plants in this family. Wait until these have flowered and at least mostly dried. Remove the seed heads and place in a paper bag to fully dry. Then rub out the black angular seeds. Use a sieve or winnow the remaining flower parts. Store in a cool dry location. Seeds of the onion family are best used within 1-2 years of saving.