Common milkweed(Asclepias syriacaL.) seed did not survive storage in water for 1, 2, and 12 months at 40, 30, and 5 C, respectively; but under dry storage seed viability was destroyed only at 40 C for 5 months. Stratifying common milkweed seed in water at 5 C was more effective in breaking dormancy than at 22 C. Mechanical scarification also caused a significant increase in germination. Exogenous 6-furfurylamino purine (kinetin) and gibberellic acid-3 (GA) increased germination while indole-3-acetic acid (IAA) had no effect. The optimum germination temperature scheme for common milkweed was an alternating 20 to 30 C, the maximum between 35 and 40 C, and the minimum between 15 and 20 C. Common milkweed seedlings were more susceptible to moisture stress than kochia [Kochia scoparia(L.) Schrad.] but less susceptible than hemp dogbane(Apocynum cannabinumL.) and sunflower(Helianthus annuusL.). In a greenhouse study common milkweed was less drought tolerant than eight other weed species. Common milkweed seedlings tolerated wider pH ranges than honeyvine milkweed [Ampelamus albidus(Nutt.) Britt.], kochia, and sunflower. Common milkweed seedlings were less tolerant to salt solutions than four other species. Common milkweed emerged from 6-cm depths in Sharpsburg silty clay loam (sicl) at 20 or 30 C but only from the 3-cm depth at an alternating 20 to 30 C. Results from root box studies showed that common and honeyvine milkweed produced less shoot and root weight than kochia and sunflower and had a lower growth rate index of roots than sunflower. Common milkweed seedlings resprouted 21 days after planting when clipped at ground level. As days from planting to clipping increased, sprouting activity increased.

An experiment was initiated in 1970 and continued through 1979 by exhuming and germinating seed of 12 economic weed species buried beneath 23 cm of soil in eastern and western Nebraska. Loss in germination of exhumed seeds over years is mathematically characterized by the formula for the rectangular hyperbola, which represents many shapes of curves that have zero as their lower limit. Of the 12 weed species, only fall panicum (Panicum dichotomiflorumMichx.) and redroot pigweed (Amaranthus retroflexusL.) seed germination did not drop significantly over the 10-yr burial period. Germination of redroot pigweed seed was higher when buried in eastern Nebraska, but was higher for smooth groundcherry (Physalis subglabrataMack&Bush.) and velvetleaf (Abutilon theophrastiMedic.) when buried in western Nebraska. Germination of the other nine species were not affected by burial location. The 12 weed species can be ranked as those showing most to least rapid loss of germination during burial for 10 yr as follows: honeyvine milkweed [Ampelamus albidus(Nutt.) Britt.], hemp dogbane (Apocynum cannabinumL.), kochia [Kochia scoparia(L.) Schrad.], sunflower (Helianthus annumL.), large crabgrass [Digitaria sanguinalis(L.) Scop.], common milkweed (Asclepias syriacaL.), musk thistle (Carduus nutansL.), velvetleaf, fall panicum, redroot pigweed, green foxtail [Setaria viridis(L.) Beauv.], and smooth groundcherry.

The root growth rate index of velvetleaf (Abutilon theophrastiMedic.) was significantly greater than that of five other weed species. The root growth rate index of sorghum [Sorghum bicolor(L.) Moench ‘RS-626’] was found to be significantly higher than that of soybeans [Glycine max(L.) Merr. ‘Ford’]. Shoot growth rate indexes of three perennial broadleaf species studied were significantly lower than shoot growth rate indexes of six annual species. Root weights of perennial broadleaf species were not significantly different from root weights of velvetleaf and soybeans. Soybeans were found to have a significantly higher shoot to root ratio than seven weed species but not higher than sorghum. All other species had larger leaf weights and leaf areas than the three broadleaf perennials. While there were significant differences in leaf weights among six species, there were no significant differences in leaf area. Seed weight was not significantly correlated with emergence rate index and root weight but was correlated to all other factors studied. Emergence rate index was correlated with root growth rate index but not with shoot growth rate index.

Common milkweed (Asclepias syriaca L.) is a deep‐rooted perennial weed that has increased in occurrence in recent years and has caused much concern among landowners. Because it occurs frequently in dryland sorghum [Sorghum bicolor (L.) Moench], a 3‐year competition study using paired plots was conducted to determine the degree of yield loss caused by common milkweed. Sorghum yield and number of heads per hectare were significantly reduced by common milkweed competition each of the 3 years. Common milkweed reduced average yield by 21% and number of sorghum heads per hectare by 14%. Even low populations (less than 12,000 plants/ha) of common milkweed resulted in severe yield losses. Grain weight per head was significantly reduced by common milkweed competition in 1970 and 1972 but not in 1971. Competition had no effect on sorghum 500‐ seed weight in 1971 or 1972. Percentage protein (Kjeldahl method) of sorghum grain was significantly increased by competition in 1972 but not in 1971.

The germination and growth of six pigweeds (Amaranthus spp.) were studied in the germinator and greenhouse. Treatments with sulfuric acid, acetone, sodium hypochlorate, and potassium nitrate variously influenced species germination. Seedlings emerged more rapidly at 100 F than at other soil temperatures. Variation in seedling characteristics was noted. Some species were more susceptible to 4-(methylsulfonyl)-2,6-dinitro-N,N-dipropylaniline (nitralin) and 2,4-bis (isopropylamino)-6-(methylthio)-s-triazine (prometryne) than others in the greenhouse. In the field little variation in susceptibility to several preplant and preemergence herbicides was found; but there were differences when postemergence herbicides were used.