Several flowers with hundreds of seeds per flower can pollute fields quickly, being dispersed on the wind. A germinating seed, if left to grow in the spring, summer or fall, will produce a plant that will be more easily controlled as a seedling in year one than a mature plant in years to come. The dandelion is not a native weed, but it has adapted well and now occurs throughout Ontario, in fertile fields and gardens, field margins and rocky hillsides. Even when weed control programs are adequate in fields, there is an abundant supply of new seeds to re-infest areas grown to field crops. Seed is produced without external pollination, from flowers which appear in May or June, and in the fall when daylight reduces to less than 12 hours.
Mature plants can develop massive tap root structures.
It is these that make the 'granddaddies' so difficult to kill. Systemic herbicides, like 2,4-D, Amitrol 240 and glyphosates must be absorbed in great enough quantities to kill this huge reserve from which the weed will recover and regrow, even when affected by a herbicide. Amitrol 240 has been particularly effective when used as a fall treatment, especially when higher rates are used. This is a time of year when mature dandelion plants are using their resources to store energy in their roots to sustain them through the harsh winter and to begin regrowth in the spring. Even if they are not killed by a spray application, their metabolism can be set back enough with the herbicide that winter hardiness is reduced enough to cause death.