Periodicity of Germination and Emergence of Some Annual Weeds

Seeds of yellow foxtail Setaria lutescens (Weigel) Hubb., ivyleaf morningglory Ipomoea hederacea (L.) Jacq., common cocklebur ( Xanthium pensylvanicum Wallr.), jimsonweed ( Datura stramonium L.), velvetleaf ( Abutilon theophrasti Medic.), and giant ragweed ( Ambrosia trifida L.) were buried in the soil November 20 and 21, 1966 at Urbana, Illinois for noting emergence of seedlings from April 1 through August 18, 1967. Similarly, seeds of yellow foxtail, ivyleaf morningglory, jimsonweed, velvetleaf, giant ragweed, common ragweed ( Ambrosia artemisiifolia L.), and Pennsylvania smartweed ( Polygonum pensylvanicum L.) were buried on October 25, 1968 for emergence observations from April 1 to August 18, 1969. Pennsylvania smartweed, giant ragweed, and common ragweed had large flushes of germination from early April through early May, with no emergence after June 1. Velvetleaf displayed similar early flushes and had additional small flushes of emergence in late May or June. Yellow foxtail seedlings also emerged in April and May in 1969 and in May and June during both years. Common cocklebur seedlings emerged abundantly in April and May but less abundantly in June. Ivyleaf morningglory and jimsonweed displayed flushes of emergence sporadically after May 1. Flushes of emergence for all species which occurred after May 1 were preceded by sufficient rainfall to bring the surface 10 cm of soil to field capacity. Cumulative heat units in the soil above 10 C were not correlated with initiation of emergence for any species. The early emergence was attributed to stimuli from general soil warming while emergence after May 1 was stimulated by favorable soil moisture from rainfall.