Answer: Most weeds can be smothered successfully. Cut the weeds off very short, cover with a layer of newspaper, and top that with a generous layer of organic mulch. Wetting the paper makes it easier to work with. Overlap the edges so the paper blocks all light. The mulch will hide the paper and both will eventually break down and help feed the soil, so it is actually helpful in the long run for your perennial planting project.
If a weed manages to come up through all that, spot treat with an herbicide containing glyphosate. Be sure to read and follow all of the label directions. If you are working between desirable plants, you may find that the wipe on application method is easier to control than spraying -- glyphosate is a nonselective herbicide and will kill desirable plants as well as weeds, the wipe on method also uses less chemical. (Trim the weed off and wipe the glyphosate on the cut stem.)
You may be able to plant the impatiens by digging individual planting holes, being sure to dig out any perennial weeds that are close to the planting spot. Then use the paper and mulch around them just as you would normally mulch between your plants.
Poison ivy is more difficult to control due to its extensive root system and root reserves. Sometimes repeated cutting and digging out the roots will control it, but usually it seems to come back. With this plant, you probably will need to use the glyphosate and even that may require more than one treatment. The best time to apply it is in late summer to early fall when the plant is building its root reserves in preparation for winter, although you can try using it now.
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