Answer: I would imagine the bleach (bleach and water solution) is intended to reduce the chance of mold and other nasties affecting the bulb over winter. If you do this, make sure the tubers are dry before you store them. Storing them in a paper bag or cardboard box plus possibly peat moss or wood shavings or even sand is fairly standard for dahlias -- it allows for some air circulation. You'll want to keep them in a cool location but do not allow them to freeze. Check them periodically to make sure they are not molding or shriveling, then replant in the spring.
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