News Release Distributed 08/23/13
By Allen Owings
LSU AgCenter horticulturist
HAMMOND, La. – Late summer is the time to begin preparations for spectacular fall blooms on your roses. Roses need disease management and proper fertilization in August through early September for best fall performance.
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 at that time of year.
You will notice during summer that the flower colors are less intense and blooms are smaller. This is simply a function of summer heat. The best flower color occurs at first bloom in spring and at peak bloom in fall. Size also is best at these times of year. You will see darker green leaf color, especially if roses are maintained under a good fertilization program.
Other things you can do to improve fall rose performance include pruning.
Hybrid tea roses can be pruned back to a height of 30-36 inches. Remove crossing and competing canes and remove canes in the centers of the plants. This thinning type of cane-removal pruning is generally recommended for late-winter pruning, but it is 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, complete pruning by the end of August. In south Louisiana, complete pruning by the first or second week in September. Fall blooms will normally peak 45-50 days after pruning – this is somewhat dependent on growing conditions after pruning, however.
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-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. This year we have been dry at times and wet at times. Roses need 1 inch of water weekly when rainfall is lacking.
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 chilly 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 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 rainfall or overwater or if plants are in partial shade or have air circulation issues, disease can occur. The amount of disease on roses largely depends on the kind of roses you have. 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.
Be sure to fertilize in late summer, too. 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 bloom size in early October.
If soil pH is wrong in your rose beds, fall is a great time to address this issue. The ideal soil pH for roses is 6.2-6.5.
Mid-September through October is a great time to plant new roses in your landscape. 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 the spring. The new Drift series roses are also great plants and are increasingly available at local nurseries in seven flower colors.
You can see more about work being done in landscape horticulture by visiting the LSU AgCenter Hammond Research Station website. Also, like us on Facebook. You can find an abundance of landscape information for both home gardeners and industry professionals at both sites.