A clone is a cutting taken from a mature mother plant to create a genetically identical copy. Unlike a seedling, this cutting lacks a functional root system, making its initial survival dependent on the environment. During this delicate rooting phase, the goal of lighting is to sustain the cutting through photosynthesis without causing undue stress or dehydration. Too much light intensity is often more detrimental than too little, as the cutting must rely on stored energy and limited water uptake through its stem until new roots form.
Recommended Light Intensity
The most important metric for measuring light delivered to a plant is the Photosynthetic Photon Flux Density (PPFD), which quantifies the number of photosynthetically active photons hitting a surface each second. For freshly taken clones, the light intensity must be kept low to prevent desiccation and stress. A typical recommended PPFD range for the initial rooting phase is between 100 and 200 micromoles per square meter per second (\(\mu\text{mol}/\text{m}^2/\text{s}\)). Some cultivators start cuttings even lower, sometimes as low as 50 to 70 \(\mu\text{mol}/\text{m}^2/\text{s}\), particularly for sensitive species or low-humidity environments.
This gentle light level is necessary because the cutting, without roots, cannot efficiently absorb the water required to support high-rate photosynthesis. Exposing unrooted cuttings to high light intensity forces them to attempt rapid photosynthesis, which quickly depletes their moisture reserves through transpiration. When stressed, the leaves may show visible signs like yellowing (bleaching) or the edges may curl or crisp up as the plant conserves moisture. Maintaining low PPFD ensures the cutting focuses its energy on developing new root structures rather than supporting excessive leaf metabolism.
Ideal Photoperiod Settings
The photoperiod refers to the total duration of light exposure within a 24-hour cycle, separate from the light’s intensity. Clones typically benefit from an extended light cycle, such as 18 hours of light followed by 6 hours of darkness (18/6), or continuous light (24/0). This prolonged exposure maximizes the total energy the cutting collects throughout the day, known as the Daily Light Integral (DLI). A higher DLI provides the energy reserves needed to fuel the metabolic processes involved in root formation.
For photoperiod-sensitive plants, maintaining a long light cycle prevents them from prematurely entering the flowering stage. The 18/6 schedule offers a short rest period, which some believe allows for cellular repair. Alternatively, the 24/0 cycle is often used to ensure maximum environmental stability by eliminating the temperature and humidity fluctuations that occur during a dark period. Both schedules are effective, but extended light duration is required during the rooting process.
Choosing the Right Light Source and Spectrum
The light source should deliver the required low intensity without excessive heat output. Low-wattage LED panels or fluorescent fixtures, such as T5 or compact fluorescent lights (CFLs), are suitable for cloning areas. These fixtures are preferred over high-intensity discharge (HID) lamps because they generate less radiant heat, preventing the cuttings from drying out prematurely.
The quality of the light, or its spectrum, also influences rooting success. A spectrum dominant in the blue wavelength range (400-500 nanometers) is preferred for the cloning phase. Blue light encourages the development of compact vegetative structures and helps inhibit the stem elongation that occurs under red-dominant light. This cooler spectrum promotes short internodal spacing and healthy leaf growth, helping the clone develop into a sturdy plant ready for the next growth stage.
Adjusting Lighting After Rooting
Once the clone has successfully developed roots, its lighting requirements must be gradually adjusted to support active vegetative growth. This transition signals that the clone is no longer relying solely on stored energy and is ready to begin absorbing water and nutrients efficiently from the growing medium. The first step is to progressively increase the light intensity from the initial low PPFD to a level suitable for young vegetative plants.
This increase should be incremental to prevent light shock, moving the intensity into the range of 200 to 400 \(\mu\text{mol}/\text{m}^2/\text{s}\) over several days. If the clone was rooted under a continuous 24/0 photoperiod, transition to an 18/6 light schedule to acclimate the plant to a typical vegetative environment. Observing the plant’s response to these changes is important, ensuring the leaves remain healthy and the growth is vigorous before moving the clone into the main grow area.