Subirrigation can reduce water loss and nutrient runoff from greenhouses, because used nutrient solution is collected and recirculated. Capacitance moisture sensors can monitor substrate volumetric water content (θ) and control subirrigation based on minimum θ thresholds, providing an alternative to timers. Our objectives were to automate an ebb-and-flow subirrigation system using capacitance moisture sensors, monitor moisture dynamics within the containers, and determine the effect of five θ thresholds (0.10, 0.18, 0.26, 0.34, or 0.42 m3·m−3) on hibiscus (Hibiscus acetosella Welw. ex Hiern.) ‘Panama Red’ (PP20,121) growth. Subirrigation was monitored using capacitance sensors connected to a multiplexer and a data logger and controlled using a relay driver connected to submersible pumps. As the substrate θ dropped below the thresholds, irrigation was turned on for 3 min followed by 3-min drainage. Capacitance sensors effectively controlled subirrigation by irrigating only when substrate θ dropped below the thresholds. Each irrigation cycle resulted in a rapid increase in substrate θ, from 0.10 to ≈0.33 m3·m−3 with the 0.10-m3·m−3 irrigation threshold vs. an increase in θ from 0.42 to 0.49 m3·m−3 with the 0.42-m3·m−3 irrigation threshold. Less nutrient solution was used in the lower θ threshold treatments, indicating that sensor control can reduce water and thus fertilizer use in subirrigation systems. The water dynamics showed that the bottom part of the pots was saturated after irrigation with θ decreasing quickly after an irrigation event, presumably because of drainage. However, the water movement among substrate layers was slow with the 0.10-m3·m−3 irrigation threshold with water reaching the upper layer 5.5 to 20 h after irrigation. The 0.10-m3·m−3 θ threshold resulted in 81% fewer irrigations and 70% less nutrient solution use compared with the 0.42-m3·m−3 θ threshold. However, the 0.10-m3·m−3 θ threshold also reduced hibiscus shoot height by 30%, shoot dry weight 74%, and compactness by 63% compared with the 0.42-m3·m−3 θ threshold. Our results indicate that soil moisture sensors can be used to control subirrigation based on plant water use and substrate water and to manipulate plant growth, thus providing a tool to improve control over plant quality in subirrigation systems.
Rhuanito Soranz Ferrarezi, Marc W. van Iersel and Roberto Testezlaf
Peter Alem, Paul A. Thomas and Marc W. van Iersel
Height regulation is crucial in poinsettia (Euphorbia pulcherrima) production for both aesthetics and postharvest handling. Controlled water deficit (WD) offers a potential alternative to plant growth retardants (PGRs) for poinsettia height regulation. We have previously shown that WD can be used to regulate poinsettia stem elongation. However, it is not clear how WD can be used to achieve different plant heights and how it affects aesthetic qualities such as bract size. Our objectives were to determine whether a range of plant heights can be achieved using controlled WD and to investigate possible adverse effects of WD on shoot morphology. Rooted cuttings of poinsettia ‘Classic Red’ were transplanted into 15-cm pots filled with 80% peat:20% perlite (v/v) substrate. Three target heights (43.2, 39.4, and 35.6 cm) were set at pinching (Day 27) and height tracking curves were used to monitor plants throughout the production cycle (77 days from pinching to finish). Substrate volumetric water content (θ) was maintained at 0.40 m3·m−3 (a matric potential of ≈–5 kPa) during well-watered conditions and reduced to 0.20 m3·m−3 (≈–75 kPa) when plants were taller than desired based on the height tracking curves. Control plants were maintained at a θ of 0.40 m3·m−3 throughout the study and had a final height of 51.2 cm. Plants with the 35.6-cm target height exceeded the upper limits of the height tracking curve despite being kept at a θ of 0.20 m3·m−3 for 70 days and had a final height of 39.8 cm. The final plant heights in the 39.4- and 43.2-cm target height treatments were 41.3 and 43.5 cm, respectively, within the 2.5-cm margin of error of their respective target heights. Relative to control plants, bract area was reduced by 53%, 47%, and 31% in the 35.6-, 39.4-, and 43.2-cm target height treatments, respectively. Our results indicate that WD can be an effective method of height control, but WD may also decrease bract size.
