Iowa gardeners successfully grow a wide variety of roses in sunny garden areas. This week Iowa State University Extension garden experts respond to common questions related to growing and caring for roses. Gardeners with additional questions can contact the experts by emailing or calling the ISU Extension horticulture hotline at email@example.com or 515-294-3108.
Modern roses, such as hybrid tea, grandiflora and floribunda roses, are propagated by budding. A single bud is removed from the desired variety and inserted onto a hardy rootstock. The bud union (the knob-like swelling at the base of the canes) is sensitive to extreme cold and rapid temperature changes in winter. Modern roses are essentially destroyed if all growth above the bud union is destroyed in winter. When planting hybrid tea, grandiflora and floribunda roses, the bud union should be positioned 2 to 3 inches below the soil surface to protect the bud union and lower portions of the canes from winter damage.
To encourage vigorous growth and abundant bloom, hybrid tea, floribunda and grandiflora roses should be fertilized two or three times a year. Fertilizer applications can be made in early spring (immediately after pruning), during the first bloom period and mid to late July. Do not fertilize after July 31. Later fertilization will produce succulent new growth that may not harden sufficiently before winter. An all-purpose garden fertilizer, such as 10-10-10, should produce excellent results. Sprinkle 1/4 cup of fertilizer around each plant per application.
Blackspot is a common fungal disease of roses. Symptoms of blackspot are circular black spots on the leaves. Infected leaves turn yellow and drop prematurely. Initially, symptoms develop on the lower leaves and gradually move upward. By late summer, severely infected plants may be nearly defoliated.
The blackspot fungus overwinters on fallen leaves and infected canes. Spores are splashed on to newly emerging foliage in spring. Blackspot development is favored by warm (75 F), wet weather.
Careful rose selection, cultural practices and fungicide treatments can be used to control blackspot on roses. Rose varieties differ widely in their susceptibility to blackspot. When purchasing roses, select rose varieties that are resistant to blackspot. When selecting a planting site, choose a site that receives full sun and provides good air movement. Full sun and good air movement promote drying of the rose foliage and discourage blackspot infections. Reduce the amount of overwintering fungi by carefully cleaning up the leaf debris in fall. When watering roses, apply water directly to the ground around the plants. Do not wet the foliage. Fungicide applications must begin at the first sign of disease symptoms.
Modern roses, such as hybrid teas, floribundas and grandifloras, require watering during hot, dry weather. The frequency depends upon weather conditions and soil type. In most gardens, a thorough watering every seven to 10 days during dry weather is sufficient. If possible, apply the water directly to the soil around each plant. Overhead watering wets the foliage and increases disease problems. If overhead watering is unavoidable, morning is the best time to water roses. Morning applications allow the foliage to dry quickly.
An excellent way to conserve soil moisture is by mulching. Possible mulches include wood chips, shredded bark, pine needles and cocoa bean hulls. Spread 2 to 4 inches of mulch around each rose or over the entire bed. Mulches also help to control weeds.
Deadheading or the removal of faded flowers is done to encourage additional bloom on hybrid tea and other repeat-flowering roses. Hybrid tea roses usually have one or two 3-leaflet leaves immediately below the flower. Next (lower down on the stem) are two or more 5-leaflet leaves. The deadheading procedure is slightly different for newly planted and established roses. During their first growing season, it’s usually recommended that the spent flower be removed above the uppermost 3-leaflet leaf. Removal of a larger percentage of the rose’s foliage reduces the plant’s ability to manufacture food and slows growth. When deadheading established roses, the stem may be cut back to a 5-leaflet leaf. Retain at least two 5-leaflet leaves on each shoot. Use sharp tools (hand shears or knife) to remove faded flowers. Cut about 1/4 inch above an outward facing bud and leaflet with the cut made parallel to the angle of the leaflet.