Since one can not prove a negative, your claim is not falsifiable and therefore unscientific. In order for a particular claim to be scientific, it needs to include the conditions under which that claim would be rendered incorrect.

Therefore, the claim that few of the 1,730 species are new rather than simply newly discovered has the same scientific weight as the claim presented that plant species are dying at the rate of about 2 or 3 per year.

It's important to note that plants evolve quickly. In fact, farming has modified plants genetically over the last few hundred years. And I don't mean the supposed GMO products. So, it's quite plausible that at least 2 or 3 new species of plant life have appeared on planet Earth over the last 250 years or so.

What we do know and can measure, is that the Earth is significantly greener than 50 years ago. Within all that new green plant life, it's quite likely that new and simultaneously undiscovered species are now thriving.