Why rose petal blackening is becoming a strategic question for growers

The busy season is on the way. Roses have to be beautiful. Climate has to be just right. Petals should be perfect.

Perfect flowers are no niche crop for Colombians. Flowers are our sixth largest export – at a value of more than US $2 billion in 2023.

But something feels different this year. Growers are reconsidering what they can tolerate in the way of quality issues.

Quality problems add to labor costs at a time when growers face some major challenges to their margins.

Not only is there a 10% import tariff in their most important market, the US, but the dollar has lost 12% of its value versus the Colombian peso over the last year. If that weren’t enough, Colombia has increased the minimum wage by almost a quarter.

Petal blackening, black edges or petal necrosis is one of the key quality challenges growers face.

In the popular Freedom rose cultivar, blackening is primarily addressed by applying a plastic bud net to the rose button.

It’s a delicate process. There’s a lot of manual work involved in applying the net, which protects or limits blackening to the outermost petal.

So what causes petal blackening and could there be other, less labor-intensive, ways to address it?

Petal blackening is often associated by growers with periods of intense radiation, especially in high-elevation production zones. But the research does not point to one simple agreed cause. In roses, studies and grower observations have linked the disorder variously to low minimum temperatures, cultivar sensitivity, and, in some cases, UV or radiation exposure.

In my experience, high radiation with high UV intensity, accompanied by large temperature deltas between day and night, intensifies the incidence of blackening.

Once the right cultivar has been selected, the most practical areas to focus on are temperature history and light/radiation conditions. Nutritional aspects have also been explored, but remain less conclusive. 

Where temperature is concerned, growers will want to have a way to avoid heat drops during the key weeks before blooms are cut. As for light, there needs to be consideration of how to protect blooms from sudden and unpredictable increases.

"Blackening decreased, the color of the petals intensified, and both the stem-length and the size and characteristics of the leaves improved"

Another practical adaptation that I am seeing is the use of Climate Screens to achieve the best PAR light, while also blocking ultra-violet rays. There is the additional benefit that temperature deltas are reduced.

In the image above, roses grown using traditional cultivation on the left are compared with roses grown under Harmony 2047 FR screens on the right. 

One grower I work with in Cayambe in Ecuador installed the Harmony 2047 FR screen during the first quarter of 2025. The grower told me that blackening decreased, the color of the petals intensified, and both the stem-length and the size and characteristics of the leaves improved.

Svensson is going further in 2026, starting trials to address petal blackening using a more transparent energy screen. The goal is to stabilize the temperature deltas, take away the most extreme radiation and all the ultra-violet exposure, but also help with one other issue.

The plants need as much energy as they can get at the far end of the growing season. The brightness of the energy screen will help here.

We hope we’ll have more to share soon about these exciting trials, which could, we hope, provide a less labor-intensive solution to petal blackening, which also promotes quality and an increase in productivity.