By LSU AgCenter Horticulturists Dan Gill, Kyle Huffstickler and Allen Owings
Late August and early September are the time to begin preparing for fall blooms on your roses. Rose flowering and overall performance aren’t great during Louisiana summers, but each year we have the potential to have great fall blooms due to the cooler conditions and typically drier weather.
During summer the flower colors on roses are less intense and the blooms are smaller. This is simply a function of summer heat. The best flower color on roses occurs at first bloom in spring and at peak bloom in fall. Size is also best at these times of the year. You will see foliage color become darker green, too, especially if roses are maintained with a good fertilization program.
Hybrid tea roses can be pruned back to a height of 30-36 inches. Remove crossing and competing canes, and remove canes in the center of the plants. This thinning-type of cane removal is generally recommended for late-winter pruning, but it’s beneficial in late summer, too.
Floribunda, grandiflora and landscape shrub roses are typically pruned in late summer to reduce plant height by one-third.
In north Louisiana, finish rose pruning by the end of August. In south Louisiana, complete pruning by the first or second week in September. Fall blooms normally will peak 45-50 days after pruning, although this somewhat depends on growing conditions.
In conjunction with pruning, clear debris from rose beds and pull any weeds that may be present. Add a granular, pre-emergent herbicide for weed control and mulch with a 2- 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 available.
Be sure to fertilize, too. Most people fertilize when they prune and mulch. A recommended rate of 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 also can be included to encourage larger bloom size in early October. If your soil pH is wrong, fall is a good time to address this issue. The ideal soil pH for roses is 6.2-6.5.
Irrigation also needs to be maintained during droughty periods. We have been dry many times this year and wet at other times. Roses need 1 inch of irrigation weekly when rainfall is lacking.
Although it’s possible insects will appear on roses in the fall, they are more of an issue in spring and summer. Examine your roses once weekly. Spider mites, aphids, flower thrips and cucumber beetles are usually the main problem insects.
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.
It’s 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 significant rainfall or overwater, have plants in partial shade or have air circulation issues, disease will be present.
The amount of disease you have on roses largely depends on the kind of roses you grow. Landscape shrub roses rarely need regular fungicide applications, but the roses most susceptible to black spot fungus – hybrid teas – need spraying on a 10-day schedule until the first killing frost.
Late summer also is a good time to plant new roses. Try low-maintenance landscape shrubs like the Knock Out varieties and Home Run. Good floribundas include Cinco de Mayo, Hot Cocoa, Julia Child, Easy Does It and Easy Goin’. You also can select lower-maintenance hybrid tea roses, but these are more available at garden centers in spring.
The LSU AgCenter will be naming a Louisiana Super Plant rose variety this fall. Watch for the announcement in September.