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Subirrigation is an economically attractive irrigation method for producing bedding plants. Because excess fertilizer salts are not leached from the growing medium, salts can accumulate in the growing medium. Fertilizer guidelines developed for overhead irrigation may not be appropriate for subirrigation systems. Our objective was to quantify the effect of the fertilizer concentration (N at 0, 135, 285, and 440 mg·L^–1) on whole-plant CO₂ exchange and growth of subirrigated pansies. Whole plant CO₂ exchange rate (net photosynthesis and dark respiration) was measured once every 10 min for 31 days. Whole-plant photosynthesis, dark respiration, and carbon use efficiency increased during the experiment. Fertilizer concentration started to affect the growth rate of the plants after approximately 7 days. Maximum photosynthesis and growth were achieved with N at about 280 mg·L^–1 in the fertilizer solution [electrical conductivity = 2 dS·m^–1]. Growth was reduced by ≈10% when the plants were fertilized with N at 135 and 440 mg·L^–1 compared to 280 mg·L^–1. Growth of plants watered without any fertilizer was greatly reduced, and plants showed symptoms of N and K deficiency. The size of the root system decreased and the shoot: root ratio increased with increasing fertilizer concentration, but the size of the root system was adequate in all treatments. These results indicate that subirrigated pansies can tolerate a wide range of fertilizer concentrations with relatively little effect on plant growth.

Fertilizer recommendations for fertilizing bedding plants are normally based on nitrogen content of the fertilizer solution. However, nutrient availability is more closely related to the concentration of nutrients in the growing medium than the concentration in the fertilizer solution. Environmental conditions can affect the accumulation of nutrients in the growing medium and optimal fertilizer concentrations are likely to depend on environmental conditions. To test this hypothesis, we grew petunias and geraniums under three temperature regimes (35 °C/27C, 25 °C/17C, and 15 °C/7 °C) and with five concentrations of fertilizers [electrical conductivity (EC) of 0.15, 1, 2, 3, and 4 dS·m^–1]. Temperature and fertilizer EC affected the plant growth. Optimal fertilizer EC decreased as temperature increased. Growth was better correlated with EC of the growing medium than with EC of the fertilizer solution. Irrespective of growing temperature, plant growth was best when EC of the growing medium was between 3 and 4 dS·m^–1. A lower growing medium EC slowed down growth, presumably because of mild nutrient deficiencies. Higher fertilizer concentrations in the growing medium (>4 dS·m^–1) decreased growth because of salt stress. The EC of the growing medium increased with increasing EC of the fertilizer solution and with increasing temperature. Because of the interactive effect of fertilizer concentration and temperature on the EC of the growing medium, plants should be grown with more dilute fertilizer solutions at higher temperatures. Fertilization guidelines for growers should be based on maintaining the EC of the growing medium within an optimal range instead of the more traditional recommendations based on the concentration of the fertilizer solution.

In the last decade, there has been an increasing focus on maintaining the electrical conductivity (EC) of leachate of bedding plants within an optimal range. However, there has been no research determining whether an optimal leachate EC results in better growth than using constant fertilizer concentrations throughout the production period. To evaluate the effects of constant fertilizer concentrations and constant leachate EC on the growth of wax begonia (Begonia × semperflorens-cultorum Hort.) ‘Cocktail mix’ and petunia (Petunia × hybrida Hort. Vilm-Andr.) ‘Gnome white’, we grew plants either with one of six different fertilizer concentrations (fertilizer EC of 0.5, 1.5, 2.5, 3.5, 4.5, or 5.5 dS·m⁻¹) or by maintaining a leachate EC close to 0.5, 1.5, 2.5, 3.5, 4.5, or 5.5 dS·m⁻¹. The leachate EC of plants fertilized with constant fertilizer concentrations increased throughout the experiment if the fertilizer EC was 2.5 dS·m⁻¹ or higher, was stable in the 1.5 dS·m⁻¹ treatment, and decreased in the 0.5 dS·m⁻¹ treatment. In treatments in which we tried to maintain the leachate EC constant, the leachate EC on average was within 0.2 dS·m⁻¹ of the target EC. As a result of the acidic nature of the fertilizer, the pH of the growing medium decreased throughout the experiment with increasing leachate or fertilizer EC. When plants were fertilized with constant fertilizer concentrations, fertilizer solution EC of 0.52 and 1.24 dS·m⁻¹ were estimated to be optimal for begonia and petunia, respectively. When the growing medium was maintained at a constant EC, 1.0 and 1.7 dS·m⁻¹ were estimated to be optimal for begonia and petunia, respectively. Growth of both begonia and petunia was greatly inhibited when high fertilizer concentrations caused accumulation of soluble salts in the growing medium. Growth was reduced more by high fertilizer EC than by high leachate EC treatments. This difference probably occurred because superoptimal fertilizer concentrations resulted in very high leachate EC (up to 10.5 dS·m⁻¹ for petunia and 12.5 dS·m⁻¹ for begonia), which in turn inhibited growth. Periodic measurements of leachate EC can be a valuable tool in fertilizer management to prevent such excess buildup of salts in the growing medium.

