Enhancing early spring microclimate to increase carbon resources and productivity in June-bearing strawberry

Effects of early spring cultural practices and microclimate manipulation on 'Jewel' strawberry (Fragaria xananassa Duch.) plant development, carbohydrate reserves, and productivity were measured in the field and under simulated early spring conditions in growth chambers. With traditional winter straw mulching practices of the northeastern and midwestern United States, starch content of overwintering leaves, crowns, and roots in the field declined by 51%, 78% and 69%, respectively, during late winter and early spring. There was also a net loss in root biomass over winter and no new leaf growth before mid-April, suggesting that carbohydrate reserves could be limiting plant performance during the critical early growth and flowering phase in spring. In growth chambers, exposure to CO2 levels between 700 to 1000 mL(.)L-1 significantly increased photosynthetic rates of overwintering and spring leaves compared to ambient CO2 levels. Elevated CO2 in growth chambers also accelerated flower development, reduced depletion of starch reserves in roots, and increased starch accumulation in crowns. In the field, early removal of straw and application of spunbonded rowcover accelerated plant development, increased starch accumulation in the leaves, and increased photosynthetic rates of overwintering and spring leaves. Elevating the CO2 levels under rowcover further increased photosynthetic rates and advanced plant development and starch accumulation, but not significantly above rowcover alone. Carbohydrate losses later in the season during flower development were reduced when rowcover was applied in early spring. Total fruit yield was as much as 48% higher for plants under rowcover in early spring than those that had no cover and an additional 9% higher when CO2 was elevated. Yield improvements were attributed mostly to an increase in the number of marketable secondary and tertiary fruit than to an increase in mean fruit size. The economics of rowcover use is favorable if the material is reused. The added expense of CO2 gas and the resulting marginal gains would not make field CO2 enrichment an economically viable practice for strawberry growers using the method herein.