The headline surprise in this category: perpetual spinach isn't spinach at all. It's Beta vulgaris -- the same species as chard (also on this hub) and beetroot (Root Vegetables & Tubers hub) -- and genuinely cross-pollinates with both. True spinach (Spinacia oleracea) is unrelated to any of them, and has its own quirk: separate male and female plants. Most of the other "spinach substitutes" here -- orache, Good King Henry, New Zealand spinach and Malabar spinach -- are four completely different, mutually unrelated plant families all doing the same culinary job as true spinach without being related to it, or to each other. Sorrel and Good King Henry are this hub's only genuine multi-year perennials, and watercress needs running or standing water -- a real growing-method departure unique on this whole site.
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Chard, beetroot AND perpetual spinach all cross-pollinate with each other -- they're the same species. All three are Beta vulgaris, wind-pollinated, and need isolation distances measured in miles rather than metres. "Perpetual spinach isn't really spinach" turns out to be literally true: it's a leaf-only chard variety. If you're growing more than one of these three for seed in the same year, treat them as one shared isolation problem.
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Orache, Good King Henry, New Zealand spinach and Malabar spinach are all called "spinach substitutes" -- and all four are unrelated to true spinach AND to each other. Four completely different plant families (Atriplex, Blitum, Tetragonia and Basella) doing the same culinary job. None of them cross-pollinate with true spinach, with the Beta vulgaris cluster above, or with each other.
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Watercress needs running or standing water -- genuinely different from every other crop on this site. Land cress gives a similar peppery flavour in ordinary soil instead, and the two don't cross-pollinate with each other despite both substituting for the same taste.
