August care improves fall rose performance

(08/22/16) HAMMOND, La. – Late summer is the time to begin preparing roses for spectacular fall blooms. Roses need disease management and proper fertilization in August through early September for best fall performance, said LSU AgCenter horticulturist Allen Owings.

“Rose flowering and overall performance in the landscape are not impressive during Louisiana summers, but each year we have the potential to have great fall blooms due to the cooler conditions and typically drier weather,” Owings said.

Flower colors are less intense and blooms are smaller during summer.

“This is simply a function of summer heat,” Owings said. “The best flower color occurs at first bloom in spring and at peak bloom in fall.”

Bloom size also is best during spring and fall. “You will see darker green leaf color, especially if roses are maintained under a good fertilization program,” he said.

Another thing you can do to improve fall rose performance is to prune.

Hybrid tea roses can be pruned back to a height of 30-36 inches. Remove crossing and competing canes and as well as those in the centers of the plants, Owings said.

“This type of cane-removal pruning is generally recommended for late-winter pruning, but it is beneficial in late summer, too,” he said.

Floribunda, grandiflora and landscape shrub roses are typically pruned in late summer to reduce plant height by one-third. In north Louisiana, finish pruning by the end of August. In south Louisiana, finish by the first or second week in September, Owings said.

“Fall blooms will normally peak 45-50 days after pruning, which somewhat depends on growing conditions after pruning,” he said.

In addition to pruning, clear debris from rose beds and pull any weeds. Then add a granular pre-emergent herbicide for weed control and mulch with a 2-inch-to-3-inch layer of baled or shredded pine straw. Any new mulch can just be added on top of old mulch already in the beds. Pine bark and other mulch materials can be used if pine straw is not locally available.

Irrigation also needs to be maintained during droughty periods. Roses need 1 inch of water weekly when rainfall is insufficient, Owings said.

It is possible that insects will appear on roses in fall, but insects are more of an issue in spring and summer. Scout plants once weekly. Spider mites, aphids, flower thrips and cucumber beetles are usually the main problem insects on roses.

A new insect causing major problems on roses in Louisiana is chilli thrips. These are foliage-feeding thrips instead of flower-feeding thrips. They are hard to identify and hard to control once a population is established. Spinosad, an organic insecticide, is one of the better chilli thrips control options.

It is important to continue disease control on roses in late summer and fall.

“If weather is dry, foliage diseases may not be a major problem. But if we have wet weather or if plants are overwatered, are in partial shade or have air circulation issues, disease can occur,” Owings said.

The amount of disease on roses largely depends on the type of rose. Landscape shrub roses rarely need regular fungicide applications, while hybrid teas, the roses most susceptible to blackspot fungus, need spraying on 10-day schedule until the first killing frost, Owings said.

“Be sure to fertilize in late summer, too,” he said. “Most people fertilize at the same time as pruning and mulching.” A slow-release fertilizer will produce nice, uniform foliage growth through September and promote October flowering.

Rose beds that have been regularly fertilized and contain soil high in phosphorus, potassium, calcium and magnesium may need less fertilization than newer beds or beds that have not been regularly fertilized. Liquid feed can also be included to encourage larger blooms in early October.

“Mid-September through October is a great time to plant new roses in your landscape,” Owings said. “Try low-maintenance landscape shrubs like the Knock Out varieties and the Drift series.”

Drift roses are Louisiana Super Plants and are increasingly available at local nurseries in seven flower colors.

Good floribunda varieties include Hot Cocoa, Julia Child, Easy Goin’ and Easy Does It, Owings said. “You also can select lower-maintenance hybrid tea roses, but these are more available at garden centers in the spring.”

The American Rose Society has named Belinda’s Dream and Cinco de Mayo as two great roses for Louisiana.

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Distorted new growth on roses, especially after pruning, is indicative of a chilli thrips infestation. Photo by Allen Owings

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Cinco de Mayo is an American Rose Society selection that does well in Louisiana. Photo by Allen Owings

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Sweet Drift is a popular low-maintenance shrub rose for Louisiana. Photo by Allen Owings

8/22/2016 6:26:33 PM
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