> these plants spread so rapidly because they offer a better nectar/pollen reward to pollinators,

That may be true for some plants but not all. For example, Black locust and Japanese knotweed propagate via underground roots or runners. In fact, in the US, most knotweed plants are male, they don't make seeds. 

Some produce chemicals which inhibit the growth of other plants (U.S. FOREST SERVICE)..

PLB

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