Yard and Garden: Rose care FAQs
AMES, Iowa — Roses are a classic flower, loved the around the world, and this week’s Yard and Garden answers common questions about rose care with expert knowledge from horticulture specialists Cynthia Haynes, Laura Iles and Lina Rodriguez-Salamanca of Iowa State University Extension and Outreach.
Question: My rose leaves have a lot of holes in them but I don’t see any Japanese beetles. What are the small green caterpillar-like things on the leaves?
Answer: Your rose may be infested with one of the two rose sawfly species that most commonly feed on roses in Iowa, the roseslug or the bristly roseslug. Rose sawfly larvae have tapered bodies, may be up to 1/2 inch in length, and are pale green in color. The larvae somewhat resemble slugs, hence the common name of roseslug, but also sawflies are frequently mistaken for caterpillars.
Rose sawfly larvae usually feed on the undersides of the rose leaves. They consume most of the green tissue of the leaf, leaving behind a thin layer of tissue and the veins. The thin layer of tissue that remains eventually turns light brown. Foliage damaged by roseslugs has a window-pane or skeletonized appearance. This window-pane damage helps differentiate rose sawfly feeding from Japanese beetle feeding damage.
Roseslugs weaken affected plants. However, the damage is mainly aesthetic. The damaged plants will continue to grow and should look better later in the growing season.
Small numbers of roseslugs can be picked off by hand and destroyed. Larger infestations can be controlled with insecticides, such as insecticidal soap, carbaryl (Sevin) or permethrin (Eight). Note that you must see larvae for treatment to be effective; if you see only damage there is no need to treat. The bristly roseslug can have several generations per year, so watch out for new damage, but the roseslug has only one generation, so damage will not reoccur.
Q: How can I control black spot on my roses?
A: Black spot is a common fungal disease of roses. Symptoms of black spot are circular black spots on the lower leaves and gradually move upward. Infected leaves turn yellow and drop prematurely. By late summer, severely infected plants may be nearly defoliated.
The blackspot fungus overwinters on fallen leaves and infected canes. Spores are water-splashed onto newly emerging foliage in spring. Black spot development is favored by warm, wet weather.
Careful rose variety selection, cultural practices, and fungicide treatments can be used to manage black spot 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 rose foliage and discourage black spot 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 prior to disease symptom development on susceptible rose varieties. For more information on rose diseases, see the free to download publication Common Rose Diseases, found at https://bit.ly/3po9g2j.
Q: There are half-circles missing from the edges of my rose leaves. What is responsible for the damage?
A: Leafcutting bees are probably responsible for the missing leaf tissue. Leafcutting bees resemble honey bees, but are often darker in color. Female leafcutting bees make nests in rotted wood or the stems of plants. The sides of the nesting cavities are lined with round pieces of foliage. After lining the cavities with leaf discs, pollen and nectar are placed in the nest cells to serve as food for the immature bees.
Leafcutting bees may remove discs of foliage from many plants. However, they prefer rose, green ash, redbud, lilac and Virginia creeper. Holes in the leaves are typically 1/2 inch or less in diameter. The cuts are clean, as if they were cut out with a scissor.
Leafcutting bees are beneficial pollinators. Damage to roses and other plants is usually minor. Control efforts are rarely justified or necessary.
Q: What is the proper way to deadhead roses?
A: 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 three-leaflet leaves immediately below the flower. Next (lower down on the stem) are two or more five-leaflet leaves. The deadheading procedure is 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 three-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 five-leaflet leaf. Retain at least two five-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.
Q: How often should I fertilize hybrid tea roses?
A: 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, is a suitable fertilizer for roses. Sprinkle 1/4 cup of fertilizer around each plant per application.
Q: How often should I water my roses?
A: 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.
Q: Japanese beetles are devouring my roses. What can I do?
A: Japanese beetles eat the foliage, fruits and flowers of over 300 plants. When feeding on foliage, the beetles consume the tissue between the veins, leaving a lace-like skeleton. Flowers and fruits are sometimes devoured completely. Roses, raspberries, grapevines, crabapples, birches and lindens are some of their favorite food hosts.
Adult beetles are present for about six weeks every summer. The adult beetles begin to emerge from the ground in late June and new adults continue to appear throughout July. Each beetle lives from 30 to 45 days.
Reducing the damage caused by Japanese beetles is difficult; the good news is that the defoliation usually does not harm the plant's health, just its appearance. Since most of us grow plants for their appearance, for the plants to be attractive we may try to limit feeding. Persistence, diligence and repeated efforts are necessary because new beetles emerge every day over a period of several weeks. Handpicking or screening of high-value plants may be of benefit in isolated situations with limited numbers of beetles. Remove beetles early and often to preserve the beauty of the plants and to reduce the attraction of more beetles. Remove beetles early in the morning while temperatures are cool and the beetles are sluggish. Collect or shake beetles into a bucket of soapy water and discard.
Spot spraying infested foliage of high value plants with a labeled garden insecticide may reduce damage, but these insecticides have the potential to harm pollinators or beneficial predators and parasites. Be sure to read and follow the labeled directions, which will include information on restrictions to minimize risk to pollinators.
Japanese beetle traps are widely available, but have been shown to be ineffective in controlling moderate to heavy infestations. In fact, they may attract more beetles than they catch!