Effects of Light, Volumetric Water Content, and Soil Nutrition in Greenhouse Propagation of the Clonal Sub-Shrub Linnaea borealis

Linnaea borealis (twinflower), a clonal groundcover native to circumboreal habitats, is declining in some areas and holds ecological value for conservationists. The plant also has commercial horticultural potential as a native alternative to more traditional shade groundcovers, but New England growers report inconsistent success at propagation efforts, with frequent failure to form roots and root rot.Two studies sought to improve propagation protocols for twinflower. In Study 1, 240 stem cuttings from 3 sites were grown over 7 1/2 weeks under three light treatments (DLI 27.6 mol•m-2•d-1, DLI 14.4 mol•m-2•d-1, DLI 5.8 mol•m-2•d-1) and 4 substrate volumetric water content levels (0= 0.30, 0.35, 0.40, and 0.45 L•L-1), then harvested and evaluated by root dry weight totals. Water treatments were significant, with 0.30 and 0.35 L•L-1 levels yielding an average root weight of 40 mg and 0.40 and 0.45 L•L-1 yielding an average root weight of 23 mg. Lower 0 treatments produced 74% more root dry mass than higher 0 treatments.Also significant were the interaction of light and 0. Higher 0 treatments produced less root dry mass across all light treatments, except under no shade, where evaporation and irrigation limitations led to many wet and dry microclimates. Conclusions indicate twinflower cuttings perform best under 0 = 0.30 and 0.35 L•L-1 (moderate to low irrigation) and DLI 14.4 mol•m-2•d-1 (light shade). Cuttings also produced high root totals under DLI 27.6 mol•m-2•d-1 (no shade), but only at high irrigation levels, and greater water usage for similar results may not be desirable for commercial growers.Study 2 implemented light and 0 regimens from Study 1 to evaluate the impact of pre-incorporated substrate fertilizer on root and shoot growth in twinflower. An additional 240 fresh cuttings from 3 sites were grown over 8 V2 weeks in a 1:1 peat:fine vermiculite substrate under moderate shade (average DLI 10.6 mol•m-2•d-1) and 0=0.35 L•L-1, then tested with 3 levels of fertilization: 0, 2.1, or 5.0 g•L-1 of incorporated, controlled release fertilizer. The 2.1 g•L-1 rate (average 14 mg) and 5.0 g•L-1 rate (average 17 mg) both significantly increased root biomass production over control (average 12 mg).Shoot biomass significantly increased at both fertilizer rates, with the 5.0 g•L-1 rate (at 127 mg) representing a 38.1% increase over the 2.0 g•L-1 rate (at 92 mg), itself a 105.5% increase over control (at 45 mg). Diy foliar nutrient concentration showed significantly higher amount of nitrogen, phosphorus, potassium, and manganese between the fertilizer treatments and control, but no significant difference in concentrations between the 2.1 g•L-1 and 5.0 g*L_1 rates. Data indicate boosts in root and shoot biomass production with light and moderate pre- incorporation of fertilizer. Increases in foliar nutrient concentration are commensurate with supplemental substrate fertilization of those nutrients. Number of stem nodes increased significantly with fertilizer, with no differences between the application rates. Similarly, root:shoot ratio of plants decreased significantly with fertilizer, but with no significant differences between ratios of the 2.1 g•L-1 and 5.0 g•L-1 rates.