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It can be quite difficult to control this without harming your soil. You can try spraying vinegar, carefully, on the plants and this can usually kill them. If this does not do the trick then you can pour boiling water on the stalks, and roots. This will kill off the plants without destroying your soil.

No, you should plant individually.
This article will help you.
https://www.gardeningknowhow.com/ornamental/flowers/astilbe/bare-root-planting-astilbe.htm

These plants are quite picky about their environment, and will not flower correctly without having near perfect conditions. This article will help you with this: https://www.gardeningknowhow.com/ornamental/flowers/astilbe/growing-astilbe-plants.htm

You can wrap it in wet newspaper, or paper towels until you get it to the location that it will be planted. I would suggest digging it up t the very last minute. The less time it spends stressed the better. As with most plants, I would recommend transplanting in early fall If you can catch it early enough, then this would be a good time to do it. Here is an article for more information: https://www.gardeningknowhow.com/ornamental/flowers/astilbe/how-to-transplant-astilbe.htm

You may want to have your soil sampled! There may be some underlying issue that is preventing growth, such as pH, infection, or nutrient deficiencies that a soil sample will reveal so that you can correct them.

This link will help you to find the closest extension service to you: https://www.gardeningknowhow.com/extension-search

Review the link below on care needed.
https://www.gardeningknowhow.com/ornamental/flowers/astilbe/growing-astilbe-plants.htm