Stratified Cutting Propagation Substrates Reduce Herbaceous Plug Structural Stability but Facilitate Similar Root Growth to a Control

Adventitious root development on cuttings may be promoted by oxygen availability in the rooting zone. Commercial propagation substrates are frequently peat-based, which can be problematic when used in propagation systems with intermittent misting due to excessive water holding. We tested top- or middle-stratifying a peat-based substrate with coarse amendments or a fine biochar. We had seven treatments, with a commercial peat-based propagation substrate as our control, then applying either a top (3 cm deep) or middle (2 cm deep) layer of coarse perlite, parboiled rice hulls, or fine coconut shell biochar in a pot with a top diameter of 6.3 cm, depth of 6.1 cm, and volume of 140 cm 3 . We stuck unrooted cuttings of four herbaceous plants [hybrid hyssop ( Agastache × ‘Sunrise Red Improved’), false heather ( Cuphea hyssopifolia ‘Maria’), blanket flower ( Gaillardia × grandiflora ‘Eclipse’), and Jame sage ( Salvia × jamensis ‘Ignition Purple’)]. We used completely randomized designs, with each plant species treated as a separate experiment. Each experiment had a replication of 7. We repeated the experiments in two series. After ∼4 weeks, we evaluated plug stability with a subjective scale (0 = no visible roots, unstable; 1 = minimal roots, one or two roots visible, unstable; 2 = moderate roots, more than two roots but large spaces with no roots, partially unstable; 3 = acceptable roots, many roots with only small gaps, stable; 4 = optimum roots, many roots with full coverage, stable), dry root weight, total root area, and total root length. Stratification significantly reduced plug structural stability. Across experiments and series, there was not a clear stratification effect on root growth. Considering that stratification with perlite, parboiled rice hulls, and fine biochar reduced plug structural stability but did not provide clear growth advantages, growers should consider alternate stratification materials that provide sufficient binding to maintain plug stability.