Answer: I wish there were something to recommend other than hand pulling, but anything you use to kill the plants popping up from the birdseed will harm your existing plants, as well. If you keep after the invaders and pull them before they set their own seeds, you'll eventually get the situation under control. You might consider laying a thick mulch over the bare soil to suffocate the invading plants, but you'll need to be able to access the soil between your existing plants. Wish I had an easier solution!
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