Fen plant biomass as growing media constituent - cultivation of Pelargonium zonale and Begonia × hiemalis in peat reduced mixtures with composted fen plant biomass

Peat is the most widely used growing media constituent in horticulture because of its favorable properties like high water and air capacity, reliable high quality as well as low pH and nutrient level. However, there are increasing environmental concerns about the use of peat and ongoing efforts are being made to find alternative growing media constituents. Fen plant biomass might be a promising peat substitute but needs to be composted before usage to stabilize its nitrogen dynamics. The aim of this study was to evaluate the suitability of composted fen plant biomass as growing media constituent. Young plants of Pelargonium zonale and Begonia × hiemalis were cultivated in growing media consisting of 20 or 40% composted fen plant biomass and 80 or 60% white peat by volume. Three composts from different fen plants were tested: Typha latifolia, Phragmites australis and Phalaris arundinacea. 100% white peat was used as control. After eight weeks (Begonia) and nine weeks (Pelargonium) of cultivation, shoot dry weight and total nitrogen content of the plants as well as soluble nitrogen in the growing media were determined. The shoot dry weight of Begonia cultivated in mixtures containing fen plant compost did not differ from the shoot dry weight of Begonia grown in 100% white peat. In contrast, Pelargonium had a significantly reduced shoot dry weight compared to the control when cultivated in growing media containing 20 and 40% composted Phragmites, respectively, or 40% composted Phalaris. The plants in the latter mixture also showed leaf chlorosis. The missing amount of nitrogen between the total given nitrogen on the one hand and on the other the nitrogen in the plant and the growing medium at the end of the experiment was in none of the compost-based growing media significantly larger than in the peat control.