Light and Fertilizer Recommendations for the Interior Maintenance of Acclimatized Foliage Plants

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University of Florida IFAS
Central Florida Research and Education Center - Apopka
CFREC-A Research Report RH-91-7

C.A. Conover, R.T. Poole and R.W. Henley*

Previous research determined that many species of foliage plants, when produced under lower light and fertilizer levels than species grown in full sun, produced a final product more able to withstand interior environments. Progressive foliage plant producers eagerly adopted the new production methods and called these plants "acclimatized". A larger, more diverse foliage plant market now exists because most of the environmental plant industry has implemented production regimes that produce acclimatized plants.

Indoor plants can be divided into three groups according to their physiological responses to light. The first group includes extreme shade plants such as Aglaonema, Maranta and Spathiphyllum. These plants usually do well in all except the brightest of interior locations, but do look best when placed in light levels of at least 75 ft-c minimum (Table 1). The second group are termed sun-shade because these plants can adapt to a wide range of interior light levels. Brassaia, Chamaedorea, Dracaena and Ficus are examples of sun-shade plants. The third group of plants, high light flowering plants, termed full-sun plants, require such high light intensities that they can not be adapted for long term indoor use. While some full-sun plants, like chrysanthemums, may be used in interior locations to provide a temporary splash of color, they will not continue to grow and bloom indoors and must be replaced periodically.

The two most important factors influencing foliage plant longevity in interiorscapes are light levels and fertilizer. application rates. These two factors are closely associated because light level determines the rate at which plants use fertilizer. Various characteristics of the potting medium also affect the rate at which nutrients are made available to foliage plants.

LIGHT

Light intensity and duration are most commonly the limiting factors when maintaining plants indoors. Plants must receive a light intensity slightly higher than the intensity at which it reaches its compensation point in order to survive. The compensation point is the point at which a plant uses as much food as it produces. A plant's compensation point is influenced by production regimes. Generally, high light and fertilizer levels during production 'set' a plant's compensation point too high for survival indoors, where light levels are much lower. However, the compensation point can be adjusted gradually when plants are moved to less light, if the difference in light levels is not too drastic. When this happens, foliage plants usually drop leaves to conserve food, with the amount of foliage loss dependant on the degree of light reduction. Production regimes producing acclimatized plants capable of adjusting to interiors with minimal leaf drop has become standard in the environmental plant industry. Plants with many small leaves having a compact growth habit were grown under higher light levels than plants of the same species having a more open canopy with fewer, but larger, dark green leaves and a less compact growth habit.

Light intensity depends on: l) the light source, natural, artificial or a combination of the two, 2) obstructions, natural and man made and 3) the amount of reflection from design elements. A locations' light intensity is determined by averaging readings taken at plant height between the hours of 11:00 a.m. and 1:00 p.m. on several days (during both cloudy and sunny weather).

Light duration refers to the amount of time during a 24 hour period that plants are exposed to light. Duration is critical because a longer exposure can compensate for unsatisfactory light intensities. When intensity is inadequate, increasing light duration will help plants adjust; conversely, when intensity is too high, a shorter exposure time may solve lighting problems. Research has shown that plants maintained indoors grew better when they received between 12 and 18 hours of light daily, while continuous 24 hour lighting was detrimental to some species. Light intensity and duration are the two primary factors affecting indoor plant survival and should be carefully determined before foliage plants are purchased and installed. Only plants able to survive in the existing light conditions should be considered suitable for that site.

GROWING MEDIA

When repotting foliage, interiorscapers can be confused by. the wide selection of prepackaged growing media on the market as well as the range of raw materials available to create individual growing mixtures. Fertilizer levels in Table l are based on utilization of potting media composed primarily of organic components with a high cation exchange capacity. The cation exchange capacity of a medium determines its nutrient retention capabilities. Most prepackaged mixes contain mainly organic materials. Potting media composed of large amounts of sand, perlite, polystyrene foam or pine bark may require slightly higher fertilizer levels because of decreased nutrient retention ability, especially where frequent or heavy leaching occurs.

pH

An understanding of potting medium pH is necessary because it controls the release of nutrients. A low pH will reduce conversion of ammonium to nitrate nitrogen, while high pH levels reduce availability of most microelements. Most foliage plants grow best when the pH of the medium is between 5.0 and 6.5 and most commercially produced potting mixes are within this range.

Low pH levels in potting media can be adjusted upward by addition of liming material such as dolomite or calcium carbonate. High pH levels can be lowered by addition of sulfur. The amount of liming material or sulfur needed to obtain a desired pH depends on the type of organic material present in the medium and the original pH. Small amounts of lime or sulfur will change phi of sandy potting mixes because of their lower cation exchange capacities. Mixes primarily made up of peat moss have higher cation exchange capacities and require large amounts of liming materials or sulfur to adjust pH.

