Cool Temps and Bright Light Treat Pansies Right

The temperature and light environment delivered to a pansy crop has a great impact on crop timing and quality. In general, the highest-quality pansies are grown under high light levels and cool temperatures, while the poorest quality occurs under low light and warm temperatures. Understanding how temperature and light impact pansy growth and development is useful to design growing environments, identify production problems and schedule crops properly.

Understanding Temperature

Low Temperatures. Cold temperatures can create problems in the landscape or inside unheated greenhouses or cold frames. Specifically, frozen soil or media, which occurs when soil temperatures drop below 28° F, can prevent water uptake by the roots. If the air temperature is sufficiently high while the soil is frozen, the shoots gradually lose water. Since no water is available from the roots, the shoots and flowers dry out, or desiccate. Desiccation can result in flower death and even plant death, depending on the duration the plants remain in these conditions. Desiccation damage occurs much more frequently than actual freeze damage: the two can be differentiated by the appearance of the damaged tissues. Desiccation damage results in dry, shriveled flowers; freeze damage results in mushy, dark brown or black flowers.

High Temperatures. Pansies experience considerable heat stress at average daily temperatures, or a 24-hour average temperature, above 80° F; however, pansies can continue to photosynthesize and grow at mid-day temperatures greater than 90° F. Therefore, high day temperatures are more tolerable when night temperatures are sufficiently 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 to root rot pathogens increases. Too little water creates drought stress problems, while  Á

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

Flower size is reduced at high temperatures, but the rate of flower development can be very fast. The time from transplanting a plug to flower can take as little as 2-3 weeks when the average daily temperatures are around 75° F.

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

Understanding Light

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

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

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

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

Impact on Growth and Flowering

Roots. Pansy root growth improves dramatically as temperatures decrease from 75-55° F. At 55° F, cell packs can be so full that the roots push the sides of the plastic outward, while at temperatures greater than 70° F, the roots of mature plants may be insufficient to hold the root ball intact. Pansy roots are more susceptible to root rot organisms (Pythium and Thielaviopsis) at warm temperatures.

Leaves. Leaf color is dark green and shiny under lower light conditions. Low-light leaves are also thinner 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 upright under low-light conditions. High-light plants have shorter petioles and horizontally arranged leaves. Also, high-light plants display increased branching and a much “fuller” appearance. Leaf size also decreases at temperatures greater than 75° F. In experiments conducted at 75° F, plants achieved good leaf size and branching, while higher temperatures resulted in poor leaf expansion as a result of heat stress.

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

Flower size increases as temperatures decrease, so flowers that develop at 55° F will be considerably larger than those that develop at 65° F or 75° F. The first flowers to open in early spring are often extremely large. The reason for this is that these flowers grow very slowly but they are able to grow for a long time. In contrast, flowers that develop under warmer temperature expand very quickly and are able to grow for only a short period of time. Thus, the “window of opportunity” for a pansy flower to expand is greatest at cool temperatures. Temperatures above 80° F will produce the smallest flowers, which can be deformed or fail to properly open. Á

The growing environment

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

Pansies grown outdoors in full sunlight can display high-light stress symptoms (see “Leaves” discussion on page 12). However, these symptoms seem most often to occur on young, newly transplanted material. As the plants grow and mature, they appear to be capable of utilizing higher light levels, so the stress symptoms diminish. If excessive light is a problem, providing shade cloth will produce more desirable results then placing the pansies inside a hot greenhouse. Thirty- to 50-percent shade cloth provides sufficient shade, while more than 60 percent shade is not desirable in most situations.

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

The bottom line is that pansies are an incredibly easy crop to grow when the temperatures are between 55 and 70° F; however, late summer production is incredibly challenging. The path of least resistance is to avoid the late summer markets, if possible. If avoidance is not an option, then a combination of retractable shade curtains, preventative fungicide drench applications and plant growth regulators will help produce the best quality possible during the heat of summer.

The authors acknowledge the Fred C. Gloeckner Foundation for their financial support of this project and Wagner’s Greenhouses for donating the plant material.