Potted Lisianthus:
Secrets of Success – Plug Production
by  Brent K. Harbaugh, Robert J. McGovern, and Jim P. Price
University of Florida, Gulf Coast Research and Education Center – Bradenton, FL

Published in Greenhouse Grower, December 1997

Part 1 - Plug Production

Part 2 - Bedding and Potted Plant Production

Part 3 - Protecting Lisianthus from Fungus Gnats and other Arthropod Pests

Part 4 - Control Diseases affecting this Crop

Although it may not be the easiest to produce, lisianthus has been gaining popularity as a potted plant each year.  We kick off our four-part series on this rising star with an article that focuses on plug production.
 

Figure 1.  State-of-the-art pot plant production in 1997.  Recently released cultivars of lisianthus, such as the Maurine series (above), have been bred for heat tolerance, basal branching, and uniform response to growth retardants.

  While many new flower crops struggle to gain consumer awareness and acceptance, this has not been the case with lisianthus.  In the U.S., lisianthus has jumped from an essentially unknown floricultural crop in the early 1980s to a very popular crop in 1997 (Figure 1).  In the past 10 years, lisianthus may well have been the fastest growing segment of the new flower category in the U.S. and worldwide.

            Lisianthus was introduced into the floriculture trade without much research on production, especially for pots.  The state-of-the-art for pot production in the 1980s was to try to force cut flower cultivars in pots (Figure 2).  Success was hampered by cut flower types that had very dominant main stems and lack basal and lower braches.

            In addition, early cultivars were not heat tolerant, resulting in rosetting and extreme variability in the number of days to flower.  Unfortunately, many of the plants in the marketplace today look like those produced in the early 1980s.

            We started doing research on lisianthus at the University of Florida’s Gulf Coast Research and Education Center in the early 1980s.  We took an interdisciplinary, or systems approach, to solving production problems.  Our research team has developed an expertise in producing lisianthus that encompasses the physiology of flowering, cultural requirements, breeding for heat tolerance, and disease and insect management.  We believe that the information presented in this series of articles will allow growers to produce quality crops of potted lisianthus for year round sales (Figure 1).

Six Lisianthus Secrets
1.)        Know the plant.  The first secret is lisianthus production is to realize that this crop is not easy to grow.  That is, production of lisianthus will require more attention to detail than many other potted plants. 

            Cultivars of lisianthus differ tremendously in performance.  We believe lisianthus eventually will be sold by ‘groups’ that perform best at different times of the year, similar to snapdragons.  If growers are to time production of uniform size plugs that will flower predictably in pots, cultivars will have to be grouped according to growth rates and responses to photoperiod and temperature.

2.)         Keep soil pH high.  High soil pH is extremely critical for production lisianthus plugs.  Lisianthus will almost always develop micronutrient toxicities if the soil pH is not kept in the 6.5 to 7.0 ranges.  Our research revealed that the ideal pH was 6.7 Zinc toxicity is especially common even when the plug mix is as high as 5.8-6.2, a soil pH commonly recommended for many other floricultural crops.

            In addition to obvious toxicity symptoms of a general chlorosis of leaves, the more subtle effects caused by lower pH are that the quality and size of plugs decrease with decreasing pH (Figure 3).

            Variability in plug growth is also one of the key symptoms of pH imbalances.  That is, within a plug tray that has a low pH soil, there will be plants with normal growth, some that are stunted and chlorotic, and some that are stunted with no chlorosis.  Without other plugs grown at a higher pH for comparison, many plug producers have not realized they could have achieved if grown at an optimal pH.

            If the soil pH starts to fall below 6.5, a soil drench of flowable limestone should be applied or other corrective actions taken to raise the pH immediately.  Liming materials which contain calcium are favorable since these materials will raise the pH and provide a source of Ca that will benefit leaf expansion.

3.)        Watch for fungus gnats.  Fungus gnats are not just nuisance pests of lisianthus.  They can cause partial or complete loss of a lisianthus crop.  Fungus gnats will eat the hypocotyls (stem) and cotyledons (seed leaves) of newly germinated seedlings.  Often it appears as if germination was poor or that damping-off was taking a toll on newly emerged seedlings.  The seedlings may fall over due to the loss of roots and/or stem damage.

