I'm not sure where you read that planting under a tree would kill it - it is simply not true! Trees growing in the natural environment routinely have other, smaller plants growing under them. Even in heavy forests, there is always undergrowth! As Ken stated, raising the soil level under existing trees can be harmful but in virtually all cases, underplanting them at the same soil level is not an issue. Some trees dislike much disruption in their root zone (Japanese maples for example) so it is best to plant things that will grow happily without the need for division or replanting. And other trees have such aggressive root systems that it is a challenge to get things to thrive within the spread of their canopy or even further. But most trees are very happy with accompanying shrubs or perennials and groundcovers in close proximity. In a small urban or suburban lot, I dislike trees planted plunk in the middle of the lawn. They have no sense of place and look like after thoughts. Plus lawn up to the base of a tree can be a hindrance to successful establishment, not to mention the potential for string trimmer or mower disasters. A good sized bed with complimentary underplantings give trees context and anchor them into the landscape.

they are conifers.. i just did a post on foo foo picea pungens.. in the conifer forum ... i would suggest.. no conifer ... over 3 foot tall was grown to size in a pot ... they are field grown.. and then dug and potted.. or BB ... try to imagine a grower with half a million small trees in pots... i would just kill myself .. lol .. soooo ... once you get to digging up any tree over say 3 feet.. you can probably assume.. 90 % of the root mass is cut off at digging ... so it just sits in its new spots for years.. trying to regrow that lost root mass ... a smaller tree.. that has grown in pots.. its whole life.. repeatedly up potting.. has an intact root mass.. though usually winding in circles.. its own problem .... so your observation is spot on in that.. a smaller tree has less stresses.. and can get re-established.. and growing to standard .. much faster than the larger.. instant gratification plant ... someone is probably going to argue with my fancy word choice.. i am sure you get the jist ... ken

There is significant overlap on the list of plants to avoid digging in the fall and the list of plants recommended to be sweated. It's quite possible the plants recommended to be sweated could benefit from fall planting, but NOT if you dig them in the fall...spring dug b&B, or container. I can't say as I've heard, or pondered, the question phrased in this way. I can say, I plant more plants in a week than most people do in a lifetime, and my list of plants that prefer fall planting would be as outlined above. Last winter, I sat in on a presentation about producing maple sugar. My background is horticulture, and before that dairy, so the maple world is new to me. What surprised me was that according to the narrative of the presentation is that just about everything I thought I know about tree biology and seasonal behavior, based on a horticultural education and years of experience in nursery production, was apparently unknown to the post graduate degreed staff of one of the only maple research centers. Which begets a series of questions about how do we know what we think we know when it comes to woody plants, physiology, seasonal behavior, and similar tangents. Before this morphs, let this be stated...the presentation was from the head of the Proctor Research Center, attached to the U of VT, and was specifically re: the new production system using high density plantings. This was the end result of a experiment supposedly designed to test how sap flowed in trees, specifically sugar maple, in the spring. So how much of the narrative was in telling a story, how much was just people pondering what could be done, and how much was truly unknown, I have no idea. But the idea of sitting in a presentation where people with advanced in plant physiology supposedly didn't know one of the basic things about tree physiology really makes we wonder how we know what we think we know. The truth is, there is a lot we think we know. No one is going to repeat every experiment on every plant...there may be that many people looking for research projects, but there's not going to be the funding for it. So what we know about this species, based on specific research, is going to be extrapolated. Which ultimately gets us to the current question, or more specifically, the application of the question in dealing with sassafras and persimmon. Ultimately, I'm not sure the nuts and bolts research has been done. If the answer to the question does exist, it's possibly buried in obscure journals that only exist in the bowels of research institutions. More likely, bits and pieces of the answer are locked away in the brains of individuals such as the head propagator for Johnson's...or Sherman's...or Bailey (yeah, I know the ownership of the two)...or Forrest Keeling...or one of several dozen other nurseries or research centers (specifically avoiding Monrovia...never can get a decent answer out of them unless it's how many they want to sell me). So at some point, we get to questions that people ask, and it's possible that question has never been asked to the right person before. the right person would be the one who is going to draft a grant proposal that gets approved to do the research. Could this be such a question? Perhaps? That's a topic in and of itself, and the kind of esoteric discussion best held in the rarified air of Colorado...and soon, maybe a state near you.

Some type of Cornus, dogwood family, perhaps Cornus sericea.