EFFECTS OF TIME OF EMERGENCE, POPULATION DENSITY AND INTERSPECIFIC COMPETITION ON HEMP-NETTLE (Galeopsis tetrahit) SEED PRODUCTION

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Abstract
Various conditions of intra- and interspecific competition may influence weed seed production thus affecting the extent of seed return to the soil. The objective of this study was to measure the effects of time of emergence, population density, and weed crop interaction on seed production of hemp-nettle (Galeopsis tetrahit) in natural field populations. Hemp-nettle seed production was studied in pure hemp-nettle stands, mixed hemp-nettle-stinkweed (Thlaspi arvense) stands, hemp-nettle-crop stands, and hemp-nettle-stinkweed-crop stands. Crop species used were alfalfa (Medicago sativa) and oats (Avena sativa). In pure hemp-nettle stands, delayed emergence and high population density reduced seed production of individual hemp-nettle plants. Hemp-nettle seed production per unit area was comparable for all but the lowest population density. Stink-weed population densities up to 200 plants m−2 had no consistent effect on average seed production of individual hemp-nettle plants. Seed production of individual plants was scarcely affected by the presence of alfalfa whereas it was largely reduced in the presence of the oat crop. Oats were usually more competitive than alfalfa, the latter crop being disadvantaged by slow initial growth. Hemp-nettle was not as prolific as other weed species but still produced enough seeds to reinfest the soil, even under highly competitive conditions.Key words: Hemp-nettle, Galeopsis tetrahit, seed production, weed reproduction

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