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The Life Cycle of Weeds in the Low Desert (February 19, 2014)
The lifecycle of weeds is normally classified as summer annuals, winter annuals, biennials and perennials. Summer annuals typically germinate in the spring, live for one season and die during the winter. If left until maturity, they will produce seed during this season. Winter annuals germinate in the late summer, fall and winter. They grow vegetatively until the spring when they develop flowers and seed. Biennials are weeds that by definition live for two years. They germinate in the spring of the first year, grow vegetatively through the summer and winter of this year and finally mature and produce seed during the second summer. There are not many true biennials in the low deserts of Arizona but many so-called annual weeds live for more than one year. It is not uncommon for some winter annual weeds to live for more than two years as biennials or even longer as a week perennial. Similarly, winter annual weeds can sometimes survive through this summer where they are shaded by a crop and received frequent irrigations. This does not occur with all species or even with all of a particular species in the same field. This has practical implications on weed management practices, four instance, where pre-emergent herbicides are used to control seedling weeds and are ineffective on those that were established in the previous seasons. The following table lists some of the common annual weeds and those that can survive year long.

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