Question
i planted my new lawn about 3 months ago and ive being cutting it for the past 2 months. the grass appears to be healthy but the lawn is littered with thistles, can i kill these without harming the grass

Answer
Thistles, Thistles everywhere -- see this link by James Altland at the Oregon State Agricultural Center Cooperative Extension Service for help identifying Thistles A to Z:

http://oregonstate.edu/dept/nursery-weeds/feature_articles/other_thistles/other_...

Note that some Thistle is NOT Thistle.  Annual Sowthistle and Spiny Sowthistle, point out the Oregon authorities, fall into the NOT category.  Bull Thistle (Cirsium vulgare), also known as Common Thistle and Spear Thistle, falls into the Thistle category.  Some Thistles are Annuals, some are Perennials, and some are Biennials.

It helps to identify these because if you can i.d. an Annual or a Biennial, you know immediately that you are looking at the light at the end of the tunnel.  These have a short lifespan.  But they are Hell-bent on rapid growth and self propogation; it is essentialy that you do not let these set seed.  If they don't, you're home free.

Knowing which Thistle is which also helps you figure out the best way to deal with them.  Bull Thistle, they maintain, 'will not survive cultivation'; they claim it cannot grow successfully in 'well-maintained nursery fields'.  Moreover, Bull Thistle (a Biennial) spreads ONLY from Seed.  If you keep it from setting Seeds, it's history.  End of story.

The best way to get rid of Thistles and Dandelions is to dig them out by hand, root and all.

Thistles are prickly above the soil line, but they are perfectly smooth in the stem just below.  Do this task on a day after you get rain, or after a Thunderstorm; the soil will be more cooperative, and you need all the help you can get -- Thistle roots have nerves of steel and will simply re-sprout stems if you leave any root pieces behind.  Dandelion forks make this job a little easier, and a more thorough.

This page on 'Lawn Weed Control' from the University of Wisconsin Extension Centers includes a bit of inside information about this technique for getting rid of Thistle and other broadleaf Weeds:

http://clean-water.uwex.edu/pubs/pdf/home.lawnweed.pdf

They note, 'This is most effective in the Spring (April or May) when Weeds have their lowest food reserves stored in roots.  In the Fall, food reserves are at their peak and Weeds are more likely to grow back after digging.'

Yes, timing is everything.

No, there is no magic bullet for this problem.  Pull them out -- COMPLETELY! -- one by one.