By Anne Skinner
UCCE Master Gardener
Unfortunately, there are a number of insect pests who enjoy roses as much as we gardeners do. In order to have beautiful blooms to enjoy this spring and summer, it is necessary to provide winter care to our rose bushes to thwart those pests.
Various insect pests overwinter in the soil at the base of the rose bush, others leave their eggs in the debris of old leaves and still others burrow into the cane to leave their eggs. Rose leaf diseases, such as powdery mildew and black spot have an advantage in dense foliage, poor air circulation and leaves left on the plant from the previous season.
Dormant rose pruning combined with horticultural oil spray can ruin the party for these pests and diseases. While the temperature in the Central Valley winter isn’t cold for long enough to cause the rose bush to become completely dormant, our bushes do demonstrate slowing of growth and reduced blooms. It is healthy for the plant to have a period of time to only produce a good root system during our “rainy” season. We can foster this by first allowing the rose blooms to die on the bush starting in November. This allows the plant to produce rose hips and slow new flower production.
Winter rose pruning should be done in December or January and be completed before Feb. 14: If winter rose pruning is done too early in the fall, it stimulates new growth in the still warm days and this tender growth can be damaged by a frost in December. If significant pruning is done after mid-February, it forces the plant to produce this year’s growth twice, stunting the plant.
Rose pruning involves science and artistry to create beautiful blooms the following spring: The basic steps of rose pruning are outlined in early in the fall, it stimulates new growth in the still warm days and this tender growth can be damaged by a frost in December. If significant pruning is done after mid-February, it forces the plant to produce this year’s growth twice, stunting the plant.
Rose pruning involves science and artistry to create beautiful blooms the following spring: The basic steps of rose pruning are outlined in Ten Steps to Beautiful Roses on the Master Gardener web site. Included are pictures of correct pruning cuts and guidelines to promote strong canes and shaping the plant to promote health and vigor. You will feel more confident pruning the roses in your own garden after reading the materials available, in addition to attending a rose pruning demonstration. Bring your gloves and clippers to work with the Master Gardeners and you will develop your skills at pruning. Rose bushes don’t grow in “by the book” shapes and are affected by various environmental stresses. When the Master Gardeners do pruning on the roses at the demonstration gardens, we frequently ask each other for opinions on pruning a particular plant. We hold the public pruning demonstrations to give you an opportunity to ask us questions.
What types of insects and diseases can be reduced by winter rose pruning?: Some insects use the rose cane as protection from cold and birds. The flathead borer beetle lays eggs on stressed plants, such as those with sunburn or disease wounds. The raspberry horntail is a wasp-like insect whose larvae caterpillars develop within the rose cane. When rose pruning, cut off all the infested cane, which will be brown and or hollow, until healthy tissue is reached.
Leaf beetles, some aphids and leafhoppers may overwinter as eggs, larvae or adults burrowing into the soil and leaf litter or hiding under leaves on the plant. Caterpillars and scale insects create a protective cocoon against the rose cane. Clearing away the debris at the base of the plant and removing all leaves from the plant eliminates their hiding places. Cocoons can be manually removed or washed off the cane with a strong stream of water.
Application of horticultural oil after pruning will also reduce these insects.
Some overwintering eggs need to be left alone, such as the egg case of the praying mantis. This beneficial insect has its eggs positioned to hatch in spring right where there will be pest insects for food. The lady beetle, which has larvae with a voracious appetite for aphids, hibernates as an adult at higher elevations in stumps or under rocks. They spend the winter as a collective to share heat. Once the aphid population starts to climb in spring, the lady beetle will come to feast and lay eggs near the aphid infestation.
Diseases such as black spot and powdery mildew, both fungal diseases, are more common in rose plants with dense leafy growth in the center of the plant, which impedes drying of the leaves. Wet leaves in the shaded interior of the plant provide an environment for fungal growth. When rose pruning, creating a vase shape, with an open center, allows for better air circulation as the plant develops new leafy growth.
The last step in dormant rose pruning is removing any remaining leaves from the plant and raking the debris of old leaves and mulch from the base of the plant: This exposes overwintering pests and their eggs, which then can be doused with a horticultural oil spray. Horticultural oil is a light oil formulated for use on plants and is available at garden suppliers. Beneficial insects are not harmed by oil spray used in winter months, nor does it harm the leafless plant. In the newspaper article archive on the Master Gardener web site, there is an article with the details of using horticultural oil on plants.
Spending some time out in the garden is pleasant on a winter day and promotes a lovely garden come spring.
Healthy, vigorous rose bushes full of beautiful blooms in spring will be your reward for a bit of time spent in winter pruning. We hope to see you at one of the pruning demonstrations.
Free rose pruning demonstrations will be held on Saturday, Jan. 19 at the Tulare County Courthouse Rose Garden and the Ralph Moore Rose Garden from 11 a.m. to 2 p.m. and at the Grangeville Church in Hanford on Monday, Jan. 21 from 11 a.m. to 2 p.m. The public is welcome to come ask the Master Gardeners questions, see a tool care demonstration, learn how to plant bare root roses, and work with Master Gardeners to become more confident in your rose pruning skills.