Everything You Need to Know About Osiria Roses
If you've ever seen an Osiria rose, then you already know why they're popular with gardeners and flower enthusiasts. "You can identify this rose by a clear color contrast between the exterior color of the rose petal and its interior," says master gardener and landscape designer Andrea Scharff. "It is most often found with a cherry red petal and a white reverse."
However, for as beautiful and interesting as Osiria roses are, growing them is no easy feat. "The Osiria rose can be challenging to grow," Benjamin Godfrey, garden manager at Cornerstone Sonoma, explains. "It's a slow grower that's susceptible to powdery mildew, black spot (fungus), and weak stems; but for those who are patient and take good care of it, the Osiria rose can reward them with florist-worthy large beautiful flowers that have a delightful fragrance."
Interested in growing these unique roses in your own garden? We asked Godfrey and Scharff for their advice about how to plant, care for, and prune Osiria roses for the best blooms.
Plant in full sun.
Since Osiria roses are a relatively short and slow-growing plant, Godfrey says it's best to plant them in an area that receives full sun; you'll also want to choose a spot that is protected from strong winds and pelting rain. "It will also do okay in partial shade, but having more sun will equate to more growth, thicker stems, and more big, beautiful flowers," he explains.
Ensure good soil drainage.
When it comes time to plant your Osiria rose, Godfrey suggests digging a hole in your garden that is one-third to twice as wide, and equally deep, as the container you're removing it from. "Before planting, it's best to slightly mound the soil so it drains well," he says. "Make sure not to plant your Osiria in a low spot, because without good drainage, it is susceptible to root rot." He also recommends planting it so that the trunk is level with the soil around the hole. "If you bury the trunk it will be susceptible to fungal infection," he adds.
Water when the top two or three inches of soil feels dry.
As a rule of thumb, Scharff says if you live in a location with extremely hot temperatures, you should water your Osiria roses every day. "However, on a standard summer day with decent heat, you will need to water every two or three days, and in warm dry weather, you will only need to water about once a week." To ensure you Osirias get thoroughly watered, Godfrey suggests waiting until the top two or three inches of soil is dry between watering.
Fertilize in the early spring and summer.
To encourage vigorous growth and abundant blooms, Scharff says Osiria roses should be fertilized two or three times a year. "Fertilizer applications can be made in early spring, immediately after pruning, during the first bloom period, and in mid to late July—but not after July 31," she says. "In early spring, raking a one-inch layer of well-composted steer manure (or a 2-1-2.4 fertilizer) into the soil around established roses provides the nutrients they need to develop foliage and flowers."
Prune about half an inch above a leaf with five petals.
When pruning or deadheading Osiria roses, Godfrey recommends cutting back one-fourth to half an inch above a leaf that has five petals. "The stem should be at least the width of a pencil so that the new growth will be able to support the weight of a flower," he explains. "Cutting the stem at an angle prevents any water from pooling on the end of the stem, reducing the risk of infection. Choose a five-petal leaf that is facing outward. Cutting above an inward facing leaf results in a cane that crosses other branches and grows into the center of the bush."