Horticulturist: Take care of roses all summer


LSU AgCenter


HAMMOND — Spring blooms on roses came late this year due to unusually cool conditions in March and April. But May flowering has been outstanding throughout Louisiana.

In most home landscapes, roses perform their best at first flowering in spring and at fall bloom in October.

Summer will be here before we know it, and it’s important to think about rose care in June, July and August. Unfortunately, the plants will not necessarily look their best during the hottest part of summer.

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

During summer, rose flower colors are less intense and blooms are smaller; petal count will decrease. A Knock Out rose with 10 to 12 petals in mid-spring, for example, may only have 5 to 6 petals in late summer.

You can also see petal counts reduced on hybrid tea roses. They typically have 40 to 50 petals per flower at peak bloom. But petal numbers are not quite as high in summer, and flower size is smaller.

Also, color pigmentation is not as good during summer due to high temperatures and less favorable growing conditions.

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.

You may see foliage burn due to fungicide application during summer.

Fertilizer should generally not be applied during the middle of summer, but a light application in late May through mid-June may be of value.

Irrigation also needs to be maintained during droughty periods. Apply water to the mulched root zone area around the plants; avoid getting the leaves wet.

Don’t 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 to 45 days after pruning.

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

—Watching roses for insect pests. Spider mites, aphids, thrips and cucumber beetles are usually the main problem insects on roses. Some insects can be easily controlled; some are more difficult to control. Monitor plants weekly for insect infestations. Also be aware of the foliage-feeding chilli thrips, although these are most prevalent in late summer and fall after pruning is completed.

—Clearing debris from rose beds and removing any weeds. You can use pre-emergent granular herbicide such as Preen or Amaze for summer weed control.

—Adding 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.

You can see more about work being done in landscape horticulture by visiting the LSU AgCenter Hammond Research Station website at www.lsuagcenter.com/ham mond.

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