Coleus Growth Response to Phosphorus Fertility
Coleus (Plectranthus scutellarioides) is a popular herbaceous annual that people grow for its wide variety of colors. When plants receive inadequate P fertility, growth can range from being compact to stunted. Excessive P results in plant stretch and other NC State University researchers have reported that for many species only 5 to 15 ppm of P is needed for healthy growth. Plants tend to visibly respond to the amount of P being applied and it is easy to chart the overall growth.
Coleus’ plant growth is highly dependent upon P levels, which is why we conducted an experiment at North Carolina State University to determine the response to six P fertility rates (2.5, 5, 10, 15, 20, and 40 ppm P) to measure the growth response across two different cultivars. This trial was completed using the cultivars ‘Salsa Verde’ (Ball Horticulture, West Chicago, IL) (green) and ‘Wall Street’ (Dummen Orange, Columbus, OH) (orange).
When examining various P fertility rates on coleus, plant height was significantly limited for plants that received 2.5, 5, or 10 ppm P. Thus, we would recommend a higher fertility rate to achieve adequate growth. Plant dry weight and diameter increased linearly on plants that received a P fertility rate of 2.5, 5, and 10 ppm P, but then a diminished rate of plant growth occurred with higher P rates. The growth of the plants plateaued. When data was subjected to nonlinear regression, ‘Wall Street’ plants exhibited a plateau point at 14.7 ppm P, and ‘Salsa Verde’ exhibited a plateau point at 13.9 ppm P in regards to growth index (GI) (which takes into account plant height, plant diameter, and shoot dry weight). Thus for these two cultivars of coleus, rates higher than 14 to 15 ppm P provided no additional benefit and growers can use these values for growing coleus.
For more information and to view photos, view the e-GRO Research Report 2022-01, Coleus: Optimizing Phosphorus.
Brian WhipkerProfessor, Horticulture, North Carolina State University
Brian E. Whipker is a professor of floriculture at North Carolina State University. He has an appointment split between extension and research, but also teaches the graduate level plant nutrition course at NC State. His specialization is plant nutrition, plant growth regulators, and problem diagnostics. He brings over 30 year of experience in diagnosing grower plant problems.