Amanda Bayer, John Ruter and Marc W. van Iersel
Controlling the elongation of ornamental plants is commonly needed for shipping and aesthetic purposes. Drought stress can be used to limit elongation, and is an environmentally friendly alternative to plant growth regulators (PGRs). However, growers can be reluctant to expose plants to drought stress because they do not want to negatively affect overall plant quality and marketability. Knowing how and when stem elongation is affected by water availability will help to increase our understanding of how elongation can be controlled without reducing plant quality. Rooted Hibiscus acetosella Welw. ex Hiern. cuttings were grown in a growth chamber set to a 12-hour photoperiod at 25 °C. Two plants of similar size were used for each replication of the study to compare growth under well-watered and drought-stressed conditions. Time lapse photography was used to determine the diurnal patterns of elongation over the course of the replications. Evapotranspiration was measured using load cells. Well-watered and drought-stressed plants had similar diurnal patterns of elongation and evapotranspiration, demonstrating that both follow circadian rhythms and are not just responding to environmental conditions. Stem elongation was greatest at night and coincided with evapotranspiration decreases, with greatest elongation shortly after the onset of darkness. Elongation was minimal between 800 and 1000 hr when evapotranspiration increases. During the drought-stress portion of the replications, elongation of drought-stressed plants was 44% less than well-watered plants. Final plant height and shoot dry weight for the drought-stressed plants were 21% and 30% less than well-watered plants, respectively. Total leaf area, number of leaves, and number of new visible internodes were greater for well-watered plants than drought-stressed plants. Average length of visible internodes and leaf size were similar for drought-stressed and well-watered plants. If growers want to use drought stress for elongation control, they should ensure that plants are drought stressed before the onset of and during the dark period, when most elongation occurs.
Amy L. Burton, Svoboda V. Pennisi and Marc W. van Iersel
Excessive internode elongation and leaf senescence are common problems with foliage plants transferred to interiorscapes. The authors’ objective was to determine whether plant growth regulators applied late in the production cycle could control growth during production and improve interiorscape performance. In addition, the authors wanted to quantify the effect of irradiance on growth and morphology during the production phase and in the interiorscape. Geogenanthus undatus C. Koch & Linden ‘Inca’ plants were grown under one of two photosynthetic photon fluxes (PPF; 50 or 130 μmol·m−2·s−1), and were treated with either α−(methylethyl)-α-[4-(trifluoromethoxy)phenyl]-5-pyrimidinemethanol (flurprimidol) or α-cyclopropyl-α-(4-methoxyphenyl)-5-pyrimidinemethanol (ancymidol) during the week 12 production, at 0.5, 1.0, or 1.5 mg/pot of active ingredient. The high PPF resulted in significantly higher leaf, stem, root, and total dry weight, and leaf area, but lower leaf area ratio (leaf area divided by total plant dry weight) compared with the low PPF. After production, plants were placed in a simulated interior environment for 4 months under a PPF of 15 μmol·m−2·s−1 and a photoperiod of 12 hours/day. Production PPF did not affect most growth parameters after 4 months in the interior environment, except for the root-to-shoot ratio. Under low-production PPF, root-to-shoot ratios were lower than under high-production PPF. For both growth regulators, the height and growth indexes were lower than for control plants, but flurprimidol offered greater control than ancymidol. Flurprimidol-treated plants had lower root dry weight and root-to-shoot ratios compared with ancymidol-treated and control plants. Applications of ancymidol or flurprimidol administered to G. undatus C. Koch & Linden ‘Inca’ late during the production cycle resulted in significant growth control and, therefore, superior plant performance throughout the postharvest period.
Svoboda V. Pennisi, Marc W. van Iersel and Stephanie E. Burnett
The growth of three english ivy cultivars in ebb-and-flow subirrigation systems was examined under three photosynthetic photon flux (PPF) treatments (low, medium, or high, corresponding to an average daily PPF of 3.2, 5.4, or 8.5 mol·m–2·d–1, respectively) and four fertilizer concentrations (0, 100, 200, or 300 mg·L–1 N) geared toward production of acclimatized foliage plants. Marketable quality english ivy can be subirrigated with 100 mg·L–1 N. Although 8.5 mol.m–2.d–1 produced the maximum shoot dry weight (SDW), good quality plants also were produced under 5.4 mol·m–2·d–1. `Gold Child', `Gold Dust', and `Gold Heart' english ivy produced with low fertility and low light may be better acclimatized and show superior performance in interior environments. Under light levels lower than 8.5 mol·m–2·d–1, `Gold Heart' had less variegation (12% or 21% for ivy grown under 3.2 or 5.4 mol·m–2·d–1, respectively). `Gold Dust' and `Gold Child' had 65% and 22% variegated leaf area, respectively, when grown under 5.4 mol·m–2·d–1 PPF. Under 5.4 mol·m–2·d–1 PPF, `Gold Dust' retains attractive foliage with overall perception of increased lighter-green coloration.
