Answer: Rather than tannic acid I would be concerned about the ability to sterilize the wooden flats, especially if they are rough wood. Sanitation is so important when starting seeds that a more practical material is plastic because it can be bleached and quickly air dried between uses. Many gardeners also find that small pans are most practical for germinating seeds and then quickly transplant the best seedlings into individual containers (either clean plastic cell packs or paper or plastic cups work well); this makes subsequent planting easier because the roots are concentrated and easy to handle. Wooden flats are also, in my experience, too heavy for easy handling.
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