Answer: I am not sure what would survive if covered by a tarp. The problem with that is there will be condensation underneath it and that plus lack of air circulation is not a healthy environment for plants. It is also possible, if the soil is very thin there, that the soil freezes. This would also kill roots. If you could leave the back side of the waterfall open to the air, then you could possibly grow some of the winter hardy grasses such as Miscanthus or even a dward Pennisetum such as you probably had, there. Your local professional nursery staff should be able to show you grasses that are reliably hardy in your area. Otherwise there is an award winning annual millet called Purple Majesty that might give you a grassy look although it does have purple leaves. That might be less expensive although you would have to replace it each year. I hope this helps.
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