Most people think about trying to control their dandelions in the spring when they see them in bloom. However, this is not the best time to try to control them, particularly if they plan to use a broadleaf herbicide to do it. Dandelions are in their most active growth stage in the spring and are actually somewhat resistant to the herbicide at that time. Yes, you can curl up the flower stems and the leaves a bit but will probably not really kill that many dandelion plants at that time of year. Another larger concern is that there are so many other broadleaf plants actively growing at that time of year too, most with lots of soft, succulent foliage that will easily take up the herbicide. The result is “collateral” damage to trees, shrubs, perennial flowers and often garden vegetable plants. Dry, granular weed and feed products that contain a fertilizer as well as a broadleaf herbicide may be somewhat safer than a liquid herbicide because it is less likely to drift. But the efficacy of a weed and feed product may not be as good, particularly if it is not applied correctly.
Fall treatment of dandelions is a much better option for a number of reasons. First, dandelion plants are much more susceptible to the effects of the herbicide in the fall. They are actively storing carbohydrates in their roots which makes it easier for the herbicide to be translocated to the roots as well. Secondly, plants of all ages will be susceptible. Spring treatments will not affect the seed that is flying around at that time of year. Once that seed lands, it starts the next generation of plants that will have avoided the spring herbicide application. So, the next spring, dandelions will still be there in the lawn. Treating in the fall is much more likely to break the cycle of new plants growing up from seed because there are usually only a few flowers forming in the fall. While most garden vegetables will be damaged by the frost, dandelions are cold tolerant enough that they can withstand freezing temperatures and still be susceptible to control with herbicides. If they are still green, you can probably still spray, but it will be best to do it on a day when the temperature is above 50°F.
Dandelions are not all bad however, I recall mother putting the flowers in a big crock to ferment with a dish towel over the top. I do not remember how the dandelion wine tasted however, probably not very good to a young child of six or so. Dandelions can also be eaten, particularly in the early spring when they are not too bitter tasting or mixed in salads. The young leaves as well as the crown of the plant can be used in salads. The leaves are often blanched to reduce the bitter flavor. The roots can be dried and ground up to make a coffee substitute. The roots are sometimes used as a component of root beer. Dandelions are credited with various medicinal properties as well.