Winter annuals germinate in the fall, and if you apply pre-emergence herbicides for winter annuals in spring, the herbicide will probably be broken down by soil microbes before the seeds germinate, and the application will be wasted. Examples of winter annuals include annual bluegrass and common chickweed. Apply pre-emergence herbicides for these weeds later in the summer. Biennials live for two years. They germinate in one season, pass the winter as a rosette of leaves close to the ground, produce flowers and seeds the second year, then die.
Dandelions are usually biennials, although many live for several years.
Pre-emergence herbicides are not usually applied strictly for dandelions because seed are produced and germinate throughout the growing season. Perennials live for two years or more and are more difficult to control because they reproduce both from seed and from vegetative buds on the roots. Pre-emergence herbicides have little affect on shoots arising from these vegetative buds, and no effect on vegetative buds of the perennials that have deep root systems. Pre-emergence herbicides are usually applied in spring for germinating perennial seeds. If you are not sure of the identification of a problem weed, take a sample to your county Extension agent or county weed office for identification.