To determine the effect of fertilizer concentration on plant growth and physiology, whole-plant C exchange rates of pansies (Viola ×wittrockiana Gams.) subirrigated with one of four fertilizer concentrations were measured over 30 days. Plants were watered with fertilizer solutions with an electrical conductivity (EC) of 0.15, 1.0, 2.0, or 3.0 dS·m^-1 (N at 0, 135, 290, or 440 mg·L^-1, respectively). Plants watered with a fertilizer solution with an EC of 2 dS·m^-1 had the highest shoot dry weight (DW), shoot to root ratio, leaf area, leaf area ratio (LAR), and cumulative C gain at the end of the experiment compared to those watered with a solution with a higher or lower EC. Shoot tissue concentrations of N, P, K, S, Ca, Fe, Na, and Zn increased linearly with increasing fertilizer concentration. A close correlation between final DW of the plants and the measured cumulative C gain (CCG) (r² = 0.98) indicated that the C exchange rates were good indicators of plant growth. There were quadratic relationships between fertilizer EC and gross photosynthesis, net photosynthesis, and dark respiration, starting at 13, 12, and 6 days after transplanting, respectively. Although plants fertilized with a fertilizer solution with an EC of 2 dS·m^-1 had the highest C exchange rates, the final differences in shoot DW and CCG among ECs of 1.0, 2.0, and 3.0 dS·m^-1 were small and it appears that pansies can be grown successfully with a wide range of fertilizer concentrations. Plants with a high LAR also had higher DW, suggesting that increased growth was caused largely by increased light interception. A detrimental effect of high fertilizer concentrations was that it resulted in a decrease in root DW and a large increase in shoot to root ratio.

To evaluate the effects of nutrient concentration and pH of the fertilizer solution on growth and nutrient uptake of salvia (Salvia splendens F. Sellow ex Roem. & Schult. `Scarlet Sage'), we grew plants with five different concentrations of Hoagland nutrient solution [0.125, 0.25, 0.5, 1.0, and 2.0× full strength; electrical conductivity (EC) of 0.4, 0.7, 1.1, 2.0, and 3.7 dS·m^-1, respectively]. In a concurrent experiment, plants were subirrigated with modified Hoagland solution at 0.5× concentration and one of five solution pH values: 4.4, 5.4, 6.4, 7.2, and 8.0. Shoot and total dry weight and leaf area increased greatly with increasing nutrient solution concentrations from 0.125 to 1.0×, while leaf photosynthesis (P_n), transpiration, and stomatal conductance decreased with increasing nutrient solution concentrations. Treatment effects on growth apparently were caused by changes in carbon allocation within the plants. Shoot: root ratio and leaf area ratio increased with increasing fertilizer concentration. Plants flowered 8 days later at low concentrations of nutrient solution than at high concentrations. Shoot tissue concentrations of N, P, K, and B increased, while C, Al, Mo, and Na decreased with increasing concentration of the nutrient solution. The pH of the nutrient solution had no effect on the growth or gas exchange of the plants, while its effects on nutrient concentration in the shoot tissue generally were smaller than those of fertilizer concentration. These results indicate that 1.0 to 2.0× concentrations of Hoagland solution result in maximum growth, apparently because the plants produce leaf area more efficiently at high fertilizer concentrations.

More efficient irrigation practices are needed in ornamental plant production to reduce the amount of water used for production as well as runoff of fertilizers and pesticides. The objective of this study was to determine how different substrate volumetric water contents (θ) affected petunia (Petunia ×hybrida) growth and to quantify the daily water use of the plants. A soil moisture sensor-controlled irrigation system was used to maintain θ within ≈0.02 m³·m⁻³ of the θ threshold values for irrigation, which ranged from 0.05 to 0.40 m³·m⁻³. Shoot dry weight increased as the θ threshold increased from 0.05 to 0.25 m³·m⁻³ and was correlated with the total amount of irrigation water applied over the 3-week course of the experiment. The daily water use of the petunias grown with a θ threshold of 0.40 m³·m⁻³ was 12 to 44 mL/plant and was positively correlated with both plant age and daily light integral. Lower θ thresholds resulted in a decrease in both leaf water (ψ) and osmotic potential (ψ_S). A decrease in turgor pressure (P) at lower θ was seen at 11, but not 20 days after the start of the treatments. There were no significant effects of θ on ψ, ψ_S, or P on fully rehydrated plants at the end of the study. Plants were able to survive and grow at all θs, although water at a θ less than 0.20 m³·m⁻³ is generally considered to be unavailable to the plants. Results show that it is possible to automatically irrigate plants with the use of soil moisture sensors, and this approach to irrigation may have applications in controlling the growth of ornamental plants.

Abscisic acid (ABA) is a plant hormone involved in regulating stomatal responses to environmental stress. By inducing stomatal closure, applications of exogenous ABA can reduce plant water use and delay the onset of drought stress when plants are not watered. However, ABA can also cause unwanted side effects, including chlorosis. Pansy (Viola ×wittrockiana) has been shown to be particularly susceptible to ABA-induced chlorosis. The objective of this study was to determine if fertilization rate affects the severity of ABA-induced chlorosis in this species. ‘Delta Premium Pure Yellow’ pansy seedlings were fertilized with controlled-release fertilizer incorporated at rates from 0 to 8 g·L⁻¹ of substrate. When plants had reached a salable size, half the plants were sprayed with a solution containing 1 g·L⁻¹ ABA, whereas the other plants were sprayed with water. Leaf chlorophyll content was monitored for 2 weeks following ABA application. Leaf chlorophyll content increased greatly as fertilizer rate increased from 0 to 2 g·L⁻¹, with little increase in leaf chlorophyll at even higher fertilizer rates. ABA induced chlorosis, irrespective of the fertilizer rate. Plant dry weight was lowest when no controlled-release fertilizer was incorporated, but similar in all fertilized treatments. ABA treatment reduced shoot dry weight by ≈24%, regardless of fertilization rate. This may be due to ABA-induced stomatal closure, which limits carbon dioxide (CO₂) diffusion into the leaves. We conclude that ABA sprays induce chlorosis, regardless of which fertilizer rate is used. However, because leaf chlorophyll concentration increases with increasing fertilizer rates, higher fertilizer rates can mask ABA-induced chlorosis.