All of these aspects were investigated in the experiments reported below.
The samples were planted in 14cm pots filled with either peat or coconut fibre and were grown in a greenhouse in Murcia, Spain, between May and mid July. One half of the plants growing in each substrate were sprayed once with paclobutrazol (CULTAR 25 per cent).
The pots were irrigated using drip irrigation and the leachate was monitored. Evapotranspiration was measured by weighing representative pots every hour. When plants were raised without being sprayed with paclobutrazol, the ones in coconut fibre were shorter than those in peat. When the growth retardant was applied, the plants were even shorter and there was less difference between them.
The best plants were those grown in coconut fibre without any growth retardant while the poorest were those sprayed with paclobutrazol and grown in coconut fibre. The plants received similar amounts of water in both substrates but water consumption was less in the coconut fibre because more water leached from this substrate.
Influence of Paclobutrazol and Substrate on Daily Evapotranspiration of Potted Geranium by Banon, Miralles, Navarro and Sanchez-Blanco (2009). Scientia Horticulturae 122 (4): 572-578. The contents of issues of Scientia Horticulturae and abstracts of papers are provided at www.elsevier.com/locate/scihorti.
Dr Ken Cockshull is emeritus fellow at Warwick HRI