Answer: The weed killer you used probably killed the weeds it touched but did not stop any weed seeds from germinating. For this you'll want to use a pre-emergent herbicide. Pre-emergents are safe to use around perennial plants. They emit a gas which stops seeds from germinating so you don't want to use it where you plan to plant seeds. Preen and WOW are two pre-emergent brands but there are others. The most earth friendly is one that contains corn gluten. To avoid problems in future, make sure the manure is aged before using it. If it is fresh, simply lay it out in a row and allow the sunshine to heat it up enough to kill the weed seeds, and rain to leach urea (salts). After aging, it should be fine to use in your garden. For right now, I'd pull the weeds out, broadcast a pre-emergent herbicide, then lay weed barrier (not black plastic) over the planting bed and cut holes in the weed barrier for the plants. Once you've finished planting, cover the weed barrier with beauty bark and the area should remain weed free for several years. The caution against using black plastic is that it does not allow air or rain water to reach the soil. A weed barrier is permeable and will allow the earth to breathe, allow moisture to penetrate, and will block sunlight, which is what seeds need in order to sprout.
Best wishes with your garden!
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