The complete spectrum of colors available in sunlight is not used for plant growth. Different wavelengths of the light spectrum or colors promote different stages of plant growth. The Rose has its unique combination of light needs.
Stages in a Plant’s Life
There are two main stages in a rose plant’s life, and these are both actively regulated by growers. One is the vegetative stage where the plant germinates, grows, and develops its morphology, i.e., its root and aerial systems. The flowering stage is the next stage. Horticulturists and gardening enthusiasts are interested in controlling growth characteristics such as stem length, compact branching, and flowering.
Photomorphogenesis or ‘light regulated development’ is an important and common feature in all plants (1). While there are some common thumb rules on usage of grow lights to maximize each stage, each species has its dos and don’ts.
As in all species, in case of rose too, blue light (430nm-450nm) and red light (640nm-680nm) are necessary to break seed dormancy and facilitate germination of the seed. Keep far-red light (720 – 740 nm) amounts low at this point of the plants’ development, as this wave-length tends to inhibit seed germination (2).
Once, the seed has germinated the seedling benefits from a mixture of blue, red, far-red and green lights. Blue grow lights are associated with vegetative growth. However, adding other light spectrum produces a well-balanced growth. In fact, overexposure to blue light alone can make a plant stunted.
Green light (500 – 600 nm) and far-red are both needed to ensure that light penetrates to the lower parts of the plant and help them grow leaves. Far-red light in encourages larger leaves, while red light produces thick leaves. Adding up to 24% of green light improves overall growth of the plant, and also increases the height of the plant. However, increasing green beyond this level can limit vegetative growth. Earlier it was believed that green light was not necessary, however various studies have shown these to be untrue (2, 3).
Ideally, the rose bushes for cut flowers should produce a number of stems that are long, and bear bigger flower buds (4). According to a 2016 scientific review, increasing the amount of far-red light can increase internode length by 21 to 38% in rose plants, as plants associate far-red light spectrum with shade. However, it is also the R to FR (red to far-red) ratio that is important. To ensure that the stems are strong, higher amounts of red and lower amounts of far-red light are optimum (5).
More far-red light will also increase the number of buds produced. Bud production at the top of the plant is increased by 17%, while overall development buds can increase by up to 100%, depending on the season. Buds are important as they lead to branching. This effect is desirable both in cases of producing more stems of cut roses as well as rose bushes for sale (5).
Thus, it is clear that vegetative growth and proper formation during the vegetative state requires the right combination of these four lights.
Photosynthesis and biomass accumulation
For plant growth to occur the most important physiological activity to consider is photosynthesis. Plants use chlorophyll, the green pigment available in leaves, to capture energy from light to combine water and carbon dioxide to produce simple sugars. These sugars are then used to produce all the other complex molecules needed for the plants such as proteins, carbohydrates, etc.
Blue and red light both contribute to the production of chlorophyll, and blue also affects other processes of photosynthesis (6). So both lights are necessary for a plant to accumulate the necessary the biomass for the plant. Plants have been shown to grow better when combinations of these two lights are used instead of just one of them (3). Green light, on the other hand, can limit photosynthesis but still does not reduce biomass accumulation (7).
Healthy and robust vegetative growth is essential to ensure abundant flowers. So during the vegetative stage, it is best to choose high amounts of blue and red, and less of green and far-red to achieve the best results.
In rose, there is no benefit or quickening of flowering time by increasing far-red light. Since increasing far-red will produce more buds, it can increase flowering in rose too. It is possible to get bigger roses using red light (7, 8).
It is important to note that rose plants will react differently to LED grow lights compared to other lights, for example, high-pressure sodium lights. Moreover, plants will yield more cut roses depending on the season, so in summer it is possible to get more roses with LEDs than winter. In winter, plants are exposed longer to LED and get warmer, and this additional heat brings down the production of flowers (9).
All plants are at risk from pests and diseases, and the rose is no exception. It is not advisable to use large amounts of far-red light in general as this makes the plants more susceptible to pests and diseases (5).
Powdery mildew is the disease that roses suffer most from in greenhouses, as its humid and warm conditions are very conducive for the spread and growth of this fungus. Brief exposure of supplemental UV-B (280 to 315 nm), for just 5 minutes each day can control 90% of mildew incidences, by preventing germination of mildew spores and its growth, according to a research finding (10).
An Environmental Friendly Alternative
Grow lights can be used to supplement traditional methods of nutrition and climatic conditions in greenhouses or indoor gardening to improve and elongate growing season for roses. Using smart LEDs lights can be environmentally friendly for growing, as nearly 40% reduction in energy consumption has been achieved in comparison to other grow lights. LED lights can also decrease dependence on chemical or even natural fertilizers by providing stimulus indoors that is not possible in open fields.
- SabineDemotes-Mainard Mainard SD, et al.2016. Plant responses to red and far-red lights, applications in horticulture. Environmental and Experimental Botany. 121: 4-21 https://doi.org/10.1016/j.envexpbot.2015.05.010
- Muneer S et al. 2014. Influence of Green, Red and Blue Light Emitting Diodes on Multiprotein Complex Proteins and Photosynthetic Activity under Different Light Intensities in Lettuce Leaves (Lactuca Sativa L.). International Journal of Molecular Sciences. 15: 4657–4670. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3975419/
- Suthaparan A et al. 2012. Suppression of Powdery Mildew (Podosphaera pannosa) in Greenhouse Roses by Brief Exposure to Supplemental UV-B radiation. Plant Disease. 96:1653-1660. https://doi.org/10.1094/PDIS-01-12-0094-RE