Jack W. Buxton

Psychrometric and ventilation constraints for vapor pressure deficit control

Constraints on air temperature, dewpoint temperature and ventilation rate in plant growth and propagation chambers under vapor pressure deficit (VPD) control are presented. These constraints apply to any controlled environment space subjected to significant radiation in which both air temperature and humidity can be simultaneously manipulated. Defining relations and logic necessary to implement two methods of VPD control (VPD air vs. VPD crop-air ) are described. Theoretical analysis and experimental results demonstrate that constraints for VPD control are inevitable, and are exacerbated by low air flow rates and high levels of incident radiation. Interactions between VPD air and VPD crop-air for rooted poinsettia cuttings showed that by maintaining VPD air constant at 1500, 2000 and 2500 Pa, the corresponding VPD crop-air increased directly with incident radiation from 50 to 300 W m -2 . Interactions between crop, air and dew point temperature for VPD air control at 1500, 2000 and 2500 Pa under different levels of incident radiation are also presented. Under the constraints identified, VPD control from manipulating dew point temperature, air flow rate and cooling is demonstrated for rooted poinsettia cuttings. This work forms a basis for defining operational constraints necessary to develop a real-time VPD controller.

Vapor Pressure Deficit Control Strategies for Plant Production

Abstract Research to develop control strategies to implement vapor pressure deficit (VPD) control for plant propagation is presented. Recently designed and constructed lowcost chambers serve as the platform for this discussion, although results can be generalized to any other equipment in which both air temperature and humidity can be simultaneously manipulated. Defining relations and logic necessary to implement two methods of VPD control are developed and described.

Ethrel studies on Petunia hybrida Vilm.

Abstract The ethylene releasing growth regulator ethrel (2-chloroethyl) phosphonic acid, in liquid and 5% granulated dry slow-release forms, was applied to petunias cultivars ‘Royal Cascade’, ‘Chiffon Cascade’, ‘White Cascade’ and ‘Pink Cascade’ as foliar spray, soil drenches or mixed in the soil medium. Granular and liquid forms of ethrel were effective as growth retardants and branch-including agents. Plant height and diameter were reduced and up to 300% increase in basal branches occurred, compared with control plants. The most effective and consistent results were observed with ethrel used as a soil drench. Best treatments for height control without phytotoxic symptoms were 250–500 p.p.m. as soil drenches, 500 p.p.m. foliar sprays and 0.1–0.4 g/pot soil incorporated granulated ethrel formulation. Branches forced by ethrel treatment remained functional. Flowers were delayed by ethrel treatments, but treated plants transplanted outdoors produced more flowers during the growing-season.

EVAPOTRANSPIRATION–BASED MISTING CONTROL FOR POINSETTIA PROPAGATION

Theoretical development and validation of a model–based misting control system for propagating poinsettias (Euphorbia pulcherrima ‘Freedom Dark Red’ Willd. ex Klotzsch) are presented. The control system predicts evapotranspiration (ET) using the Penman–Monteith combination model, with the canopy surface resistance term replaced by an equivalent surface resistance that includes the combined resistance effects of plant stomata, water uptake, propagation medium, and leaf adaxial surfaces. Different combinations of incident radiation (Ri) and air vapor pressure deficit (VPDair) were used to validate this technique by comparing measured and estimated values of ET of partially rooted cuttings under partially wetted conditions. Under dark conditions, four ranges of VPDair were applied (0.8 to 1.1, 1.3 to 1.6, 1.8 to 2.1, and 2.3 to 2.6 kPa). Under light conditions, three levels of Ri (100, 200, and 300 W m–2) were combined with three ranges of VPDair (1.3 to 1.6, 1.8 to 2.1, and 2.3 to 2.6 kPa). Observed and predicted values of ET under dark conditions were in remarkable agreement, with the ET model overestimating actual ET by 6% and 8% at root emergence and after one week, respectively. During light conditions, predicted ET was at most 14% larger than observed values. The use of predicted ET in misting control for propagation of vegetative cuttings has several advantages over other techniques, such as timer–based scheduling and techniques that account for only Ri or VPDair, by applying only enough water to maintain water status of the cutting.

Energy conservation by ventilating a greenhouse with deep-mine air

Abstract Use of deep-mine air to heat or cool a greenhouse has the potential to reduce significantly the energy required for environmental control. The environment is also potentially well suited for growth of many greenhouse crops. Temperatures within the greenhouse were generally maintained at 10–13°C during the night and 16–24°C during the day when solar energy was available. The relative humidity remained at 100% day and night during cloudy weather and fell as low as 40–50% on days of high solar intensity. In addition, the carbon dioxide concentration was near 2400 mg 1−1 throughout the year. Flowering bedding-plants and vegetable transplants of high quality were produced during the spring season. High - quality lettuce was produced throughout most of the year. The production time of all crops was equal to that of crops grown commercially in the area.