Cool Temps and Bright Light Treat Pansies Right By James E. Faust & Kelly P. Lewis

The temperature and light environment delivered to a pansycrop has a great impact on crop timing and quality. In general, thehighest-quality pansies are grown under high light levels and cooltemperatures, while the poorest quality occurs under low light and warmtemperatures. Understanding how temperature and light impact pansy growth anddevelopment is useful to design growing environments, identify productionproblems and schedule crops properly.

Understanding Temperature

Low Temperatures. Coldtemperatures can create problems in the landscape or inside unheated greenhousesor cold frames. Specifically, frozen soil or media, which occurs when soiltemperatures drop below 28¡ F, can prevent water uptake by the roots. Ifthe air temperature is sufficiently high while the soil is frozen, the shootsgradually lose water. Since no water is available from the roots, the shootsand flowers dry out, or desiccate. Desiccation can result in flower death andeven plant death, depending on the duration the plants remain in theseconditions. Desiccation damage occurs much more frequently than actual freezedamage: the two can be differentiated by the appearance of the damaged tissues.Desiccation damage results in dry, shriveled flowers; freeze damage results inmushy, dark brown or black flowers.

High Temperatures.Pansies experience considerable heat stress at average daily temperatures, or a24-hour average temperature, above 80¡ F; however, pansies can continue tophotosynthesize and grow at mid-day temperatures greater than 90¡ F.Therefore, high day temperatures are more tolerable when night temperatures aresufficiently cool to bring the 24-hour temperature below 80¡ F.

Watering poses a real challenge during high temperatures.Pansy root growth is relatively poor at high temperatures and susceptibility toroot rot pathogens increases. Too little water creates drought stress problems,while ç

over-watering creates root rot problems. Additional shadehelps growers better manage watering, but too much shade can reduce growth andthe quality of leaves and shoots.

Flower size is reduced at high temperatures, but the rate offlower development can be very fast. The time from transplanting a plug toflower can take as little as 2-3 weeks when the average daily temperatures arearound 75¡ F.

Late summer pansies tend to be "thin" and have a"stretched" appearance. This appears to be a result of rapidflowering caused by long days, high light levels and warm temperatures. Theresult is a plant that quickly "bolts," or quickly produces aflower on the primary shoot before the lateral shoots begin to develop. RegularPGR applications are required to keep plants compact and to maintain plantquality. Bonzi and Sumagic are the most effective PGRs on pansies, but usingthe proper application method is critical. B-Nine/Cycocel tank mixes and A-Restare also effective. We have not had much success using Florel on late summerpansies. In our trials, we observed that Florel consistently delayed flowering,which might be desirable, but did not improve branching or reduce stemelongation. Spring pansies do not usually require plant growth regulators toproduce compact plants.

Understanding Light

Both the amount of light, or light quantity, and the daylength impact pansy quality.

Photoperiod. Pansiesare facultative, long-day plants, thus flowering occurs fastest under longdays, but most varieties will flower under short days. Winter floweringimproves, in terms of flower number and time to flower, when night-interruptionlighting is provided. Night-interruption lighting can be provided with incandescent,fluorescent or high-pressure sodium lighting. Incandescent lighting results inadditional plant stretch, so further plant growth regulators may be required.High-pressure sodium lamps can be mounted on irrigation booms and allowed tomove over the crop throughout the night to provide a night-interruptiontreatment to the entire bay.

Light Quantity.Pansies are a relatively high-light-requiring crop. Whereas many floriculturecrops perform well receiving 2,000-4,000 foot candles or 10 moles per day (20percent of outside summer light levels), pansies perform best when receivinggreater than 4,000 foot candles or 20 moles of light per day (greater than 40percent outside summer light levels). Time to flower is reduced, while flowernumber and branching increase, under high light quantities.

Excessively high light levels can cause leaf curling, leafcupping or leaf discoloration. The margins of the leaves will bend upwardsrelative to the mid-vein, forming a V-shaped leaf. Occasionally, the outer edgeof the leaf will curl upwards, creating a spoon-like appearance. Typically,leaf curl is more pronounced on small plants. Pansies will usually grow out ofthese symptoms as they get larger and are able to utilize higher light levels.Excessively low light levels result in floppy plants that flower and branchpoorly.

