khobeza

some plants compartmentalize heavy metals in the soil and fix them in root tissues, if the heavy metals can even cross the soil-root barrier. there is generally compartmentalization/chemical barriers between root-shoot (below and above ground growth) and then again between shoot-fruit (vegetative and reproductive tissues).

i don't know about this plant specifically (seems to be in the okra family) and generally leafy greens have one less barrier between contaminated soil and the eater. it says its typical ecology is disturbed sites, including agricultural fields, so they could be harvesting in marginal areas well away from buildings / brownfields.

but i think ultimately, it's an issue of immediate needs for nutrition and worrying about contamination later. i mean, you can die or get terrible metabolic disorders from malnutrition in a short amount of time. but as we see in the states, you can hang on for several decades with severe lead toxicity and even be the president!

but for any of you comrades considering urban gardening on contaminated sites, this is a short and sweet publication with good tips.