Stephanie E. Burnett, Marc W. van Iersel and Paul A. Thomas
French marigold (Tagetes patula L. `Boy Orange') was grown in a peat-based growing medium containing different rates (0, 15, 20, 30, 42, or 50 g·L–1) of polyethylene glycol 8000 (PEG-8000) to determine if PEG-8000 would reduce seedling height. Only 28% to 55% of seedlings treated with 62, 72, or 83 g·L–1 of PEG-8000 survived, and these treatments would be commercially unacceptable. Marigolds treated with the remaining concentrations of PEG-8000 had shorter hypocotyls, and were up to 38% shorter than nontreated controls at harvest. Marigold cotyledon water (ψw), osmotic (ψs), and turgor (ψp) potentials were significantly reduced by PEG-8000, and ψp was close to zero for all PEG-treated seedlings 18 days after seeding. Whole-plant net photosynthesis, whole-plant dark respiration, and net photosynthesis/leaf area ratios were reduced by PEG-8000, while specific respiration of seedlings treated with PEG-8000 increased. Marigolds treated with concentrations greater than 30 g·L–1 of PEG-8000 had net photosynthesis rates that were close to zero. Fourteen days after transplanting, PEG-treated marigolds were still shorter than nontreated seedlings and they flowered up to 5 days later. Concentrations of PEG from 15 to 30 g·L–1 reduced elongation of marigold seedlings without negatively affecting germination, survival, or plant quality. It appears that marigold seedlings were shorter because of reduced leaf ψp and reductions in net photosynthesis.
Lucas O’Meara, Marc W. van Iersel and Matthew R. Chappell
Irrigation is an essential component of ornamental plant production, yet relatively little is known about how much water nursery crops require to maintain optimal growth rates. Our objectives were to precisely determine the daily water use (DWU) of Hydrangea macrophylla and Gardenia jasminoides grown in 6-L containers, quantify how this is affected by environmental conditions, develop a quantitative model describing DWU, evaluate this model with an independent data set, and determine the feasibility of using crop coefficients (Kc) for irrigation scheduling. In 2010, we quantified the DWU of two Hydrangea macrophylla cultivars, Fasan and Pia. There was little difference in DWU of the two cultivars, which ranged from 50 to 300 mL/plant/day depending on plant age and environmental (hoophouse) conditions. In 2010, daily light integral (DLI) had the greatest influence on DWU with DWU increasing with increasing DLI. The combination of plant age, final leaf area, DLI, and their interactions explained 83.2% and 90.8% of day-to-day variation in DWU of ‘Fasan’ and ‘Pia’, respectively. In July 2011, a follow-up study was conducted using Hydrangea macrophylla ‘Fasan’ and Gardenia jasminoides ‘Radicans’. DWU of ‘Fasan’ ranged from 50 to 200 mL/plant/day and DWU of ‘Radicans’ ranged from 50 to 560 mL/plant/day. The lower DWU of ‘Fasan’ in 2011 compared with 2010 was the result of stunted growth of the hydrangeas, a result of elevated temperatures within the hoophouse during the plants initial growth flush. Vapor pressure deficit (VPD) explained more of the daily fluctuations in DWU in 2011 compared with 2010. Predicting DWU of the 2011 ‘Fasan’ crop using 2011 environmental conditions and the model developed using the 2010 data resulted in DWU estimates that were on average 64% too high. This discrepancy is likely the result of slower overall growth rate and a 15.4% reduction in ‘Fasan’ total growth in 2011 compared with 2010 and points to the challenges of modeling DWU. There were distinct seasonal changes in Kc values for the crops, but the trends differed between 2010 and 2011. Our results suggest that an accurate measure of canopy size may improve performance of predictive water use models.
