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Jonathan Foster, Stephanie Burnett, and Lois Stack

Twinflower (Linnaea borealis) is an understory subshrub native to northern regions of North America, Europe, and Asia. Some growers report that this native plant is difficult to propagate. Although twinflower prefers partial shade and grows in areas with naturally variable moisture, there has been no greenhouse propagation work testing the impact of light or soil moisture conditions on root development of this plant or whether fertilizer impacts root development or root:shoot ratios during propagation. The goal of the first experiment was to propagate twinflower under a variety of daily light integrals (DLI)—27.6, 14.4, or 5.8 mol·m−2·d−1—and soil volumetric water content values (θ = volume of water ÷ volume of soil) 0.30, 0.35, 0.40, and 0.45 L·L−1, both parameters aimed at reproducing a range of natural conditions. The largest roots were grown at DLIs of 5.8 and 14.4 mol·m−2·d−1 and θ values of 0.30 and 0.35 L·L−1. In the second experiment, twinflower plants were grown in substrates with 0, 2.1, or 5.0 g·L−1 of incorporated controlled-release fertilizer (14N–6.1P–11.6K). Root and shoot dry weight increased at both treatment rates. The relative percentages of nitrogen, phosphorus, and potassium, and the total concentrations of manganese in parts per million, increased in foliage, as well. In both experiments, the source of cuttings impacted results. In the first experiment, cuttings taken from the source that was in the most light were least likely to survive (26% survival rate) compared with cuttings taken from stock plants growing in partial shade (65% or 82% survival rates, by site). In the second experiment, cuttings taken from source plants that were most intensively managed for removal of weeds and competing plants had the highest survival rate and the greatest shoot and root dry weight. We recommend propagating twinflower with moderate rates of fertility (i.e., 2.1 g·L−1 of incorporated controlled-release fertilizer) under some shade (5.8–14.4 DLI) and a moderate θ (0.30–0.35 L·L−1).