Chapter 14

Weed Management for Organic Crops

Weed Biology

Knowledge of weed biology is essential for development of both economically and environmentally acceptable weed management systems. Weed biology relates to plant attributes such as morphology, seed dormancy and germination, physiology of growth, competitive ability and reproductive biology. Concepts of population biology such as seedbank dynamics for annuals and root reserves, dormancy, and longevity of vegetative propagules for perennials can be used to predict weed infestations better and to evaluate sustainable management strategies.

Life Cycle of Weeds

Most weeds of organic cropping systems are herbaceous, but a few species that can become established in reduced-tillage fields are woody. Weeds can be categorized according to their life cycle, or how long they live: annual, biennial, and perennial. Knowledge of life cycles is important to reducing the potential for weeds to produce viable seed or vegetative structures that aid in weed dispersal.

Annuals

Plants that complete their life cycle in one year are annuals. They germinate from seed, grow, mature, produce seed and die in one year or less. Annuals reproduce by seed only and do not have any vegetative reproductive parts. Within the category, there are several subgroups: summer and winter annuals. Summer annual weeds germinate in the spring, then grow, flower and produce seeds during one growing season.

Biennials

Biennial species propagate through seeds and have a two-year life cycle. They germinate and grow vegetatively during the first year, overwinter, and flower, set seeds and die during the second year. Soil cultivation effectively prevents biennial species to flower and set seed.

Perennials

Perennials are plants that live for two or more years. Perennials can reproduce by seed or vegetatively. The plant parts that allow perennials to spread without producing seeds are stolons (creeping aboveground stems—e.g., white clover and strawberries), rhizomes (creeping belowground stems—e.g., milkweed, quackgrass), tubers (enlarged underground stems—e.g., potato, yellow nutsedge) and bulbs (underground stem covered by fleshy leaves – e.g., tulip).

Reproduction in Weeds

Plants have two main modes of reproduction, by seed or vegetatively. Most annuals and biennials reproduce by seed, and in the case of weeds, the production is often quite prolific.  Perennials can reproduce by seed as well as by vegetatively via rhizomes and stolons. A rhizome is an underground stem that sends out roots and shoots from its nodes.

Weed Seed Banks

Once a weed has produced seed and dispersed them in the soil, the majority of the seeds remain for a long period of time. This reservoir of viable seeds in the soil is called a seed bank. In any given year, only a small percentage of seeds in the seed bank germinate due to a variety of seed dormancy mechanisms.

Weed Emergence

Weeds rarely emerge in a single uniform flush. Emergence for each weed species is based on a wide variety of factors depending on the weather, soil type, tillage system, prior crop, and crop rotation. But year-to-year emergence and the duration of emergence, of a known species are fairly consistent.

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