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Although it will tolerate light frost, if you want to try to keep this plant over the winter in a greenhouse or a bright window, try to bring it indoors when nighttime temperatures are in the high 30’s.

https://www.gardeningknowhow.com/ornamental/flowers/popcorn-cassia/popcorn-cassia-information.htm

You might try just cutting down to a couple of feet until you get new side shoots that can take over. At least then you can be sure the the tree will have some way to regenerate. Here is an article for the best way to prune the trees: https://www.gardeningknowhow.com/ornamental/trees/cassia/cassia-tree-pruning.htm

Care for trees indoors can be easy, as long as you provide a window that gets sun as long as possible. Just care for it as you would outdoors. The big thing here is to watch your watering. Don't water until the top of the soil is dry down to two inches.

Here is an article on propagating trees from cutting: https://www.gardeningknowhow.com/ornamental/trees/tgen/how-to-root-cuttings-from-various-shrubs-bushes-and-trees.htm

Unfortunately, it is not likely to germinate. It doesn't appear to be fully mature. There is no harm in trying but, don't be disappointed if they fail to pop. I'm not sure if you have read this particular article yet, but this will describe the process: https://www.gardeningknowhow.com/ornamental/trees/cassia/growing-cassia-trees.htm

This is true. The best way to combat this is by making sure they get watered a little more than normal while they are still trying to grow roots. This could also have something to do with your growing zone too. If you can include this information, I can better help you. If you can include pictures, then it would be very helpful.

In the meantime, here is an article for more information: https://www.gardeningknowhow.com/ornamental/trees/cassia/growing-cassia-trees.htm

The seeds will form on the plants as the flowers fade and petals drop. If you removed the flowers before the seeds formed, you may not have seeds in the box that you saved.

This article has a clear image of Cassia seeds.

https://www.gardeningknowhow.com/ornamental/trees/cassia/cassia-tree-propagation.htm