Lighting Recommendations for Successful Plant Propagation

Young plants need far less light than a finished crop, and giving them too much is as damaging as giving them too little. Cuttings root best at a daily light integral of 4 to 5 mol·m^-2·d^-1 with peak intensity held under 200 µmol·m^-2·s^-1 before roots appear, while seedlings need a daily light integral of 8 to 12 mol·m^-2·d^-1 for quality plug production (general guidance, not cultivar specific). A plant’s growing cycle is shaped during the early vegetative stages of growth, and those early stages directly influence the rest of its life. Supplemental lighting during propagation allows for more predictable and faster crop cycles while improving overall quality and root structure. Air temperature, and especially medium temperature, matter alongside light, because temperature controls the rate of callus and root development. This article covers the basics behind cutting and seedling production in relation to light.

Propagation Light Levels at a Glance

These are starting points for quality plug and liner production. Individual light levels vary by cultivar and by the natural outdoor light available, so treat the ranges as a baseline to adjust from rather than a fixed prescription.

If the terms above are unfamiliar, our guide to horticultural lighting metrics explains how PPFD and DLI relate to each other and to fixture specifications.

Propagation Lighting for Young Plants: Seedlings and Cuttings

Propagation Lighting and Seedlings

For the most part, many young plants are started in the winter months in order to be ready for spring, meaning you often deal with limited light intensity and duration. Making sure seedlings are getting the recommended amount of light is essential to growing quality plants with well developed root systems. The benefits of adding supplemental lighting to young plants include improved branching, thicker stems, increased rooting, more rapid growth and more compact size.

Supplemental lighting influences plant growth by increasing the Daily Light Integral (DLI) and therefore photosynthesis. DLI is the amount of photosynthetic light delivered to plants each day. It is expressed as moles of light per square meter per day and is a function of light intensity and duration. PPFD (Photosynthetic Photon Flux Density) is another way to measure light intensity and is a reading of the instantaneous light level being delivered to the crop canopy at a certain time, measured in micromoles per square meter per second (µmol·m^-2·s^-1).

Germination of seeds requires either dark or light conditions, and some dormant seeds also need a period of cold, moist stratification before they will germinate. Stratification is distinct from vernalization, which is a cold exposure that prepares a plant to flower rather than one that breaks seed dormancy. Researchers have found that red light is most effective at breaking photodormancy in light dependent seed crops. Where darkness is required for germination, it can be provided by covering seeds with a fine layer of growing substrate, covering with a plastic film or simply turning the lights off. During propagation, light should be managed based on the instantaneous intensity (PPFD), but we can also consider DLI, as research has shown that rooting of some crops can be delayed when the DLI is below 4 or 5 mol·m^-2·d^-1.

Four Stages to Plug Production

1. Radicle (root) emergence
2. Cotyledon spread
3. Unfolding of three to four leaves
4. Finished plug with more than 4 expanded leaves

For seedlings, a DLI of 8 to 12 mol·m^-2·d^-1 or PPFD of 100 to 300 µmol·m^-2·s^-1 is recommended as a minimum for quality plug production. Timing matters as much as total light. In petunia and pansy plugs, supplemental lighting delivered over the last two-thirds of the plug stage increased shoot dry mass and leaf number and accelerated flowering, compared with lighting supplied only during the first third. Worth knowing before you plan the schedule: those same plants reached first flower with a lower shoot dry mass and fewer flower buds, so earlier flowering is a trade rather than a free gain. As plants move into stage 4, the light levels can further be increased to 500 to 800 µmol·m^-2·s^-1, or a DLI of 12 or more depending on what you are growing.

Plant responses to supplemental lighting are higher in winter months than in spring, because winter provides a much lower naturally occurring DLI. DLI follows the law of diminishing returns, so as DLI continues to increase it has less of an effect on flowering time and development. A change in DLI from 7 to 11 mol·m^-2·d^-1 will see a more pronounced difference than a change from 12 to 16.

All plant species respond differently to light intensity. While a higher light intensity will provide faster growth, some species will benefit from a lower DLI and others from a higher one. Supplemental lighting can also extend the photoperiod, and early flowering can occur in long day flowering plants. Our guide to managing photoperiod for better crop production covers how day length is controlled separately from intensity.

Propagation Lighting and Cuttings

Adding supplemental lighting to cutting (clone) production can greatly increase the chances of a successful crop, speed up root emergence and shorten crop times. If the DLI is too low during propagation, leaves will be unable to get enough light for photosynthesis and rooting can be delayed. However, if light levels are too high, leaves can become bleached, and leaf temperature and transpiration will increase, which can hinder rooting.

Four Stages to Cutting Production

1. Cutting insertion into substrate
2. Callus development
3. Root development
4. Preparation of rooted cutting for finishing, plus further root and shoot development

Having appropriate light levels in your cutting production can help provide energy for callus development, along with the growth of adventitious roots. The clearest way to manage it is to target a daily light integral for each stage and hold peak intensity under a ceiling, rather than aim at a single PPFD number.

