How many hours of direct sunlight does the roses require? This is the most common question. I asked the same question when I started growing roses.
After doing some research on plant psychology, I learned that ensuring the minimum hours of indirect sunlight is as important as determining the minimum direct sunlight. The reason is in Malaysia we are blessed with long hours of sunlight, having direct sunlight is not so crucial.
Before we determine the number of hours of sunlight for our roses, we need to understand how and what is the optimum rate of photosynthesis.
A commonly used slightly simplified equation for photosynthesis is:
6 CO2(g) + 12 H2O(l) + photons → C6H12O6(aq) + 6 O2(g) + 6 H2O(l)
carbon dioxide + water + light energy → glucose + oxygen + water
The equation is often presented in introductory chemistry texts in an even more simplified form as:
6 CO2(g) + 6 H2O(l) + photons → C6H12O6(aq) + 6 O2(g)
What this means is that during photosynthesis, the plant will use light energy to manufacture glucose. At the end of this process, glucose is stored for the purpose of plant growth and production of carbohydrate.
2 H2O + 2 NADP+ + 2 ADP + 2 Pi + light → 2 NADPH + 2 H+ + 2 ATP + O2
While knowing photosynthesis is at work during sunlight, there is another process that is at work all the time. This process is known to be light-independent process. That means during night or darkness, the plant will draw energy stored to use CO2 for the production of carbohydrate.
3 CO2 + 9 ATP + 6 NADPH + 6 H+ → C3H6O3-phosphate + 9 ADP + 8 Pi + 6 NADP+ + 3 H2O
This process of respiration happens all day and night.
Another critical factor to take note is that when sun light is very intensed and temperatures rises, photosynthesis rate shall decline. However respiration rate will continue to rise, therefore reduces the net photosynthesis (lower photorespiration - in shaded red below).
Therefore understanding this effect is important to determine the right amount of direct sunlight and at which of part of the day is crucial, especially trying to grow roses in tropical country like Malaysia. As Malaysia weather is known to have average of 32 to 33 degress C outdoors, it is crucial not to over exposed the roses with too much heat. It doesn't harm that much if its for short period, as roses are known to be quite tolerant of direct sun.
I would prefer to place roses at a location where it can draw direct morning sun for about 2 to 4 hours as temperature is at lowest of the day, and indirect sunlight for 5 to 7 hours. The rose plant must get indirect sunlight from opened sky and not from opened window (or any location that is shielded, it can never grow indoor). In such condition, you provide a controlled temperature environment within 28 to 32 degress C.
A practical way to artificially create this effect is by growing roses next to a wall or a taller tree/plant that will shield your roses from the intensed afternoon sun.
How much light does my roses get?
My garden usually gets direct sun between 10am to 12pm, my estimation is about 100,000 lux (light intensity) at clear sky. It is believed that at this light intensity, photosynthesis will reach light saturation (photosynthesis will cease if reach light saturation of 32,000 lux for upper leave and 100,000 lux for whole plant)
Thankfully Malaysia's weather is usually cloudy, therefore will diffuse the direct sunlight. However it is necessary to take pre-caution if weather is dry and hot for long periods as the plant will be exposed to high temperature thus raise the rate of respiration. That is why, I feel it is necessary to place roses at location that is shielded from hot afternoon sun, but not totally shielded from indirect sun.
Just to ensure that your roses receives sufficient sunlight, especially if your garden has limited space and suspicion of insufficient light, just do a simple calculation.
8am to 5pm - 9 hours x 30,000 lux (indirect sunlight) = 270,000 lux hours
10am to 12pm - 2 hours x 100,000 lux (direct sunlight) x 30% (below upper leave) = 60,000 lux hours
Total is 330,000 lux hours.
In case you are unsure how many lux (light intensity) in your garden, in general you can use 100,000 lux for direct sunlight, 30,000 for indirect sunlight and 5,000 for low light (indoor light. You will need to multiply the ratio of 0.3 (30%) for direct sunlight, as only the leaves below the upper leave will benefit from the sunlight. It is believed that at high intensity the upper leaves will cease photosynthesis due to higher light saturation.
For the plant to survive it requires a minimum of 160,000 lux hours of sun light.
If your light calculation exceeds the minimum of 160,000, your rose should thrive. If you have read some books on rose growing, you may notice that most say that roses require direct sunlight, that is because the weather is different from our tropical weather in Malaysia. Averagely Malaysia outdoor weather is at 32 degrees C, if at direct sunlight, the temperature may increase for another 2 to 3 degrees C, which lower the net photorespiration rate.
Also if the heat reaches the soil, the good bacterias in the soil will also cease their activities thus will affect the plant in the long run. It won't do much harm if exposed for short periods, as it reduces the risk of fungal infections. But if for long periods, the plant roots will deplete in its nutrition as the bacterias stop breaking down the nitrogen components.
You may want to check the earlier posting that is extracted from external site. I posted in this blog, in case you wish to know about plant leaves anatomy, photosynthesis and light.
This posting explains the impact of diffuse light on photosynthesis rate. It dispel the myths of having direct sunlight leads to increased in photosynthesis rate.