The best time to pull up weeds with long tap-roots, like dandelion and yellow dock, is just after a good soaking rain. Even without a special tool, the tap-root can be wiggled a bit and then tugged right out of the ground. If even a bit of root is left behind it will regrow, but the plant will be weaker and will eventually die if the tops are cut off repeatedly. A good article on controlling dandelions organically is available from Northwest Center for Alternatives to Pesticides.