Summer heat and drought stress can cause roses to lose their visual appeal. (Photo by Allen Owings. Click on photo for downloadable image.)

News Release Distributed 06/01/12

By LSU AgCenter Horticulturists Dan Gill, Kyle Huffstickler and Allen Owings

Spring bloom on roses is just about completed. Roses in most home landscapes have their best bloom performance at first flowering in the spring (mid-April) and at the fall bloom in October.

We are now rapidly approaching the summer months. Rose bushes will not necessarily look their best during summer. This is especially true as we get to July and August. We should be thinking now about how to maintain our roses through summer to enjoy them to their fullest.

Most gardeners now are planting landscape or shrub roses in abundance. This includes the popular Knock Out varieties. However, many home gardeners in Louisiana still enjoy growing the traditional modern roses like hybrid tea, floribunda and grandiflora varieties.

During summer, flower colors on roses are less intense and blooms are smaller. Petal count will decrease. A Knock Out rose with 10-12 petals in midspring may only have five to six petals in late summer.

You can also see petal count reduction on hybrid tea roses. They typically have 40-50 petals per flower, but petal numbers are not quite as high in summer, and flower size is smaller. Also, flower pigmentation is not as good during summer due to the high temperatures and unfavorable growing conditions.

Heat stress was common on all roses last summer with exceptionally high temperatures and drought conditions from May through September. Heat stress can be identified by flower performance, but also just by an overall reduced, almost stagnant, growth rate and pale green foliage. Leaf crinkling can occur when heat stress is severe.

Roses need disease management from now through the end of summer for good flowering and performance into fall. Normally, the varieties with high to moderate susceptibility to blackspot disease need to continue to be sprayed with fungicide on a 10-to-14-day schedule. Even though it is hotter than ideal for the blackspot fungus to reproduce as fast in summer, the disease inoculum from spring will still be present.

You may see foliage burn due to fungicide application during summer. Fertilizer should probably not be applied during the middle of summer, but a light application may be of value in some situations. Irrigation also needs to be maintained during droughty periods. Apply water to the mulched, root zone area around the plants. Avoid getting water on the plant foliage.

Do not prune roses in June and July other than taking off old flowers as they fade. If you continually “deadhead” roses during summer, you’ll need to do less pruning later. The recommended late-summer pruning of modern rose varieties such as hybrid tea, floribunda, grandiflora and shrub roses should be completed by early September in south Louisiana. Bloom will return on the pruned bushes 40-45 days after pruning.

Other rose gardening work that needs to be done during summer includes:

– Watch roses for insect pests. Spider mites, aphids, thrips and cucumber beetles are usually the main problem insects on roses. Some can be easily controlled; some are more difficult to control. Monitor your plants weekly for insect infestations.

– Clear debris from rose beds and pull any weeds that may be present. Add pre-emergent granular herbicide such as Preen or Amaze for summer weed control.

– Add new mulch if you did not refresh the bed earlier in the year (pine straw is an excellent material). Even if you did add mulch earlier in the season, a new application on top of the older mulch may be beneficial. Three inches of pine straw is ideal.

Visit LaHouse in Baton Rouge to see sustainable landscape practices in action. The home and landscape resource center is near the intersection of Burbank Drive and Nicholson Drive (Louisiana Highway 30) in Baton Rouge, across the street from the LSU baseball stadium. For more information, go to www.lsuagcenter.com/lahouse or www.lsuagcenter.com/lyn.