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delayed weed seed germination

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Date and time: Fri, 25 Jun 2021 05:16:01 GMT

If the weed seed bank includes weeds that generally emerge after the stale or false seedbed period, the practice may not reduce weed pressure, but only change weed species composition. For example these techniques may work well for soybean in the north-central and northeastern states if the main weeds are common lambsquarters, Pennsylvania smartweed, ragweed, and others that normally emerge before soybean planting. However, if the dominant weeds include later-emerging species like redroot pigweed, common cocklebur, and large crabgrass, a false seedbed would either miss these weeds or entail an unacceptable delay in crop planting. Stale and false seedbed may be especially well suited for late plantings of vegetable crops like lettuce, snap bean, or cucumber, for which sufficient time is available to deplete weed seed populations in the germination zone before vegetable planting. In the southern states, carrots, beets, and some other root crops can be planted in the latter half of July, which allows time for several weeks’ cultivated fallow during the peak emergence period of many weeds.

In the false seedbed approach, weeds emerging in response to tillage are killed by two or more additional shallow cultivations at weekly intervals. The crop is planted immediately after the final cultivation. Because small weed seeds germinate better when the soil is firmed to enhance seed–soil contact, rolling is recommended after all except the final cultivation.

Depleting the Seed Bank by Stimulating Germination

Seeds require adequate seed–soil contact in order to take up the moisture needed to initiate germination. For small seeds such as those of lambsquarters, galinsoga, or Canada thistle, a fine tilth and firmed soil surface optimizes seed soil contact and promotes germination, whereas a coarse, loose seedbed can significantly reduce their germination. This is why higher weed densities sometimes occur within rows of crops seeded by mechanical planters with press wheels (Caldwell and Mohler, 2001), and is one of the mechanisms by which incorporated cover crop residues can reduce weed emergence (Gallandt, 2006). Gallandt et al. (1999) further suggest that moving loose soil over planting rows after mechanical seeding can reduce within-row populations of small-seeded weeds.

Multiple stimuli can break the dormancy of at least some seeds in the more persistent weed seed bank, and researchers continue to explore means by which these can be effectively delivered in the field to lower weed seed banks further (Egley, 1986). Whereas some of the methods investigated are not appropriate for organic systems (for example, applications of soluble N fertilizers or the synthetic ethylene-generating compound ethephon), other strategies may emerge that utilize organic soil management practices to provide multiple seed germination stimuli.

Since soluble N can stimulate germination of seeds of many weeds including pigweed and lambsquarters (Table 1), manipulation of soil fertility has been extensively explored as a tool for reducing weed density (Dyer, 1995). Practices that avoid large pulses of soluble N early in crop development, such as delayed or split N applications, or use of slow-releasing N sources such as mature compost, can delay weed emergence and reduce weed density in the crop. Conversely, incorporation of leguminous cover crops or applications of materials that release N rapidly, such as chicken manure, can promote weed emergence and growth. For example, ammonium released from decomposing hairy vetch can stimulate germination of smooth pigweed (Teasdale and Pillai, 2006). Mixing such legumes with grasses can reduce the concentration of soluble N released after incorporation and may be beneficial for weed management.