Impact on Growth and Flowering

Roots. Pansy rootgrowth improves dramatically as temperatures decrease from 75-55¡ F. At55¡ F, cell packs can be so full that the roots push the sides of theplastic outward, while at temperatures greater than 70¡ F, the roots ofmature plants may be insufficient to hold the root ball intact. Pansy roots aremore susceptible to root rot organisms (Pythium and Thielaviopsis) at warmtemperatures.

Leaves. Leaf coloris dark green and shiny under lower light conditions. Low-light leaves are alsothinner and more pliable than high-light leaves. Under high-light conditions,the leaves are lighter green, slightly duller and thicker.

Petiole length increases and the leaves are held uprightunder low-light conditions. High-light plants have shorter petioles andhorizontally arranged leaves. Also, high-light plants display increasedbranching and a much "fuller" appearance. Leaf size also decreasesat temperatures greater than 75¡ F. In experiments conducted at 75¡ F,plants achieved good leaf size and branching, while higher temperaturesresulted in poor leaf expansion as a result of heat stress.

Flowers. The timefrom transplant to flower is influenced by both temperature and light quantity.The most rapid flowering occurs at 75¡ F and at high-light levels (greaterthan 50 percent of full sunlight or 5,000 foot candles). Lowering the lightlevels with shade cloth or naturally low-light levels from late fall to earlyspring will increase production time and reduce flower number per plant.Temperatures below 70¡ F or above 80¡ F will also noticeably increasethe time to flower.

Flower size increases as temperatures decrease, so flowersthat develop at 55¡ F will be considerably larger than those that developat 65¡ F or 75¡ F. The first flowers to open in early spring are oftenextremely large. The reason for this is that these flowers grow very slowly butthey are able to grow for a long time. In contrast, flowers that develop underwarmer temperature expand very quickly and are able to grow for only a shortperiod of time. Thus, the "window of opportunity" for a pansyflower to expand is greatest at cool temperatures. Temperatures above 80¡ Fwill produce the smallest flowers, which can be deformed or fail to properlyopen. ç

The growing environment

For summer/fall pansy production, the outdoor environment isoften superior to the greenhouse environment. Greenhouse temperatures arefrequently hotter than outdoor temperatures, and the light levels are muchlower inside the greenhouse. High temperature combined with high shade is theworst environment in which to produce high-quality pansies.

Pansies grown outdoors in full sunlight can displayhigh-light stress symptoms (see "Leaves" discussion on page 12).However, these symptoms seem most often to occur on young, newly transplantedmaterial. As the plants grow and mature, they appear to be capable of utilizinghigher light levels, so the stress symptoms diminish. If excessive light is aproblem, providing shade cloth will produce more desirable results then placingthe pansies inside a hot greenhouse. Thirty- to 50-percent shade cloth providessufficient shade, while more than 60 percent shade is not desirable in mostsituations.

Retractable-roof greenhouses or retractable shade curtainsystems provide an ideal environment for growing pansies. Growers can providethe benefits of outdoor conditions when the weather is suitable, and they canprovide the protection of a greenhouse during rains or midday heat. Shading fora few hours in the early afternoon, e.g., 11:30 a.m.-2:30 p.m., can reducetemperature stress while allowing the plants to continue receiving high lightlevels.

The bottom line is that pansies are an incredibly easy cropto grow when the temperatures are between 55 and 70¡ F; however, latesummer production is incredibly challenging. The path of least resistance is toavoid the late summer markets, if possible. If avoidance is not an option, thena combination of retractable shade curtains, preventative fungicide drenchapplications and plant growth regulators will help produce the best qualitypossible during the heat of summer.


The authors acknowledge the Fred C. Gloeckner Foundationfor their financial support of this project and Wagner's Greenhouses fordonating the plant material.

James E. Faust & Kelly P. Lewis

James E. Faust is an assistant professor and Kelly P. Lewisis a research specialist in the Department of Horticulture at Clemson University,Clemson, S.C. They can be reached via phone at (864) 656-4966 or E-mail

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