Onion, shallot and spring onion are all the same species -- Allium cepa -- and cross-pollinate with each other if left to flower nearby. Leek and elephant garlic are a second cluster, both Allium ampeloprasum -- yes, despite the name, elephant garlic is genuinely a leek relative, not true garlic. True garlic (Allium sativum) is its own species and, unusually, essentially never flowers in cultivation, so it carries almost no practical cross-pollination risk at all. Welsh onion and chives round out the hub as two genuine multi-year perennials, each its own species with no cross-partner here.
Onion
Guide available
Leek
Guide available
Shallot
Guide available
Spring Onion
Guide available
Garlic
Guide available
Elephant Garlic
Guide available
Welsh Onion
Guide available
Chives
Guide available
Onion, shallot and spring onion will cross-pollinate with each other -- they're the same species. Shallot was classified as its own species (Allium ascalonicum) until 2010, when it was reclassified as a group within Allium cepa, the same species as bulb onion and the White Lisbon-type spring onion this site models. If you're saving seed from more than one of these three, treat them the way the Brassicas hub treats kale and cabbage -- as genuine cross-partners needing real isolation, not separate crops that happen to look similar.
Despite the name, elephant garlic isn't true garlic -- it genuinely crosses with leek instead. Leek and elephant garlic are both Allium ampeloprasum. In practice elephant garlic rarely flowers reliably in cultivation, but if it does, treat it as a real cross-partner with leek, not with true garlic.
True garlic, Welsh onion and chives are each their own species, and true garlic barely flowers at all. True garlic (Allium sativum) is propagated from cloves and essentially never sets viable seed in cultivation -- genuinely the lowest cross-pollination risk on this whole hub. Welsh onion (Allium fistulosum) and chives (Allium schoenoprasum) have no cross-partner here either, and are this hub's only two genuine multi-year perennials.