Answer: A brand new flower bed is always a fun project. Now that you have removed the grass, I'd suggest incorporating some organic matter (compost, aged-manure, shredded leaves, etc.) to help enrich the soil. After digging in the organic matter, level the planting area with a rake and then roll some weed barrier over the entire bed, anchoring it down. Then choose your plants and cut the weed block fabric to dig holes for each. When you're finished, cut away any excess weed barrier and cover the fabric with decorative bark or rocks. When you prepare and finish a flower bed in this way, you'll have minimal weeds for a very long time. When you water, direct the water into the area immediate around each of the plants.
Spring is usually the best time to plant, and that's when you'll find the largest assortment of plants at the garden centers. Fall is also a good time to plant, especially autumn blooming plants such as chrysanthemums, African daisies, Sedum Autumn Joy, and pansies. Roses are usually sold bare-root in February and March but you can find them in nursery containers this time of year. When choosing a rose, don't just choose based upon color - read the label to see if it is disease-resistant. In the Pacific Northwest roses can be a real challenge so choosing hardy, disease-resistant types will save you a lot of trouble in the future.
Best wishes with your new garden!
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