From sticking through callus formation and root initiation, current guidance from Michigan State University is a DLI of 4 to 5 mol·m^-2·d^-1 with peak intensity not exceeding 200 µmol·m^-2·s^-1. Shading should be used on bright days to avoid dehydrating the cuttings. Once roots are developing, raise the target to a DLI of 6 to 8 mol·m^-2·d^-1 with a maximum of 400 µmol·m^-2·s^-1. When roots have filled about half the liner and the crop is being toned for transplant, a DLI of 8 to 12 mol·m^-2·d^-1 is appropriate and plants can tolerate more light than at any earlier stage. This higher intensity helps acclimate plants to the post-propagation environment.

Temperature, Humidity and Sanitation During Rooting

Managing the entire growing environment is critical to growing well rooted cuttings, and good sanitary practices help prevent the spread of insects, diseases and viruses. A good practice when receiving cuttings is to use a preliminary dip, immersing cuttings in a low-risk biopesticide such as insecticidal soap, horticultural oil or BotaniGard to cover the whole plant. Treat it as a way to reduce incoming pest pressure rather than eliminate it. In poinsettia trials, dipping removed roughly 70% of a starting whitefly population, enough for biocontrol agents to suppress what remained.

A rooting hormone containing an auxin, applied promptly, improves rooting speed and uniformity in crops that respond to it. Response varies by species, cultivar and cutting type, and excessive rates can inhibit root formation, so follow the validated rate and method for the crop rather than treating it as a universal step. During the early stages of rooting, the humidity should be maintained quite high, around 90 to 100%. Depending on your setup, misters or foggers can be utilized to keep the air moist and humidity high. Providing bottom heat, or a media temperature between 22 and 24 degrees Celsius, will also help to speed up rooting.

Where that bottom heat comes from depends on your fixtures. High pressure sodium luminaires radiate a significant amount of heat downward, and growers running HPS over propagation benches have long relied on it to warm the media. LED luminaires deliver less radiant heat downward, so media temperature should be measured rather than assumed as a by-product of the lighting. If it falls short of the 22 to 24 degree target after a retrofit, it can be brought back up with bench or root-zone heating or with a higher air temperature setpoint. This is worth planning for before a retrofit rather than discovering it in the first crop.

Why Light Uniformity Matters in Propagation

Light uniformity is critical at this stage in development, because it becomes difficult to manage plants of varying sizes and at different stages of development within the same tray or bench. Uneven light produces uneven plants, and in propagation that unevenness carries forward into every downstream crop stage. When plants are ready for production, the PPFD can be further increased depending on the desired PPFD or DLI for your crop. Ensuring young plants are getting the right amount of light during the early stages of growth will make later transplant into the greenhouse environment faster and easier on your crops. At the same time, more supplemental lighting can increase how quickly cuttings dry out, so a misting program matters.

How evenly a fixture spreads light across a bench varies by luminaire type and layout, which we compare in our article on light uniformity between LED and HPS luminaires. On multi-tier benches, where fixtures sit close to the canopy, mounting distance and uniformity need to be checked at the design stage rather than assumed.

Common Questions About Propagation Lighting

How Much Light Do Seedlings Need?

For quality plug production, seedlings need a PPFD of 100 to 300 µmol·m^-2·s^-1, or a daily light integral of 8 to 12 mol·m^-2·d^-1. Once plants reach stage 4 of plug production, levels can be increased to 500 to 800 µmol·m^-2·s^-1 and a DLI of 12 or more, depending on the crop.

How Much Light Do Cuttings Need Before They Root?

From sticking through callus formation and root initiation, target a daily light integral of 4 to 5 mol·m^-2·d^-1 and keep peak intensity below 200 µmol·m^-2·s^-1. Too much light at this stage bleaches leaves and raises leaf temperature and transpiration, which hinders rooting. Once roots are developing, raise the target to 6 to 8 mol·m^-2·d^-1 with a ceiling of 400 µmol·m^-2·s^-1.

Do Cuttings Need Light to Root?

Yes. Cuttings still photosynthesize while they form callus and adventitious roots, and if the DLI is too low, rooting can be delayed. The requirement is low light rather than no light, which is why shading on bright days is used alongside supplemental lighting on dark ones.

What DLI Is Too Low for Propagation?

It depends on what you are propagating. For unrooted cuttings in the callus and root-initiation phase, 4 to 5 mol·m^-2·d^-1 is the target, and rooting of some crops is delayed below it. For seedlings, that same figure is only a floor, because quality plug production calls for 8 to 12.

Does Supplemental Lighting Help More in Winter?

Yes, and the reason is diminishing returns. Because winter light is the scarcest, the same added light buys a bigger response than it would in spring, which is why winter and early spring propagation is where supplemental lighting pays back hardest.

Getting Propagation Lighting Right

Supplemental lighting and light intensity play an important role during the propagation stages of production. Responses to DLI differ among cultivars, with shade loving plants generally requiring less light than full sun plants. Propagation requires the proper balance between light, temperature, and humidity. Using supplemental lighting for plugs during low light intensity times of the year will increase photosynthesis, speed up flowering, increase root structure, produce stronger and sturdier plants and shorten crop cycles.

If you are sizing lighting for a propagation range or a multi-tier bench system, P.L. Light Systems can put together a light plan for your crop and structure.