Post-production Fertilization Research
Lane Greer
NCSU Floriculture
 

     High fertilizer rates, particularly high levels of nitrogen, decrease postharvest longevity and increase bud drop in crops such as potted roses and mums (Nielsen and Starkey, 1999; Ter Hell and Hendriks, 1995). Because plant demand for nutrients decreases at flowering, supplying unneeded nutrients results in a build-up of soluble salts. These higher soluble salt levels cause root injury, reducing the plants' ability to absorb water and nutrients (Nell et al., 1989).
     Type of nitrogen used is important to post-production life (Nell et al., 1997). Lower levels of ammonium lead to longer post-production life in potted roses and mums (Roude et al., 1991; Ter Hell and Hendriks, 1995). Nell (1993) states that the ratio of ammonium to nitrate nitrogen should be approximately 40% ammonium and 60% nitrate for flowering potted plants. In the past, lowering the amount of N relative to the amount of K applied during the last month of production was a recommended practice for mums. Newer research suggests, however, that it may be more important to switch to a nitrate based fertilizer during the last three weeks of production (Roude et al., 1991; Ter Hell and Hendriks, 1995).
     In mums, fertilizing until flowering provides better immediate plant quality, but this practice results in reduced longevity overall (Nell et al., 1989). Stopping fertilization 3 weeks before flowering will yield attractive plants that will also have a good shelf life (Nell et al., 1989).
     For potted roses, high calcium concentrations and low ammonium concentrations are more important to post-production life than plant spacing, air humidity, using capillary mats on benches, or adding lime to the peat medium (Nielsen et al., 1999). Roses with higher levels of calcium are also less likely to be infected with gray mold (Starkey and Pedersen, 1997).
     Harbaugh and Waters (1982) conducted an interesting study, in which potted Exacum plants were placed in a simulated home condition. After four weeks, the plants that had received high levels of fertilizer had fewer chlorotic leaves but also fewer flowers.
     In a study conducted by Staby and Kofranek (1979), terminating fertilization of poinsettia two weeks prior to flowering provided better plant quality than stopping fertilization four, one, or zero weeks before harvest. This study was complicated, however, by the fact that a controlled-release fertilizer (CRF) was incorporated into the potting media at planting (Staby and Kofranek, 1979).
     In another study on poinsettias, researchers found that 3 g/pot of a 3 to 4 month release formula (analysis 14N-6.1P-11.6K) provided wider plant diameters, darker bract color, and longer postharvest life than did a 3 g/pot application of an 8-9 month release formula with an analysis of 18N-2.6P-10K (Scott et al., 1984). The authors attribute better postharvest performance to the depletion of the 3-4 month release formula, compared to the continued presence of the 8-9 month CRF. Similar results were obtained with Exacum (Harbaugh and Waters, 1982). Plants fertilized with the 3-4 month CRF (14N-6.1P-11.6K) had fewer chlorotic leaves and higher floral display ratings.
     In petunias and ornamental cabbage, however, discontinuing fertilization is not a recommended practice, since it results in less attractive plants that have poor post-production life (Armitage, 1986; Gibson and Whipker, 2001). Continued nitrogen fertilization at a lower concentration is beneficial to these and several other crops.
Mums, petunias, and ornamental cabbage are all moderate to heavy feeders, but the research suggests that fertilization strategies differ among them. Growers, then, have to realize that fertility programs are crop-specific. Tables 1a, b, and c provide crop-specific recommendations. Table 3 summarizes previous post-production nutrition research.

Literature Cited

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