Answers to questions on pruning, overwintering roses and more.
A. The black spots are a disease known as Black spot and can be prevented by either spraying with Copper Spray, Benomyl or with a mixture of baking soda and water. Mix the baking soda and water at a rate of 1 tablespoon to one gallon of water mix and spray thoroughly. It is best to do any spraying before symptoms are noticed. Keep your roses pruned properly to allow for maximum air circulation and always water early in the morning so that the leaves can dry by mid day. For more information click here.
A. This is one of the worst rose diseases: blackspot. A fungal infection, it causes the leaves to get these black spots with fringed margins which then turn yellow and fall off. Rose bushes are defoliated from the lower leaves upward. It is most prevalent on warm humid days at temperatures between 20-24 degrees Celsius. The spores are spread by splashing water, and quickly develop when they land on wet leaves, so try to avoid wetting the foliage by watering from the base of the plant. Pick up and dispose of fallen leaves as they will harbor the fungal spores. Spray your plants with Garden Sulfur as a control measure.
A. These distinct holes are most often caused by the larvae of rose sawflies, which eat holes in leaves from the undersides, causing a skeletonized effect. Most gardeners are usually unaware of their presence as they are small and difficult to spot. The damage they cause can be very rapid, so quick action is very imperative. Doktor Doom House and Garden is one of the best controls for rose sawfly.
A. This is powdery mildew, a fungus that grows on the surface of foliage, coats the plant with white felt-like spores, causes leaves to curl up, and makes flower buds die without opening. The disease spreads most quickly when evenings are cool and the air is humid and stagnant (not moving). Fungal spores germinate rapidly on leaves that are moist, so do whatever you can to keep the foliage dry. Spacing your rose bushes far enough apart to allow good air circulation and keeping them as vigorous as possible will help to deter the problem. As a preventative, spray with a fungicide such as Garden Sulfur, Later’s Folpet, or Bordo Copper Spray.
A. These spots are caused by a fungal disease, called rust, which covers leaves with tiny spore cases that resemble curry powder. Affected leaves always wilt and then fall off. Rust can defoliate and seriously weaken or even kill rose bushes. The disease occurs most frequently at temperatures between 18-21 degrees Celsius accompanied by 4-12 hours of rain, fog, or dew. At the first sign of rust, remove and destroy all infected leaves. Spray plants with Garden Sulfur as a control measure.
A. These are rose galls which are caused by certain species of wasps. The wasps secrete chemicals that cause the swelling, and each gall has a characteristic shape that identifies the wasp species. As eggs hatch and larvae begin to grow, the galls increase in size. Prune off and destroy infected canes (stems) to eliminate the larvae before they emerge to complete their life cycle.
A. Spider mites, related to spiders, are minute pests that suck juices from the undersides of leaves. They are most prevalent in hot, dry weather. If you do not have a major infestation, spray the undersides of the plant with a strong spray of water from a garden hose. If the problem is more severe, use an insecticidal soap or Doktor Doom House and Garden Spray, making sure that you spray the undersides of the leaves as that’s where the spider mites are hiding.
A. These tiny holes are caused by the rose weevil (rose curculio), a very small red and black insect about ¼ inch (5 mm) in length. With their long snouts rose weevils bore into flower buds to feed and lay their eggs. Remove and destroy all affected flowerbuds. An occasional light spraying with Doktor Doom House and Garden Spray over all flower buds can help to alleviate the problem. Some Rugosa roses appear to be susceptible to curculio beetle infestations.
A. Harmless, leafcutter bees cut precise ovals or near-perfect circles out of leaves to line their nests and cap their egg cells. Leafcutter bees are important pollinators of plants such as alfalfa, clover, and forage crops, so no chemical controls should be used.
A. In all likelihood, this is being caused by rose thrips, tiny slender insects about 1/8 inch in length. Thrips can damage, distort, or completely destroy opening flower buds. If you suspect thrips, try this: cut off a couple of flowers, give them a hard rap overtop of a piece of white paper, and see if any thrips fall out. You can then spray your rose bushes with an insecticide such as Sevin or Doktor Doom House and Garden Spray. It is a good idea to cut off and destroy the affected flowers.
A. Wait until mid-May when new growth is evident and the earliest leaves are beginning to emerge. Remove only the dead canes (branches) and branch tips. A general rule of thumb is to cut the roses back to a height of 8 to 10 inches above the ground. Using a pair of sharp garden shears, always make your pruning cut about ¼ of an inch above an outward-facing bud, to avoid ending up with crossed-over, inward-facing canes, a situation that can promote disease.
A. Roses require a minimum of 7-8 hours of direct sunlight for good flower production, so although a south-facing location is the best situation, an eastern exposure will do, provided the light is direct and not filtered. With 10 or more hours of direct sunlight, your roses will flourish to their maximum potential.
A. Aphids are small, soft-bodied insects that suck plant juices from flower buds or new leaves. Total eradication is almost impossible as they multiply so prolifically, but you can control their numbers by using Doktor Doom House and Garden Spray or any of the different insecticidal soaps. Ladybugs and their larvae are excellent biological controls.
A. Wait until late in the fall when the rose becomes dormant, and then put it into storage, pot and all, in a cold-room or slightly heated garage. To keep the rose dormant throughout the winter, it is imperative that the temperature remains constant, between 23-32 degrees F (-5 to 0 degrees Celsius). Roses can also be overwintered in an unheated, drywalled, attached garage provided the temperature does not fall below -7 degrees C for an extended period of time. To achieve maximum protection, place the rose adjacent to the garage wall which attaches to the dwelling (the house wall). Soak the pot thoroughly just before you put it into storage and if it dries out before spring, give it a light watering.
A. Almost all of the low-maintenance, hardy shrub roses from the Explorer or Parkland Series will bloom continuously throughout the summer. That’s what makes them so popular with gardeners. Some, like “Winnipeg Parks” will even bloom after the first mild frosts of autumn, well into October. However, freezing temperatures around -8 degrees Celsius or lower the buds will not open.
Q. How do I encourage my roses to continue blooming throughout the summer? Do I prune off the rose hips?
A. Regular deadheading (removal of spent/dead blooms) will encourage reblooming throughout the summer months. To prepare the roses for the onset of fall and winter, do not deadhead after the end of August. Many of the hardy shrub roses such as rugosas produce large, red rose hips that add winter interest. The Red-leaf rose blooms only once so it should not be dead-headed if you want the bright red-purple rose hips for winter interest.
A. A common rose fertilizer is 28-14-14. Other fertilizers that one can apply are 20-20-20 and 15-30-15. On average, roses should be fertilized once a month, in late May, late June, and late July. The latest applications of fertilizer should be in mid-August to allow the rose bushes to slow down growth in preparation for winter. There are two types of fertilizers: granular fertilizers, which are added directly to soil, and water-soluble fertilizers, which you mix with water. Granular fertilizers work in a slow-release fashion, while water-soluble fertilizers release their nutrients quickly and contribute to more rapid growth than the granular fertilizers.
A. The most preferred soil for roses is loam: a balanced mixture of sand, silt, clay, and organic matter. Loam is well-drained soil, which means that water will neither puddle on top nor drain through too quickly. It is advisable to work lots of organic matter, such as peat moss, well-rotted manure or compost, into your soil before planting to improve both water and nutrient retention.