Growth of Pelargonium x hortorum and Euphorbia pulcherrima in rubber-containing substrates

Geranium (Pelargonium xhortorum L.H. Bailey) 'Freckles' and poinsettia (Euphorbia pulcherrima Willd. ex Klotzch) 'Freedom' were grown in six peat and shredded-rubber substrates formulated to contain 75:25:0, 50:50:0, 25:75:0, 75:0:25, 50:0:50, 25:0:75 sphagnum peat : fine-grade rubber : coarse-grade rubber (by volume). Additionally, plants were grown in a 50 peat : 30 perlite : 20 loam (by volume) control substrate. Shredded rubber-containing substrates had higher bulk densities, lower total pore space, and higher total solids than the control substrate, Fine rubber-containing substrates had lower air-filled pore space (AFP) and lower water-holding capacities (WHC) than the control substrate. Substrates containing 25% coarse rubber had lower AFP and WHC than the control, but substrates containing 50% and 75% coarse shredded rubber had higher AFP and lower WHC than the control, Shredded rubber-containing substrates had significantly higher levels of Zn than the control substrate, Plants grown in rubber-containing substrates had tissue Zn levels significantly higher than the control and at levels reported to be phytotoxic in other species. Geraniums grown in rubber-containing substrates had lower root and shoot fresh mass, were shorter, and had fewer axillary branches than those grown in the control substrate. Poinsettia plants grown in rubber-containing substrates were shorter, had lower shoot fresh mass, fewer bracts, and lower bract area as compared to plants grown in the control substrate.