Managing light for rooting cuttings

© Danny | Adobestock

Rooting cuttings to produce high-quality liners as efficiently as possible should be every propagator’s goal. While poinsettias are rooted during the summer when light is not a limiting factor in greenhouse crop production, cuttings are regularly rooted during light-limiting times of the year.

Traditionally, cuttings have been kept under low light to reduce any stress. Lower light intensity is appropriate for the early phases of liner production, but not for later phases of production. In fact, increasing photosynthetic light during cutting propagation can enhance root growth and development, resulting in a higher quality liner in a shorter period compared to cuttings grown under a traditional low-light paradigm. This article will review how to manage photosynthetic light for all stages of liner production.

Stage 1: Sticking

Why even discuss light intensity for the first stage of cutting propagation, when Stage 1 focuses on sticking cuttings into substrate?

First, consider that cuttings are usually placed in a cooler when they are received to keep them hydrated and turgid. When it comes time to stick the cuttings, they are removed from the cooler. Cuttings left on the bench in the greenhouse can wilt quickly from warmth — not only the air temperature, but the radiant energy in light also warms them up. A best management practice is to only take as many cuttings as can be stuck in a timely manner from the cooler, leaving the rest until you are ready to stick more. Another option is to keep any cuttings in the greenhouse waiting to be stuck inside a cooler to keep the sun off them in the meantime.

Stage 2: Callusing

Once cuttings are stuck and placed on benches in the greenhouse to root, the goal is to minimize cuttings desiccating. Humidity and mist play a large role in keeping plants turgid. Managing temperature also plays a large role, since too much heat can promote excessive transpiration and wilting. Managing photosynthetic light during callusing is a balancing act. Enough light should be provided so cuttings can photosynthesize, producing the carbohydrates required to sustain the cutting as well as promote new growth, including root development. However, if too much light is provided, the radiant energy from light can raise the temperature of plant tissue, which can increase transpiration too much and cause wilting, much like supra-optimal air temperatures. Maintaining a daily light integral (DLI) of ~5 mol·m^–2·d^–1 is recommended for Stage 3. The steps taken to achieve this target DLI will depend on the time of year. Shade may be required to reduce the light intensity if ambient DLIs in the greenhouse are above 5 mol·m^–2·d^–1.

Stage 3: Root development

Once the new adventitious roots are visible, having broken through the outer cambium on the cutting stem, the cuttings have entered Stage 3 — root development. Now the goal is to promote root growth and development as much as possible. Not only do more roots increase the quality of rooted cuttings, it also reduces the time until the liner is fully rooted-in and pullable. Unlike Stage 2, when no root initials are reaching the substrate, roots starting to take up water from the substrate begins in Stage 3. As a result, cuttings can be grown under more intense light than during callusing. Research has shown increasing the DLI to 10 to 12 mol·m^–2·d^–1 during Stage 3 improves root growth for many herbaceous species, including both annuals and perennials. With increased photosynthesis, more carbohydrates are produced that can be dedicated to developing roots. Supplemental lighting may be required to achieve target DLIs, and either high-pressure sodium (HPS) lamps or light-emitting diodes (LEDs) are suitable for lighting cuttings. Providing supplemental light in the morning or evening is generally better than midday, as the DLI can be increased while avoiding supra-optimal instantaneous light intensities when solar radiation is greatest.

Stage 4: Toning

Once cuttings have entered Stage 4 — toning, they can start to be grown much like finished plants — harder. This includes lower temperatures and less substrate moisture, and more fertilizer and light. While light intensities above those recommended for Stage 3 may be beneficial for cuttings during Stage 4, it can be hard to justify providing more than 12 mol·m^–2·d^–1 with supplemental lighting, as the improved growth may not justify the increased costs. As a result, striving to maintain a 10 to 12 mol·m^–2·d^–1 DLI remains a good goal for Stage 4.

Developing strong root systems on cuttings in propagation should be every producer’s goal. Increasing photosynthetic light above levels that have been traditionally used during root development and toning can improve liner quality and crop time. Additionally, any investments made for vegetatively propagated crops can be more affordable in propagation as opposed to finishing, considering the higher planting density.

Christopher is an associate professor of horticulture in the Department of Horticulture at Iowa State University and a regular contributor for Greenhouse Management. ccurrey@iastate.edu