This is a new program begun by Dr Doug Tallamy to promote cultivation of native plants for biodiversity retention.
Watch his videos to get inspired and join the network.

I'm always fascinated by a "native plants" discussion.

At what point in time has Dr. T. declared that plants should be considered native?

I'm assuming prior to European contact.

That sounds like a lot of different dates.

How about European colonization? Essentially what was there before something was built there. That's the standard that most (all?) native plant societies use. No exceedingly fine hair-splitting.

Sure, if that's the chosen benchmark. I don't think all colonization happened on a single day though. Did other people move plants around before then? Are native plants defined differently in USA than other countries? Does it matter if the plant was moved by humans, or some humans and not others? What if a plant has expanded its range as a result of animal activity? Or weather? Evolution? How far from a spot determined on the chosen benchmark day does a plant need to move to be determined that it is no longer in native territory?

That's some excellent hair-splitting.
In general I think it means without direct assistance from humans. What you would find somewhere before people moved things around. What evolved in that location.
For instance; I might plant a White Alder in a riparian area nearby, but I wouldn't plant a White Birch or even a River Birch. Around San Francisco I would plant a Sequoia sempervirens but not a Sequoiadendron giganteum. Near London you would plant Quercus robur but not Quercus nigra.
Can you give some examples of confusion?

I don't have any examples, just curiosity. Those seem like questions that need answers in order to form a working definition of native plants. Humans manipulated plants for a long time before Europeans. As did animals, weather, geology. Seashell fossils are found on the tops of mountains, things are always changing. It seems difficult to say which manipulation is only good or only negative. Are humans capable of determining positive biomass increase vs. negative? I don't have many answers, but a lot of questions.

Just looked him up, I did not remember him when I was at the U of D, he showed up 10 years later. The University was into native plants when I attended the school. Interesting guy!

https://transitiontownmedia.or...

It's essentially human introduced species, not other sources of introduction. Humans have conscious knowledge of their actions which other species and non-biotic forces do not. That's the determination of native/non-native. That's where positive/negative biomass identification is made. Human manipulation of species already native to a new territory does not affect classification. If humans moved the plants they are non-native, period. Those species did not evolve in those locations and the other species in those areas did not evolve with the introduced species.
I don't think there were many species brought to North and South America by immigrants from Asia 13,000 years or so ago, so it's fairly easy to identify native plants on those continents. Same goes for Australia before Europeans arrived there as well. When it comes to Europe, Africa and Asia it gets a little murkier since they've been inhabited so long.

Does that mean that the basic premise is that humans aren't part of nature? Or that only humans can negatively affect nature? Or that anything humans do "to" nature is negative? Is it all humans, or just some?

How far does a plant need to move to cross the threshold into non-native territory? A foot? A mile? Or just any distance if the move can be attributed to humans?

It's pretty simple.
If a plant wasn't there before people, any people, moved it, it is non-native. Don't overthink it. It's not about 'part of nature', negative/positive or anything that complicated or philosophical. Just, did it evolve there or was it introduced by people.

The typical definition of a native plant is something that grows naturally in a particular region without direct or indirect human intervention. In North and South America that means before European settlement of an area.
he U.S. government defines them as "naturally occurring, either presently or historically, in any ecosystem". The operative word is "naturally". Naturally means without human input. That would preclude any species brought to North America by people.

naturally occurring, either presently or historically, in any ecosystem
This sounds more like a literal definition of volunteer plants. If a seed blows away from a garden and sprouts, did it do so "naturally" without human intervention?

Naturally means without human input.
Does that not imply that humans are not part of nature?

Don't history and evolutionary observations teach us that things are always changing? If it's natural for things to change, would it be unnatural to try to interfere?

I'm not sure I subscribe to the premise that Europeans were the first to move plants around, or that only the plant moves caused by Europeans are those which determine a plants' heritage. Was there a particular day in the past when nature's ideal manifested, if things are always changing?
https://www.scientificamerican...

I can't understand the borders of the ecosystems, or how far a plant needs to move to be described as non-native to that spot. How was it determined that a particular plant did not exist in a particular spot before a particular day?
https://beta.nsf.gov/news/anci....

I understand the concept of native plants, but I've never been able to pin it down. It does sound simple on the surface, but so many questions develop upon additional consideration. There are so many underlying premises and assumptions. When investigating anything, examining the foundational principles is always a sound practice. If any of that is weak, the conclusions drawn inherit the weakness. Are there weaknesses in the concept of native plants? IDK. I just have a lot of questions. I can't move on and fully accept the concept without more info.

You are indeed making this waaaaayyyyy too complicated. For a plant to be native to North and South America it must have been here on or before January 1st 1492. Before that date there was no continuous European, African, or Asian contact or influence. Yes pre-Colombian people modified and moved plants around before then, but those species mostly haven't established wild populations without continuing human assistance. Other areas have other dates for determining 'native' vs 'introduced'.
It's not that humans are 'bad' or 'good', it's that they have altered the environment (flora and fauna) of an area from what it was.
Native plants evolved in the locations they are 'native' to, along with attendant insect, mammal and bird species that are adapted to feeding on them or each other. That English ivy, kudzu, Chinese Tallowtree, Tree of Heaven and other assorted species of exotic plants have established populations is not a reason to identify them as 'native'. They are exotics that were brought here by humans and have escaped cultivation. You can call them 'naturalized', but they are 'native' to somewhere else and there are insect, bird and mammal species that adapted to them there.
I hope this clarifies things a little better.

I've heard a lot about Tallamy in this area, and the HNP from a Master Naturalist who is very active in my county and speaks at the library for us.
In my mind , key is encouraging people to choose plantings that help insect and bird populations around you. That some plants straight species are better food sources than cultivars. That helping insects is vital to helping birds, because birds rely heavily on insects.
Making small choices to benefit nature when you can.
I am not ripping out my Berberis bealei (Mahonia) because in my yard it the only source of flowers this time of year, and I will see bees on it. But in new plantings, I chose shrubs for berries that birds like versus something I know won't get berries.

Tallamy's overall message focuses on the here and now. Perhaps some of the inquiries can be answered in his books (he has authored three books). You can always go to your local library and rent one of his books or e-book. Otherwise he primarily focuses on the plant's ability to provide food and energy in a particular ecoregion in the present moment.