Substrate Stratification Can Reduce Peat Requirement Associated with Young Plant Production

Production of young plants from cuttings and seed relies heavily on peat and frequent, but light, irrigation. Interest in reducing peat usage as well as a propensity for short container heights to inhibit drainage have led to the exploration of alternative techniques to improve substrate airspace in young plant production. Substrate stratification has been shown to be effective for reducing excessive moisture content and improving root growth in the lower strata of larger containers. This research evaluated the effects of substrate stratification in 5.1-cm tall, 37-cm3 cell plug trays using two plant propagation substrates: a bark-based vegetative cutting substrate (16% peat) and a peat-based seed germination substrate (65% peat). Each substrate was stratified by layering over either a commercially available wood fiber or horticultural grade perlite and was compared with an unstratified control. Substrate physical properties were measured on unplanted substrate treatments. Cuttings of two common bedding plants [coleus (Solenostemon scutellarioides ‘Salsa Verde’) and evolvulus (Evolvulus glomeratus ‘Blue Daze’)] were grown in the vegetative bark-based substrate treatments, and seeds of three common seed-started taxa [basil (Ocimum basilicum ‘Thai Towers’), hibiscus (Hibiscus moscheutos ‘Luna Pink Swirl’), and zinnia (Zinnia elegans ‘Zesty Purple’)] were grown in the seed peat-based substrate treatments. Finished plants were assessed for plug integrity and various growth parameters. Stratification with perlite increased airspace in the vegetative substrate only, not in the seed substrate. Stratification with wood fiber resulted in reduced airspace and increased container capacity in both substrate types. Stratification with perlite decreased plug integrity compared with nonstratified treatments, whereas wood fiber stratification resulted in similar or improved plug integrity, even in treatments in which root growth was reduced. Dry root biomass was greatest in both nonstratified substrates, with perlite stratification generally associated with the lowest root biomass. Perlite stratification was also associated with the lowest total root length and total root surface area, whereas wood fiber stratification resulted in values equivalent or greater than nonstratified treatments. Despite decreases associated with perlite stratification, however, both perlite- and wood fiber–stratified treatments produced quality plugs that established successfully post-transplant. The results demonstrate that using stratification in young plant production may provide growers with an opportunity to reduce peat consumption in propagation operations.