Annual vinca [Catharanthus roseus (L.) G. Don] is intolerant of high fertility, cool temperatures, and wet soil conditions, making vinca difficult for growers to produce alongside other, more tolerant bedding plants. Our objective was to develop better recommendations for producers. Growth of `Grape Cooler' vinca was compared using different production inputs, including type of media (with or without bark), form of micronutrient source, and form of N. Optimal root and shoot dry weights occurred in peat-lite media with either sulfated or chelated micronutrients adjusted to pH 5.5. Root and shoot dry weights were greatest when high nitrate-N to ammonium-N ratio fertilizers were used. Root and shoot dry weights were negatively affected by high levels of ammonium-N in the fertilizer solution. Root development is the critical factor in the production of high-quality vinca. Our data suggest that root development may be optimized by using fertilizer products that have a high nitrate to ammoniacal nitrogen ratio. Micronutrients in the sulfate form also seem to enhance growth when medium pH is maintained near 5.5. Use of high-porosity, peat-based mixes appears to provide an optimal root growth environment.
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