Answer: Mid summer is the hottest and most stressful time of year to plant, but you could plant now if you are using container grown plants. If you are planning to use divisions you should wait until early September when the weather begins to cool and it tends to be naturally wetter with seasonal rain -- much better for digging and dividing and replanting.
Allow a week or two for the beds to settle, preferably have them go through a good rain or a heavy watering or two. Then re-check that they are level and even out any low spots. Use the time to run some basic soil tests and check the fertility and pH. Then add fertilizer and/or lime as indicated by the soil tests.
You may also find that there are many weed seedlings that appear in the first two weeks. Rake these out or hoe them off. This will make less work for you weeding later on. Keep in mind that every time you disturb the soil you bring more weed seeds to the surface, so do not till it to remove the weeds.
After planting, mulch the beds with several inches of organic mulch. This will help keep the soil evenly moist and also help keep down weeds. It will also help feed the soil slowly over time as it breaks down.
Good luck with your new flowers!
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