Answer: Using herbicides on soil that might contain roots of trees and shrubs is not the best approach. The herbicides can translocate down into the soil and be taken up by the roots of your trees. The damage may not show up for a year so cross your fingers that what you used degraded before it traveled too far into the soil. It sounds as though you're dealing with perennial weeds with deep roots and that the landscape fabric either had holes in it, or you didn't overlap the edges of the fabric and sunlight was able to penetrate. That's all the encouragement perennials weeds need - just a sliver of sunlight and they'll grow like crazy. The best control measure is the most physically demanding, but it will be worth the effort. Remove the fabric/plastic and dig out those nasty weeds, roots and all. When the entire area is weed free, place new weed barrier over the bare soil and anchor it down so it makes good contact with the soil. Weeds won't sprout if you also add 2-3" of mulch material - chipped bark or rocks. Eliminating sunlight penetration will keep weeds from sprouting and overtaking your flower bed. Once you've put all that hard labor into renewing your flower bed, it should remain maintenance free for many years to come.
Best wishes with your landscape!
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