The effect of organic matter addition and irrigation rates on the growth of bedding plants was found to vary with species. Marigold and sweet alyssum were field-grown with or without added peat moss under normal or 50 percent reduced irrigation.
Regardless of organic matter treatment, marigolds with reduced irrigation were shorter than those with normal irrigation. Under normal irrigation, adding organic matter had no effect on height. Under reduced irrigation, incorporating organic matter was beneficial to marigolds: plants in these plots were 10% taller than plants under reduced irrigation without added organic matter.
Sweet alyssum, a relatively drought-tolerant plant, was wider under reduced than under normal irrigation. It did not benefit from added organic matter: plants grown with added organic matter were 17% narrower than those without added organic matter, regardless of irrigation level. Blanket recommendations to add organic matter to conserve water should be avoided.