Denise L. Olson, Ronald D. Oetting and Marc W. van Iersel
Coconut coir dust is being marketed as a soilless medium substitute for sphagnum peat moss that inhibits fungus gnat (Bradysia sp.) development. However, little information is available on the effects of coconut coir dust on Bradysia sp. In a laboratory study we examined the effect of substituting coconut coir dust for peat moss, with or without a food source, on the development of fungus gnats. An average of less than one adult emerged when 20 fungus gnat eggs were provided with pure or sterilized peat moss or coconut coir. A significantly higher number of adults (11.5-13) emerged when a food source of 1 g of yeast was added to either soilless potting medium type. The adults required up to 10 fewer days to emerge when food was provided, compared to sterilized and pure media, except for the pure peat moss. In a greenhouse study examining the effects of coir and peat at different textures and different moisture levels on fungus gnat survival, there were significant differences at the different levels of moisture. There was a higher population of larvae in the coarse medium containing peat. In the coir-based media, the fine-textured medium had the highest population level of fungus gnats. There were no significant effects on fungus gnat populations among the different levels of moisture within a medium type. However, there was a tendency for lower populations in the most moist and the driest media and the highest survival in the media that were maintained at 52.5% moisture. Plant growth was best in the media with the lowest number of fungus gnats (coarse coconut coir dust-based and fine and medium peat-based media). These results suggest that it is possible to select growing media that minimize fungus gnat populations, while optimizing plant growth. However, contrary to claims made by growing media producers, coconut coir dust does not necessarily inhibit fungus gnat development.
Amanda M. Miller, Marc W. van Iersel and Allan M. Armitage
Light and temperature responses of whole-plant CO2 exchange were determined for two cultivars of Angelonia angustifolia Benth., `AngelMist Purple Stripe' and `AngelMist Deep Plum'. Whole crop net photosynthesis (Pnet) of `AngelMist Purple Stripe' and `AngelMist Deep Plum' were measured at eight temperatures, ranging from 17 to 42 °C. Pnet for both cultivars increased from 17 to ≈20 °C, and then decreased as temperature increased further. Optimal temperatures for Pnet of `AngelMist Purple Stripe' and `AngelMist Deep Plum' were 20.8 and 19.8 °C, respectively. There was no significant difference between the two cultivars, irrespective of temperature. The Q10 (the relative increase with a 10 °C increase in temperature) for Pnet of both cultivars decreased over the entire temperature range. Dark respiration (Rdark) of both cultivars showed a similar linear increase as temperature increased. As photosynthetic photon flux (PPF) increased from 0 to 600 μmol·m-2·s-1, Pnet of both cultivars increased. Light saturation was not yet reached at 600 μmol·m-2·s-1. The light compensation point occurred at 69 μmol·m-2·s-1 for `AngelMist Purple Stripe' and at 89 μmol·m-2·s-1 for `AngelMist Deep Plum'. The lower light saturation point of `AngelMist Purple Stripe' was the result of a higher quantum yield (0.037 mol·mol-1 for `AngelMist Purple Stripe' and 0.026 mol·mol-1 for `AngelMist Deep Plum'). The difference in quantum yield between the two cultivars may explain the faster growth habit of `AngelMist Purple Stripe'.
Rhuanito Soranz Ferrarezi, Sue K. Dove and Marc W. van Iersel
Substrate volumetric water content (VWC) is a useful measurement for automated irrigation systems. We have previously developed automated irrigation controllers that use capacitance sensors and dataloggers to supply plants with on-demand irrigation. However, the dataloggers and accompanying software used to build and program those controllers make these systems expensive. Relatively new, low-cost open-source microcontrollers provide an alternative way to build sensor-based irrigation controllers for both agricultural and domestic applications. We designed and built an automated irrigation system using a microcontroller, capacitance soil moisture sensors, and solenoid valves. This system effectively monitored and controlled VWC over a range of irrigation thresholds (0.2, 0.3, 0.4, and 0.5 m3.m−3) with ‘Panama Red’ hibiscus (Hibiscus acetosella) in a peat:perlite substrate. The microcontroller can be used with both regular 24-V alternating current (AC) solenoid valves and with latching 6- to 18-V direct current (DC) solenoid valves. The technology is relatively inexpensive (microcontroller and accessories cost $107, four capacitance soil moisture sensors cost $440, and four solenoid valves cost $120, totaling $667) and accessible. The irrigation controller required little maintenance over the course of a 41-day trial. The low cost of this irrigation controller makes it useful in many horticultural settings, including both research and production.