Adjustment of pH levels should be made prior to repotting plant material since changing the pH is more difficult once plants are growing in the medium. When plants do not need repotting but require pH adjustments, the best material to raise pH is calcium hydroxide (hydrated lime), 1 lb/100 gallon. Plants can be retreated every 4 weeks until desired pH is obtained. Calcium carbonate applied to the medium surface will also raise pH.

When pH is too high, sulfur can be applied at a rate of 1 lb/100 ft² to lower pH. Sulfur may be applied once every 4 weeks until the desired pH level is obtained. Do not apply liming material or sulfur more often than once every 4 weeks. Irrigation after application of liming materials or sulfur application will remove residues from foliage if present.

In addition to raising pH, dolomite also provides the essential elements calcium and magnesium. The addition of sulfur to lower the pH will provide that essential element. If these elements have not been added to the media during the manufacturing process or while adjusting the pH, they need to be added by some other means to ensure proper plant growth.

SOLUBLE SALTS

Soluble salts levels of the growing medium should be determined prior to utilization. Many interiorscapes were not designed to accommodate large amounts of leachate, therefore plants must be watered carefully so that minimal leaching occurs. When media have high soluble salts concentrations, they should be heavily leached to remove excess salts prior to utilization. The addition of unnecessary fertilizer ions into the medium can be avoided by careful monitoring of fertilizer programs and plant damage can be reduced by using less fertilizer.

FERTILIZER

The recommended interior fertilizer application rates for selected foliage plants are listed in Table 1. Application rates are recommended for four light intensities because plant's nutritional requirements change with a change in light intensity. When light levels are very low, below 75 ft-c, just the natural decomposition of peat moss and/or bark in potting mixes can provide sufficient nitrogen for most foliage plant species. At such low light levels, plants grow very slowly and use very small amounts of fertilizer. A common mistake interior technicians make is preparing one liquid fertilizer solution for all plants in an interior environment, disregarding the greatly divergent light levels in which the various plants are maintained. Typically, this causes the plants in the lower light levels to be damaged by excess salts accumulating in the growing medium, while plants in the highest light levels produce pale, weak, unattractive growth caused by under fertilization.

Other factors which must be considered when establishing the amount of fertilizer plants use are temperature, watering frequency and the amount of water applied per irrigation. Most foliage plants originated in the tropics and therefore grow very slowly when growing medium temperatures drop below 65°F or night air temperatures are 70°F or below. Maintenance of fertilizer levels listed in Table 1 under these lower temperature conditions could cause unused fertilizer salts to accumulate in the growing medium; therefore, application rates should be reduced at least 50% until warmer conditions return.

Slow-release fertilizers are temperature dependent. The specified release period, the amount of time fertilizer pellets actively release nutrients (the Osmocote listed in Table 4 has a release period of one year), is usually based on a median temperature of 70°F but may be set higher by the manufacturer if the product is to be used in a warmer area. The release period is listed on the product packaging and should be noted. As growing medium cools, the rate at which fertilizer is released slows; as medium warms the release rate increases. Slow-release fertilizer application rates can be manipulated by lengthening the time between applications when plants are placed in cool interiors and decreasing the time between applications when foliage is maintained under warmer conditions.

The amount of water applied to the pot at each watering affects the amount of fertilizer leached from the potting medium and thus affects fertilizer application rates. Recommended amounts of the fertilizers listed in tables 2, 3 and 4 are for plants that are watered so that the potting medium is thoroughly saturated, but little or no leaching occurs. In most interiorscapes, moderate to heavy leaching is impractical and/or impossible to perform, consequently care must be taken to avoid soluble salts buildup in the medium.

Fertilizer selection is economically important. Nitrogen is available from three primary sources, nitrate (N0₃^-), ammonium (NE₄⁺) and urea [CO(NH₂)₂]. In the past, most fertilizers contained 20-50% nitrate nitrogen and the remainder came from ammonium or urea nitrogen forms. Today, fertilizer manufacturers are substituting urea for much of the nitrate nitrogen because of its lower cost. Recent research on foliage plants has shown that fertilizers containing ammonium and urea nitrogen performed as well or better than fertilizer containing nitrate nitrogen. Although no problem was found when 100% urea or ammonium nitrogen sources and combinations of the two were tested on selected foliage, tests on all genera have yet to be conducted; therefore a small amount of nitrate nitrogen (10-15%) is still recommended for foliage. On flowering crops the use of 25-50% nitrate nitrogen is advised.

Relative levels of nitrogen, phosphorous and potassium in a fertilizer analysis are referred to as the N-P₂0₅-K₂O ratio. Research on plants maintained indoors for extended periods show that plants grow equally well with 1-1-1 ratio fertilizers such as 14-14-14 and 20-20-20 or 3-1-2 ratio fertilizers like 9-3-6 and 18-6-12. The use of 3-1-2 ratio fertilizers is recommended because of the cheaper cost per unit of nitrogen and also because the lower total medium soluble salts levels generated improves a plants' ability to adjust to interior environments.

