One year Seeding seven years Weeding

Dormancy is a state of seeds and buds in which they are alive but not germinated. If all weed seeds were to germinate at one time, their seedlings could be destroyed. Dormancy allows storage of millions of weed seeds in soil and enables them to grow in flushes over years. In this context, the old gardeners saying “One year Seeding seven years weeding” is very appropriate. In fact, weed seeds have been found viable even after 20-80 years of burial in soil.

Weed seeds exhibit three types of dormancy.

1. Enforced dormancy - It is due to deep placement of weed seeds in soil during ploughing of the field. Weed seeds germinate readily when they are restored to top 3-5 cm. Enforced Dormancy is a non-specific character of seed. Cultivation encounters enforced dormancy by bringing the weeds to surface where they are exposed to light besides better aeration. High soil temperature and NO3 content of surface soil may further help in breaking seed dormancy.

2. Innate dormancy - It is a genetically controlled character and it is a feature of specific weed seeds, which fail to germinate even if they are present in the top 3–5 cm soil, and adequate soil moisture and temperature provided to them. The possible reasons are the presence of (i) hard seed coats e.g., Setaria, Ipomoea, Xanthiums pp. and (ii) immature embryos e.g., Polygonum. In certain weed seeds particularly of Xerophytic origin, presence of inhibitors is responsible for innate dormancy. It can be overcome with passage of time, or under the influence of some climatic pressure.

3. Induced dormancy - Induced dormancy results from some sudden physiological change in normally non-dormant weed seeds under the impact of marked rise in temperature and or CO2 content of soil, low O2 pressure, water logging etc. Wild oat (Avena fatua) seeds exhibit all three kinds of dormancy.