Answer: Based on your description I think the collection probably included perennials, plants that return for several years. When planted as small, bare root plants, most perennials will not bloom until the second year in the garden. Also, the collection may have included perennials that do not until mid to late summer. (Most perennials bloom for only a week or two once a year.) So it may still be a bit early to expect flowers.
There are some pre-emergent weed preventers that stop weed seeds from germinating, but these must be applied before the weeds come up and are most effective against annual weeds such as crabgrass. Corn gluten is an example.
If you have a problem with perennial weeds, a pre-emergent will not stop them from growing. You could dig them out by the roots, or you might be able to spot treat them with an herbicide containing glyphosate, using a wipe-on application method rather than the spray method. This is a nonselective herbicide and will kill all kinds of plants so you do not want it to contact your flowers.
Once you have controlled the existing weeds, prevent new ones from growing by using a layer of organic mulch in between your plants. It should be about three inches deep. This will also help reduce watering needs and feed the soil slowly as it breaks down.
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