These plants are self-sterile, cross-pollinating, or outbreeding. They will cross with other plants of their species. To save seeds from these plants you must allow only one variety in each species to flower at a time let multiple plants of one variety flower to ensure pollination.
In our dense urban environments, some crossing can occur with our neighbors' plants, but these plants will not cross over great distances. Many are rarely allowed to flower anyway.
Lily or Onion Family (Amaryllidaceae or Alliaceae)
chives, garlic, leeks, onions
They are biennial, which means they won't flower until the second year, after winter. Let the seeds dry on the plant. Collect. With bulbing varieties, replant bulb when it sprouts.
Goosefoot or Amaranth Family (Chenopodiaceae or Amaranthaceae)
amaranth, beet, chard, lamb's quarters, orach, quinoa, spinach
Beet and Chard are the same species, so only let one variety flower at the same time.
Spinach is dioecious meaning each plant is either male or female, so let many plants flower at once for pollination. Let the seeds dry on the plant. Collect.
Parsley Family (Umbelliferae or Apiaceae)
carrot, celery, caraway, chervil, cilantro (coriander), dill, fennel, parsley, parsnip
Carrot, unfortunately, will cross with Queen Anne's Lace, so don't save carrot seeds if Queen Anne's Lace grows nearby. Many of this family are biennials, so flowering may not occur until the second year. Let the seeds dry on the plant. Collect.