By James R. Feucht, Colorado State University Cooperative Extension specialist, Landscape Plants
Roses have peaked for the year, but with a little care your bushes will continue to bloom while they also prepare for cold weather. While winter seems a long way off, roses need several months of pre-conditioning for freezing weather.
Remove dead or wilted flowers and weak or diseased canes. This helps keep the bushes strong by diverting all food energy to root, rather than fruit, production. To remove canes, make the cut just above a strong side branch or a leaf that has five leaflets.
You won't need a high-nitrogen fertilizer at this time of year, if your bushes are deep green and are producing stout healthy stems. Late fertilizing with high nitrogen induces too much soft, succulent growth in the latter part of the season. This growth will not survive Colorado winters.
If your roses are yellowed, however, and are producing weak growth, you can increase their vigor by applying a low-nitrogen (5-l0-5) fertilizer. (The first number on a fertilizer bag is the percent of nitrogen, the second is the percent of phosphorus and the third, the percent of water-soluble potash.) Failure to thrive and continued yellowing, also may signal iron deficiency. Correct this by spraying plants with liquid iron available under various brand names or by dissolving four tablespoons of ferrous sulfate and two tablespoons of detergent in three gallons of water. Apply during the cool of the day. You may want to apply ferrous sulfate to the soil. Spread ten pounds of the material for every 100 square feet of ground, cultivate and water.
Regular cultivation promotes healthy roses. Loosen soil frequently to aerate, keep down weeds and provide better utilization of the ground moisture.
To help roses "harden off" for winter, avoid frequent light waterings and, as summer progresses, gradually withhold water for longer periods. Don't withhold water so long, however, that the plants actually wilt.
Control insect pests and fungus diseases throughout the summer. Most of today's rose pest sprays, if used regularly and according to directions, will guard against the major insect problems. Many are a combination insecticide and fungicide and thus will control insect pests, mildew and black spot. Mildew, this area's most frequent fungus disease in roses, seriously weakens the bushes and reduces their ability to withstand winter.
Thin out climber and ramble roses at this time of year. After flowering, remove side shoots that have flowered and all canes three years or older.
At this time, young canes will develop from or near the plant's base. Train these to replace the older canes, taking care to prevent damage to the young growth. Avoid high-nitrogen fertilizers at this time. Climbers, however, frequently yellow from iron deficiency, and treatment may be necessary.
Photograph courtesy of Judy Sedbrook.
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010