The List - Try #1
Light - Your rose should get at least six hours of direct, unfiltered light. Some hybrid teas need more than eight to bloom. A few old roses and hybrid musks might get by on just a bit less.
Water - Soil at six inches and deeper (established roses) should not dry out. Some well-established plants do fine through long dry spells; but repeating roses that fail to bloom in summer or fall might sometimes be reacting to stress from drought. Give them a deep, slow watering.
Standard Nutrition - Fertilize with an organic fertilizer such as Mills Magic. Or use a slow-release fertilizer like Osmocote. Too much nitrogen will cause growth at the expense of bloom.
Non-Standard Nutrition - Have your soil checked, and have it amended to fix imbalances. Sometimes Epsom salts or bone meal are used to encourage bloom.
Bud Union - If it's a grafted rose, check the bud union. Sometimes a graft is bad or the scion dies and the rootstock pushes up: and sometimes rootstock doesn't bloom very well.
Pruning - Pruning the wrong way can have a negative effect on blooming. Prune once-blooming roses within a few weeks of when they have bloomed. Bloom repeat-flowering roses such as floribundas and hybrid tea roses in spring before they start to leaf out. If you live in a place with a lot of freezing and thawing in spring, you may want to delay doing this until hard frosts have passed - especially with frost-tender plants.
Frost - Late frosts can kill rose plants. Or they can freeze spring rose buds. By planting cold hardy roses this effect can sometimes be ameliorated. It is possible, too, that applying a few extra inches of mulch will slow the warming of the soil, possibly delaying bud development just a tiny bit.
Training - Climbing roses, bourbon roses, hybrid perpetual roses, and some other roses make strongly vertical growth and will typically bloom at the end of a vertical shoot. Train canes at a more horizontal angle, about 45 degrees, or peg the long cane to the ground. This causes them to make 'laterals' and these lateral branches each form a bloom. The number of blossoms formed by a long can be multiplied by ten or twenty or more using this method. And the rose likes it.
Check References Some roses can take up to five years to get established, Sunset Celebration, for example. Others put a lot of energy into making a few roses , Parole, for example. Still others are just stingy. Check ARS Handbook for Selecting Roses, or the online resource HelpMeFind. (This item added at Cindi's suggestion below.)
