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Here is an article that will help you to solve this common, and annoying issue:

https://www.gardeningknowhow.com/ornamental/trees/tgen/how-to-kill-a-tree-killing-trees-in-your-garden.htm

This happens where they are not fully hardy. In these areas, they are treated as fast growing annuals. If you can offer more winter protection to the roots, this may not happen every year. Mulching, or burlapping the tree over the coolest parts of the winter can help.

Here is an article that will help you with the care of the tree:

https://www.gardeningknowhow.com/ornamental/trees/royal-empress/worlds-fastest-growing-shade-tree.htm

Unfortunately, if it is not already mature enough to germinate, it may not mature much more off of the tree. You can lay it in a place to dry in a paper sack, and that may help them to mature a little.

This collection of articles will help you with the care of the tree:

https://www.gardeningknowhow.com/ornamental/trees/royal-empress

Every tree and plant has the capacity to become invasive if given the correct conditions.

It is up to each individual to do research in the area in which one lives in order to ensure that a tree that is blacklisted is not planted. Otherwise, it would be very unfair to include information for some gardeners around the world, but not others.

Please ensure that you do proper research before planting anything to avoid hefty legal fines, or worse.

Not only do we have articles for those that live in areas where the tree is NOT invasive, but we also have articles on how to control the tree where it IS invasive.

https://www.gardeningknowhow.com/ornamental/trees/royal-empress/getting-rid-of-royal-empress-trees.htm

In order to be fair and provide information to everyone, we must provide information on ALL plants and trees. All species have the capacity to become invasive, given the proper environment. It is the direct responsibility of each gardener to research and sure that anything planted is not invasive or blacklisted in an area. Each of us must do our part to ensure that we are gardening, responsibly, as to not destroy the ecosystem.

I would, actually, do what you have just done, here! In fact, I have done this before, myself. I think that what you have asked, here, would either get a yes or no response on if they have what you are looking for, or not. If not, then you can move on to another provider. I doubt anyone would think anything of it to request something specific.

I would start, locally, and move outward if you don't find what you are looking for.