Invasive species (not just plants) theory, and the biological knowledge to use it offensively, is relatively new. And, even if attempted, the granting of beneficial outcomes (to the attacker) would lag the initial issue by years, while providing a casus belli from the start if caught. If the situation did not necessarily develop to the attacker's advantage, i.e. if they lost, they'd be stuck paying reparations for decades as they'd have no easy way to "turn off" their attack vector.
Not to mention that, in a number of instances, invasive species were originally intentionally introduced to improve the local situation. So, if miscalculations are common, the reverse (an unintentional benefit to the attacked party) - or at the very least a relative lack of negative effects - could be somewhat expected.
to steal a point from B. Lorentz excellent answer: in the past most enemies were right next to each other, sharing the same ecosystem or with adjacent ecosystems, so the scope for novel invasive species would have been limited.
Last, but not least, historically a good proportion of wars were prosecuted to annex territory. So seeding ecological disruption in what's hopefully soon to be your territory is an additional disincentive.
So a) never heard of that, b) historically not something countries would have thought to try (unlike say flinging plague-ridden corpses into besieged cities) and c) there are many good reasons a country would think twice before using it, unless they were ready to commit to a deeply adversarial relationship for the long haul with limited immediate short term benefits.
re. the article:
But, what’s actually inside seem to be random plant seeds.
Darn sloppy weapon production quality assurance ;-)
Edit: None of this answer is written from the viewpoint that there is ecological warfare going on in the originally linked news story or that the OP believed there was. It's just looking at the practical limitations accounting for the likely lack of historical use.