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The Preen will not affect the bulbs at all. It prevents seeds from sprouting, but doesn't affect bulbs at all. You wouldn't want to use it if you were planting seeds (like marigold, zinnia seed, etc.) in your landscape but it won't affect the plants at all, even small ones. I would sprinkle it over top of the mulch since mulch also sometimes contains weed seeds.
Good luck with your beautiful new landscape.
All the best,