            On older seedlings, fungus gnats can almost completely destroy the root system.  They also will eat holes in leaves touching the soil surface.  Of course, without a normal root system, plants may wilt and develop nutrient deficiencies that prompt growers to increase irrigation and fertilization rates, further damaging the plants.

            We cannot stress enough that fungus gnats are a major problem for seedling production.  When other crops are in the greenhouse, fungus gnats appear to prefer lisianthus.  We have never visited a production facility and not detected fungus gnats in their seedling production.  We will provide suggestions for control measures in a future article, but be prepared to battle fungus gnats if you plan to grow lisianthus.

4.)        Feed them well.  Lisianthus are heavy feeders.  A light to moderate fertilization program for the first 2 weeks after germination will ensure that seedlings get off to a good start.  For the remainder of the plug production cycle, a heavy fertilization program is needed to optimize root and shoot growth.

         The confusion over the need to use high fertilization rates appeared to develop for at least two reasons.  First, when plants are grown in low pH soil, the root system is poor and the plants will not respond to heavy feed due to micronutrient imbalances.

            Secondly, low fertilization rates result in small plugs without inducing obvious symptoms of foliar nutritional deficiencies.  That is, one will not detect visual symptoms of foliar deficiencies such as chlorosis, necrotic lesions or spots, or interveinal chlorosis that might be typical of other pot plants if grown at low fertilization levels.

            However, if plugs are not grown with a heavy fertilization program at the correct soil pH, one can expect as much as a 50% - 75% reduction in leaf size and number, severe reduction of the root system, and a delay in initiation of bolting (Figure 4).

5.)        Watch high temperatures.  We have shown that high temperatures cause rosetting in lisianthus.  A rule-of-thumb definition for “high temperature” is an average temperature [(the day time high + night time low)/2] of 70º - 75ºF for sensitive cultivars, 75º - 82ºF for most cultivars, and above 88ºF for heat tolerant cultivars.

            The greater number of days and the higher the temperature, the greater the percentage of rosette plugs that will develop.  A few days at an average of 82ºF would result in similar stress as several weeks at 75ºF.

            We have also determined that there is an interaction of temperature with photoperiod and cultivar on growth and flowering of lisianthus.  High temperatures and short days cause the greatest problems.  Lighting cultivars grown under high temperatures will result in plants that will bolt, but plant quality will probably be poor at flowering.

            The answer to these problems is to grow plugs only during those seasons, or at locations where optimal temperatures can be achieved.  Or alternatively, to use heat-tolerant lines, such as the Maurine series developed at the Gulf Coast Research and Education Center, which has been bred to flower year-round even at average temperatures as high as 82º-88ºF (Figure 1.)

            Unfortunately, many plugs have been sold that have been heat stressed and are rosetted.  An early indication that plugs have rosetted occurs when plugs have greater than 12 leaves and have not bolted.  While heat-stressed plugs may actually look more vigorous because they have more and/or larger leaves than plugs grown under lower temperatures, the result for the pot forcer is disastrous (i.e. vegetative plants or plants that flower sporadically over a period of 6 months).

            It is imperative that plug producers understand the effects of high temperatures, cultivars, and day length so that those forcing plugs to flower are able to do so in a predictable and economically desirable fashion.

6.)        Consider plug size.  Seedlings grow slow initially and the first leaves are relatively small compared to many other pot plants.  However, lisianthus quickly produce long roots, and plugs that have been root-bound for even short periods will not perform well during subsequent forcing.

            In addition, we have found that plugs grown in the 200- to 400-cell size should be transplanted before they have developed two or more internodes or performance will be less than optimal (Figure 5).

            Since most cultivars bolt at the 8- to 10-leaf stage (including cotyledons), plugs should be marketed at the 8 to 10-leaf stage.  At this stage, plugs may be smaller than customary for some growers producing potted flowering plants (Figure 6) and they may think the plugs are not mature.  Thus, there may be a need for increased communication to inform those who force plugs to flower that these relatively small plugs are actually at the optimal stage for transplanting.

            To achieve a favorable balance between small plugs and deep root systems, many growers use plug trays with deep cells in the 200 to 406 sizes.  If smaller plug trays are sued, then transplanting at the 6- to 8-leaf stage will be necessary.  Since the leaves will be very small, these plugs will be difficult to work with.

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