When preparing fertilizer solutions, remember that liquid fertilizer formulations containing more than 9 grams/gallon nitrogen may bum foliage. When it is not possible to rinse off irrigation water containing high concentrations of nitrogen splashed on leaves, consider dividing the fertilizer application rate in half and making two applications instead of one. Example: (rates taken from Table 3) 14.2 grams 20-20-20/14 inch pot/4 months can be adjusted to 7.1 grams 20-20-20/14 inch pot/2 months.

MICROELEMENTS

All microelements necessary for foliage plant growth are usually added to most commercially prepared growing mixes during production. When creating a medium, microelements must be added and should be thoroughly incorporated. Micromax (Grace - Sierra Co., Milpitas, CA 95035) and Perk (Estech General Chemical Corp., Chicago, IL ) are just two examples of microelement sources available. Both gave excellent results in experimental plots when added at the rate of l to l-l/2 lbs/yd3. If micronutrients have not been mixed into the potting medium, they may be added separately or incorporated into the fertilizer program, either as a periodic treatment or along with every fertilizer application.

Incorporation of superphosphate into potting media for foliage plants was once a common practice but is no longer recommended because superphosphate contains 1 to 2% fluoride, a contaminant which has been shown to cause serious phytotoxicity on the following genera; Calathea, Chlorophytum, Cordyline, Dracaena, Maranta, and Yucca. Research on foliage plants has shown that incorporation of superphosphate is unnecessary for the production and maintenance of good quality foliage when other sources of phosphorus are used.

*Professor and Center Director (retired 7/96), Professor, Plant Physiology and Professor, Environmental Horticulture, respectively, Central Florida Research and Education Center-Apopka, 2807 Binion Road, Apopka, FL 32703-8504.

ADDITIONAL READING

l. Braswell, J.H., T.M. Blessington, and J.A. Price. 1982 Influence of cultural practices on postharvest interior performance of two species of schefflera. HortScience 17(3):345 347.

2. Collins, P.C. and T.M. Blessington. 1985. Keeping quality of Ficus benjamina as affected by production light levels and post production light quality and level. HortScience 20(3):390-39 l .

3. Conover, C.A. and R.T. Poole. 1980. Interior quality of Dracaena angustifolia Roxb. 'Honoriae' as influenced by light and fertilizer during production. HortScience 15(1):24-26.

4. Conover, C.A. and R.T. Poole. 1981. Incremental growth of two foliage plants maintained under artificial light levels for one year. Foliage Digest 4(7):5-7.

5. Conover, C.A. and R.T. Poole. 1981. Influence of light and fertilizer level and fertilizer sources on foliage plants maintained under interior environments for one year. J. Amer. Soc. Hort. Sci. 106(5):517-574.

6. Conover, C.A., R.T. Poole and T.A. Nell. 1982. Influence of intensity and duration of cool white fluorescent lighting and fertilizer on growth and quality of foliage plants. J. Amer. Hort. Sci. 107(5):817-822.

7. Conover, C.A. and R.T. Poole. 1990. Light and fertilizer recommendations for production of acclimatized potted foliage plants. University of Florida, IFAS, Central Florida Research and Education Center - Apopka, CFREC-A :Research Report RH-90-1.

8. Dunn, S. 1975. Lighting for plant growth or maintenance. Florist's Review 156(4054):41, 86-90.

9. Henley, R.W. 1989. Selected foliage plants grown for retail markets. Foliage News 14(8): 8pp. University of Florida, Florida Cooperative Extension Service.

10. Henley, R.W. 1989. Major foliage plants utilized by the interior landscape industry. Foliage News 14(9): 8pp. University of Florida, Florida Cooperative Extension Service.

11. Marchant, Brent. 1982. Basic lighting techniques for interior landscapes. Amer. Nurseryman 155(S):87-95.

12. Turner, Melanie A., David L. Morgan and David Wm. Reed. 1987. The effect of light quality and fertility on long term maintenance of selected foliage plants. J. Environ. Hort. 5(2):76-79.

13. Weiler, T.C., G.M. Pierceall and J.A. Watson. 1982. Light requirements of interior plants. American Nurseryman. 156(11):39-42.

^ZRate m g N/ft²/yr. Categories are defined as 1 = 2g, 2 = 4g, 3 = 6g, 4 = 8g.

1. ^aCategories are defined as 1 = 2 g, 2 = 4 g, 3 = 6 g and 4 = 8 g N/ft2/yr.
   ^bOne teaspoon 14-14-14 equals approximately 5 grams.

1. ^aCategories are defined as 1 = 2g, 2 = 4g, 3 = 6g and 4 = 8g N/ft²/yr.
   ^bOne teaspoon 20-20-20 equals approximately 5 grams

When preparing soluble fertilizer solutions always remember that liquid fertilizer formulations containing more than 9 g/gal N may burn foliage. Foliage should be rinsed with clear water to remove the fertilizer.

1. ^aOne level teaspoon = approximately 5 grams.
   ^bCategories are defined as 1 = 2g, 2 = 4g, 3 = 6g and 4 = 8g N/ft²/yr.