The best time to buy roses is in late winter, when they're available as dormant bare-root plants, or during the first bloom flush of spring, when they're sold as flowering container plants (shop early for the widest possible selection).
Roses appreciate well-amended soil. If you know you'll be planting bare-root roses at some time in winter, clean up the area and amend the soil in fall. That will leave you with less work to do come planting time, when the weather is often cold and unpredictable and you may be rushing to set in a bare-root plant between storms.
Because most modern roses put out new growth and flowers throughout the growing season, they need regular water and consistent fertilizing during that time. In general, a rose needs constantly moist (but not soggy) soil to the full depth of its roots. This can take up to 5 gallons of water per rose in sandy soil, almost 8 gallons in loam, and up to 13 gallons in clay. Water again when the top few inches of soil are dry - usually within a week for sandy soil, 10 days for loam, and up to 2 weeks for clay. Mulch around plants to enhance moisture retention.
Roses are heavy feeders. Many gardeners prefer to work a controlled-release complete fertilizer into the top few inches of soil at the start of the growing season (before applying a mulch). If you don't go the controlled-release route, plan on feeding your repeat-flowering roses every 6 weeks (with a dry granular fertilizer) or every month (with a liquid fertilizer). Stop fertilizing about 6 weeks before the first frost date - or in September, if you live in a mild-winter climate.
With repeat-flowering kinds, deadhead spent blooms regularly, cutting back several inches to a five-leaflet leaf. If the rose bears attractive hips, stop deadheading in September. You'll be able to enjoy the brightly colored hips during autumn, and you'll also be sending a signal to the plant that it's time to slow down and prepare for dormancy. There's no need to deadhead roses that flower just once a year.