Roses are popular throughout the mainland. In Hawaii, growing roses is always a rewarding challenge for the gardener. Roses are cool climate plants that do best at elevations of 1,000 feet or higher. At lower elevations, insects and disease problems are aggravated. At best, roses require specific care or they will not do well. The most spectacular plantings of roses I have seen are at the homes in Kaloko Mauka.
Roses come in two main classes, dormant and container grown. Dormant rose plants are usually shipped in from the mainland in early spring and are available now at some nurseries and garden centers. Container grown plants are available after the nursery has given them a good start. Rootstocks vary, but research in Hawaii suggests that roses grown on fortuniana rootstock are best for low elevations. Other rootstocks used are Dr. Huey, manetti and multiflora.
Plant roses for cut flowers in a separate bed because they must be pruned properly, covered most of the time with protective fungicides, and denuded of old flowers.
Select a sunny site. In the event the bed is located near large plants, protect the rose bushes from marauding roots with a sheet of metal roofing buried vertically along the side of the bed.
Prepare the planting site prior to securing the rose bushes. If the soil is sandy or rocky, remove it to a depth of 15 inches and replace with compost of well-rotted leaves, aged manure, peat moss and soil. For best growth, be sure the material is on the acid side. Roses do better in soils that have a generous amount of organic matter.
Be sure the planting holes are large enough to accommodate the roots without crowding. In the bottom of each hole, mix in a handful of slow-release garden fertilizer. Dip the roots of each plant in water just before planting. Install the new bush so the root system will stand at the same level where it stood in the nursery container.
Keep an eye out for diseases such as black spot and powdery mildew. Insect pests that may harm the bushes include aphids, thrips, cottony-cushion scale, red spider mites and assorted night-feeding rose beetles.
Your garden supply store has specific insecticides and fungicides available. Apply these on a regular program. Garden shops also have fertilizers labeled as “rose food.” These should be applied as directed.
Since roses require continuous feeding and pest control, they are placed in the high maintenance category. High, at least, when compared to the care required by the average flowering shrubs. The requirements also include regular “grooming” or removing unwanted or unsightly parts from rose plants to improve their growth and appearance.
Roses bloom throughout the year in Hawaii. This does not mean that they are constantly covered with flowers, but there are five to seven periods during the year when many flowers open on a plant at once. These peaks of flower production or “flushes” of bloom are separated by periods of four to eight weeks when flowers are opening. It is after each flush of bloom that we find rose bushes in need of grooming.
Approach the grooming job armed with a pair of sharp pruning shears and a container to carry away unwanted parts trimmed from the bush. Spent flowers should be removed by cutting just above a well-formed leaf. Usually the first or second five-leaflet leaf under the flower is a good place to make the cut. Just as in harvesting flowers for indoor use, this trimming to remove spent flowers must include consideration of the development of the new flowering shoots. Making a clean, sloping cut just above the leaf bud will permit its best possible future growth. Long or ragged stubs should be avoided.
Preventing fruit development is one of the reasons for grooming the rose plant after each main period of flowering. If left on the plant, the base to which the petals are attached will usually enlarge, forming a small apple-like fruit called a rose-hip. By removing the faded flowers, food materials needed to form the fruit can be diverted to production of additional flowering growth.
At the same time spent flowers are being removed, the bush should be inspected for any dead wood that can serve as a reservoir for parasitic organisms that cause die-back of rose canes. Dead wood should be cut from the plant whenever it is found. Any shoots developing from the root stock below the graft union also should be cut off.
Picking off yellow or brown leaves is the final step in the practice of grooming. Since these spent leaves will soon fall off naturally, their removal is of less importance than removing dead wood, spent flowers and root suckers. However, picking leaves that have completed their life span and are ready to drop will improve the plant’s appearance.
Grooming, pest control and fertilization are time consuming practices, but your plants will respond by giving you loads of flowers in return.
This information is supplied by the University of Hawaii College of Tropical Agriculture and Human Resources. For Further information, contact the office near you.