Best Management Practices for Reducing Greenhouse Gas Emissions in Ornamental Plant Production

Abstract

In 2018, agriculture accounted for nearly 10% of total greenhouse gas (GHG) emissions in the United States. Nearly half of N2O emissions in that 10% were attributed to the management of agricultural soils, through the application of synthetic and organic fertilizers, the cultivation of N-fixing crops, the drainage of organic soils, and irrigation practices. Other contributions include production of livestock (>25% of agricultural emissions), wetland production of rice, tillage, and burning of crop residues. The greenhouse, nursery, and floriculture industry was estimated to generate $16.7 billion in direct industry output in 2018, employing more than 134,000 people. While the plants produced generally serve as C and N sinks, the production process contributes to the increase in atmospheric GHG concentrations through its reliance on plastics, synthetic fertilizers, managed irrigation, and allied industry (electricity, transportation, etc.). The objective of this research was to evaluate GHG emissions during the plant production process as a factor of several best management practices, including irrigation method, fertilizer application method, light requirement, and use of alternative substrates. This work seeks to understand the mitigation potential of these practices in an effort to provide growers with quantitative data detailing the ‘worth’ of their efforts in anticipation of legislation regulating emissions, as well as any C credit trading system that could have financial benefits. In order to evaluate the impact of irrigation method (overhead impact or micro-emitted) and fertilizer placement (incorporated, dibbled), GHG emissions from the production of Japanese boxwood (Buxus microphylla) grown in 3-gal containers were measured once or twice weekly over nine months. Total cumulative CO2 emissions were unaffected by differences in irrigation or fertilizer delivery method. Emissions of N2O were highest for plant-pot systems receiving overhead impact irrigation and utilizing incorporated fertilizer. Regardless of fertilizer placement, the utilization of drip, or micro-emitted, irrigation significantly reduced N2O efflux over the course of the evaluation period. However, when limited to overhead impact irrigation, N2O emissions may be mitigated through the use of dibbled fertilizer. Fertilizer placement (incorporated, dibbled, or top-dressed) was also evaluated as a factor affecting GHG emissions in the production of two perennial species [full sun-grown daylily (Hemerocallis  ‘Stella D’Oro’ L.) and shade-grown hosta (Hosta  ‘Royal Standard’, or Hosta plantaginea Aschers  Hosta sieboldiana N.Fujita). Similar to the boxwood study, plant-pot systems utilizing the dibbled fertilizer method had the lowest CO2 and N2O emissions over the duration of the five month study. While no differences were observed between cumulative N2O emissions for plants fertilized with either incorporated or top-dressed fertilizer, cumulative CO2 emissions were lower for plants top-dressed with fertilizer, as compared to plants fertilized by incorporation. In the final study, wood fiber-amended substrates were evaluated for their effects on GHG emissions in the production of three annual species [coleus (Solenostemon scutellarioides Thonn. ‘Redhead’), vinca (Catharanthus roseus L. ‘Cooler Grape’, and impatiens (Impatiens walleriana Hook. f. ‘Super Elfin XP White’)]. Portions of peat and perlite were replaced in a traditional 80:20 peat:perlite blend to form substrates with either 20% WholeTree (80:20 peat:WholeTree) or 40% WholeTree (60:40 peat:WholeTree). No differences were observed for the main effect of species or media for either N2O or CH4. As expected due to its superior size, coleus had the highest cumulative CO2 emissions over the course of the 52-day evaluation period. As a main effect, substrate had a significant impact on CO2 emissions, as the 60:40 peat:WholeTree treatment had significantly higher cumulative CO2 emissions as compared to the other two substrates, regardless of plant species. Treatments with only 20% WholeTree were similar to the 80:20 peat:perlite industry standard. These results suggest that the utilization of a more sustainable material such as WholeTree in a substrate designed for greenhouse crop production could yield plants that are similarly sized with no impact on GHG emissions. Data from these studies builds on previous research in the pursuit of quantifiable GHG mitigation strategies for ornamental plant producers.