Question
I have three plants that I believe are Moon Flower.  Each plant now is about the size of a dinner plate and spread out flat on the ground.  The leaves are similar to those of a dandylion but longer and narrower.  The Yellow flowers come out of the middle of the plant and open up in the evening and  by morning looking pinkish in colour they are closed and dying.  I started these plants in my garden in early Spring and they were the size of a small teacup.  They have since grown larger and have produced many flowers.
I think that they reseed themselves.  I live in Southern Ontario on the shores of Lake Erie.  I hope that you can give me a name for these plants.  Thanks so much.

Answer
Pat, it sure does sound like you are growing at least one plant called Moonflower in your neck of the woods, right down to the reseeding behavior and the hardiness factor.

Several species carry large, fragrant flowers that open at dusk are referred to as Moonflowers.

Wikipedia (en.wikipedia.org/wiki/Datura_stramonium) posts an excellent illustration and description of Moonflowers - Datura species - including foliage that resembles a Dandelion.  You can see a double yellow Datura metel, sold as "Angel's Trumpet" in the color they call Ballerina Yellow, at Park's Seed, which describes the seedpods as "jazzy".  Yours, perhaps?

Ipomoea initially appears to be your flower.  Like Datura, it has a shady reputation in law enforcement circles, appealing to teenagers who eat the seedpods for kicks and instead find themselves becoming poisoning victims.

Ipomoea muricata (Purple Moonflower) appears on certain lists for causing "Moonflower Intoxication".

Ipomoea alba (White Moonflower) does not have a reputation for hallucinations or other scary side effects. Blue Ipomoeas are called Morning Glories(protophoto.com/picture.html?pic=6312). The shape of the flower may look close, but even in yellow or white, these do not sound like your Dandelion-leaved, dinnerplate-proportioned Moonflower.

So... Scratch Ipomoea.  The more I think about it, the more this looks like Daturas you are growing.

Datura species are highly toxic but used nevertheless by fun-seekers to get high, and can be ingested by pets with disturbing consequences. You should be careful where you grow them.

The American Brugmansia and Datura Society (www.abads.net/datura_metel.htm) will give you any advice you might need about these plants.  Just make sure you don't eat them, and don't let children play near them - the "jazzy" seedpods might look too juicy to resist.