Expert: John Locke - 6/10/2008

Question
There are a lot of dandelions in our yard and neighborhood. All at once I seemed to notice a major decline around the neighborhood. They seem to be gone now. Is there a certain length of time that dandelions grow. Many people had sprayed toxic chemicals on their yard. My wife, Rosemary, did not want to poison the soil in our yard

Answer
Thanks for using AllExperts, Rolland.

Dandelion growth depends upon the ambient temperature, with cooler temperatures leading to slower growth. The seeds will only germinate when the soil temperature is above 50 degrees Farenheit, meaning that dandelion growth will stop when the weather gets consistently cold; the plants grow most quickly when the soil is around 77 degrees.

Generally, dandelions will begin growing when the soil temperature reaches 50 degrees, generate leaves a few weeks from the start of growth, then produce flowers 8-15 weeks later. The flowers will continue to form indefinitely until stopped by a drop in temperature, meaning that it's unlikely that all the dandelions in your neighborhood stopped growing because of their life cycle. I would suspect that the herbicides used by your neighbors leached into your yard by wind and rain. This probably killed your dandelions as well, but these chemicals must be applied repeatedly and at the appropriate time to be effective in the long-term. These compounds are also not free of health risks to humans.

If you don't wish to use herbicides, the manual removal of dandelions will control existing infestations, though it will not prevent new ones and is labor-intensive. Good luck!

The distinctive leaves of the dandelion:
http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pni7469-1b.html

These leaves form when the seedling germinates, which lasts about 8-15 weeks from the start of growth. The flowering stem then grows from the seedling, as shown here:

http://www.ipm.ucdavis.edu/PMG/T/W-CO-TOFF-MP.001.html

More information can be found here:

http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7469.html