Some plants travel from continent to continent, migrating together with the people who are fond of them. Like the garlic mustard that travelled to the US because the French immigrants didn't want to miss this wild edible plant. Where it became a very invasive species by the way, and in some states it's a threat to local flora.
I have pondered that, as I am a herbalist, and therefore my wisdom is very local. Drop me in a forest in Indonesia or Brazil and this wild edibles expert would be lost. What if I'd move to the other part of the world? I'd probably feel delighted to get to know some new wonderful flora, but wouldn't I also feel lost and alone, away from my trusted allies that I know and I have worked with for quite some time? I remember doing a weed walk in the US, in an area in (roughly) the same climate zone as here in Belgium, and I was thrilled and excited to be introduced to some new plants, but noticed just how relieved I was seeing all the plants I recognized and knew very well. They made me feel a bit more at home.
So yes, even though I know how hard this can be on an ecosystem, and by no means want to minimalize the effects foreign species can have on local wildlife, as I see it happening every day, I can understand that people who leave almost everything behind do want to take a trusted plant. Maybe it's a food plant they love, or a medicinal plant, or an ornamental plant, or a psychoactive plant, or a fiber plant, or a plant to dye with. Or just a plant species you're attached to for emotional reasons. Perhaps because your grandmother used it, or because it was a daily company when you walked the dog.
What about you? Can you think of a plant you would take with you when migrating? What is the one plant you wouldn't want to miss?