Answer: Any herbicide applied to the mulch will degrade in the presence of sunlight and fresh air, so don't replace it for that reason alone. You can leave the mulch in place and add new to cover over it, or you can completely remove it and apply new. As for plants, I only know of a few annuals that are super low pollen plants and unlikely to cause allergic reactions. These are petunias, begonias, and geranium. You might look for perennials with low pollen or semi-closed flowers to add to your garden. I'm sure your doctor will have a list of appropriate plants, as well. Good luck with your garden.
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